Network Working Group P. Karn
Request for Comments: 2522 Qualcomm
Category: EXPerimental W. Simpson
DayDreamer
March 1999
Photuris: Session-Key Management Protocol
Status of this Memo
This document defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). Copyright (C) Philip Karn
and William Allen Simpson (1994-1999). All Rights Reserved.
Abstract
Photuris is a session-key management protocol intended for use with
the IP Security Protocols (AH and ESP). This document defines the
basic protocol mechanisms.
Table of Contents
1. IntrodUCtion .......................................... 1
1.1 Terminology ..................................... 1
1.2 Protocol Overview ............................... 3
1.3 Security Parameters ............................. 5
1.4 LifeTimes ....................................... 6
1.4.1 Exchange LifeTimes .............................. 6
1.4.2 SPI LifeTimes ................................... 7
1.5 Random Number Generation ........................ 8
2. Protocol Details ...................................... 9
2.1 UDP ............................................. 9
2.2 Header Format ................................... 10
2.3 Variable Precision Integers ..................... 11
2.4 Exchange-Schemes ................................ 13
2.5 Attributes ...................................... 13
3. Cookie Exchange ....................................... 14
3.0.1 Send Cookie_Request ............................. 14
3.0.2 Receive Cookie_Request .......................... 15
3.0.3 Send Cookie_Response ............................ 15
3.0.4 Receive Cookie_Response ......................... 16
3.1 Cookie_Request .................................. 17
3.2 Cookie_Response ................................. 18
3.3 Cookie Generation ............................... 19
3.3.1 Initiator Cookie ................................ 19
3.3.2 Responder Cookie ................................ 20
4. Value Exchange ........................................ 21
4.0.1 Send Value_Request .............................. 21
4.0.2 Receive Value_Request ........................... 22
4.0.3 Send Value_Response ............................. 22
4.0.4 Receive Value_Response .......................... 23
4.1 Value_Request ................................... 24
4.2 Value_Response .................................. 25
4.3 Offered Attribute List .......................... 26
5. Identification Exchange ............................... 28
5.0.1 Send Identity_Request ........................... 29
5.0.2 Receive Identity_Request ........................ 29
5.0.3 Send Identity_Response .......................... 30
5.0.4 Receive Identity_Response ....................... 30
5.1 Identity_Messages ............................... 31
5.2 Attribute Choices List .......................... 33
5.3 Shared-Secret ................................... 34
5.4 Identity Verification ........................... 34
5.5 Privacy-Key Computation ......................... 36
5.6 Session-Key Computation ......................... 37
6. SPI Messages .......................................... 38
6.0.1 Send SPI_Needed ................................. 38
6.0.2 Receive SPI_Needed .............................. 39
6.0.3 Send SPI_Update ................................. 39
6.0.4 Receive SPI_Update .............................. 39
6.0.5 Automated SPI_Updates ........................... 40
6.1 SPI_Needed ...................................... 41
6.2 SPI_Update ...................................... 43
6.2.1 Creation ........................................ 44
6.2.2 Deletion ........................................ 45
6.2.3 Modification .................................... 45
6.3 Validity Verification ........................... 45
7. Error Messages ........................................ 46
7.1 Bad_Cookie ...................................... 47
7.2 Resource_Limit .................................. 47
7.3 Verification_Failure ............................ 48
7.4 Message_Reject .................................. 49
8. Public Value Exchanges ................................ 50
8.1 Modular Exponentiation Groups ................... 50
8.2 Moduli Selection ................................ 50
8.2.1 Bootstrap Moduli ................................ 51
8.2.2 Learning Moduli ................................. 51
8.3 Generator Selection ............................. 51
8.4 Exponent Selection .............................. 52
8.5 Defective Exchange Values ....................... 53
9. Basic Exchange-Schemes ................................ 54
10. Basic Key-Generation-Function ......................... 55
10.1 MD5 Hash ........................................ 55
11. Basic Privacy-Method .................................. 55
11.1 Simple MaSKINg .................................. 55
12. Basic Validity-Method ................................. 55
12.1 MD5-IPMAC Check ................................. 55
13. Basic Attributes ...................................... 56
13.1 Padding ......................................... 56
13.2 AH-Attributes ................................... 57
13.3 ESP-Attributes .................................. 57
13.4 MD5-IPMAC ....................................... 58
13.4.1 Symmetric Identification ........................ 58
13.4.2 Authentication .................................. 59
13.5 Organizational .................................. 60
APPENDICES ................................................... 61
A. Automaton ............................................. 61
A.1 State Transition Table .......................... 62
A.2 States .......................................... 65
A.2.1 Initial ......................................... 65
A.2.2 Cookie .......................................... 66
A.2.3 Value ........................................... 66
A.2.4 Identity ........................................ 66
A.2.5 Ready ........................................... 66
A.2.6 Update .......................................... 66
B. Use of Identification and Secrets ..................... 67
B.1 Identification .................................. 67
B.2 Group Identity With Group Secret ................ 67
B.3 Multiple Identities With Group Secrets .......... 68
B.4 Multiple Identities With Multiple Secrets ....... 69
OPERATIONAL CONSIDERATIONS ................................... 70
SECURITY CONSIDERATIONS ...................................... 70
HISTORY ...................................................... 71
ACKNOWLEDGEMENTS ............................................. 72
REFERENCES ................................................... 73
CONTACTS ..................................................... 75
COPYRIGHT .................................................... 76
1. Introduction
Photuris [Firefly] establishes short-lived session-keys between two
parties, without passing the session-keys across the Internet. These
session-keys directly replace the long-lived secret-keys (such as
passWords and passphrases) that have been historically configured for
security purposes.
The basic Photuris protocol utilizes these existing previously
configured secret-keys for identification of the parties. This is
intended to speed deployment and reduce administrative configuration
changes.
This document is primarily intended for implementing the Photuris
protocol. It does not detail service and application interface
definitions, although it does mention some basic policy areas
required for the proper implementation and operation of the protocol
mechanisms.
Since the basic Photuris protocol is extensible, new data types and
protocol behaviour should be expected. The implementor is especially
cautioned not to depend on values that appear in examples to be
current or complete, since their purpose is primarily pedagogical.
1.1. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC-2119].
byte An 8-bit quantity; also known as "octet" in
standardese.
exchange-value The publically distributable value used to calculate
a shared-secret. As used in this document, refers
to a Diffie-Hellman exchange, not the public part of
a public/private key-pair.
private-key A value that is kept secret, and is part of an
asymmetric public/private key-pair.
public-key A publically distributable value that is part of an
asymmetric public/private key-pair.
secret-key A symmetric key that is not publically
distributable. As used in this document, this is
distinguished from an asymmetric public/private
key-pair. An example is a user password.
Security Association (SA)
A collection of parameters describing the security
relationship between two nodes. These parameters
include the identities of the parties, the transform
(including algorithm and algorithm mode), the key(s)
(such as a session-key, secret-key, or appropriate
public/private key-pair), and possibly other
information such as sensitivity labelling.
Security Parameters Index (SPI)
A number that indicates a particular set of uni-
directional attributes used under a Security
Association, such as transform(s) and session-
key(s). The number is relative to the IP
Destination, which is the SPI Owner, and is unique
per IP (Next Header) Protocol. That is, the same
value MAY be used by multiple protocols to
concurrently indicate different Security Association
parameters.
session-key A key that is independently derived from a shared-
secret by the parties, and used for keying one
direction of traffic. This key is changed
frequently.
shared-secret As used in this document, the calculated result of
the Photuris exchange.
SPI Owner The party that corresponds to the IP Destination;
the intended recipient of a protected datagram.
SPI User The party that corresponds to the IP Source; the
sender of a protected datagram.
transform A cryptographic manipulation of a particular set of
data. As used in this document, refers to certain
well-specified methods (defined elsewhere). For
example, AH-MD5 [RFC-1828] transforms an IP datagram
into a cryptographic hash, and ESP-DES-CBC [RFC-
1829] transforms plaintext to ciphertext and back
again.
Many of these terms are hierarchically related:
Security Association (bi-directional)
- one or more lists of Security Parameters (uni-directional)
-- one or more Attributes
--- may have a key
--- may indicate a transform
Implementors will find details of cryptographic hashing (such as
MD5), encryption algorithms and modes (such as DES), digital
signatures (such as DSS), and other algorithms in [Schneier95].
1.2. Protocol Overview
The Photuris protocol consists of several simple phases:
1. A "Cookie" Exchange guards against simple flooding attacks sent
with bogus IP Sources or UDP Ports. Each party passes a "cookie"
to the other.
In return, a list of supported Exchange-Schemes are offered by the
Responder for calculating a shared-secret.
2. A Value Exchange establishes a shared-secret between the parties.
Each party passes an Exchange-Value to the other. These values
are used to calculate a shared-secret. The Responder remains
stateless until a shared-secret has been created.
In addition, supported attributes are offered by each party for
use in establishing new Security Parameters.
3. An Identification Exchange identifies the parties to each other,
and verifies the integrity of values sent in phases 1 and 2.
In addition, the shared-secret provides a basis to generate
separate session-keys in each direction, which are in turn used
for conventional authentication or encryption. Additional
security attributes are also exchanged as needed.
This exchange is masked for party privacy protection using a
message privacy-key based on the shared-secret. This protects the
identities of the parties, hides the Security Parameter attribute
values, and improves security for the exchange protocol and
security transforms.
4. Additional messages may be exchanged to periodically change the
session-keys, and to establish new or revised Security Parameters.
These exchanges are also masked for party privacy protection in
the same fashion as above.
The sequence of message types and their purposes are summarized in
the diagram below. The first three phases (cookie, exchange, and
identification) must be carried out in their entirety before any
Security Association can be used.
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
Value_Request ->
pick scheme
offer value
offer attributes
<- Value_Response
offer value
offer attributes
[generate shared-secret from exchanged values]
Identity_Request ->
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
<- Identity_Response
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
[make SPI session-keys in each direction]
SPI User SPI Owner
======== =========
SPI_Needed ->
list SPI attribute(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
<- SPI_Update
make SPI
pick SPI attribute(s)
make SPI session-key(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
Either party may initiate an exchange at any time. For example, the
Initiator need not be a "caller" in a telephony link.
The Initiator is responsible for recovering from all message losses
by retransmission.
1.3. Security Parameters
A Photuris exchange between two parties results in a pair of SPI
values (one in each direction). Each SPI is used in creating
separate session-key(s) in each direction.
The SPI is assigned by the entity controlling the IP Destination: the
SPI Owner (receiver). The parties use the combination of IP
Destination, IP (Next Header) Protocol, and SPI to distinguish the
correct Security Association.
When both parties initiate Photuris exchanges concurrently, or one
party initiates more than one Photuris exchange, the Initiator
Cookies (and UDP Ports) keep the exchanges separate. This results in
more than one initial SPI for each Destination.
To create multiple SPIs with different parameters, the parties may
also send SPI_Updates.
There is no requirement that all such outstanding SPIs be used. The
SPI User (sender) selects an appropriate SPI for each datagram
transmission.
Implementation Notes:
The method used for SPI assignment is implementation dependent.
The only requirement is that the SPI be unique for the IP
Destination and IP (Next Header) Protocol.
However, selection of a cryptographically random SPI value can
help prevent attacks that depend on a predicatable sequence of
values. The implementor MUST NOT expect SPI values to have a
particular order or range.
1.4. LifeTimes
The Photuris exchange results in two kinds of state, each with
separate LifeTimes.
1) The Exchange LifeTime of the small amount of state associated with
the Photuris exchange itself. This state may be viewed as between
Internet nodes.
2) The SPI LifeTimes of the individual SPIs that are established.
This state may be viewed as between users and nodes.
The SPI LifeTimes may be shorter or longer than the Exchange
LifeTime. These LifeTimes are not required to be related to each
other.
When an Exchange-Value expires (or is replaced by a newer value), any
unexpired derived SPIs are not affected. This is important to allow
traffic to continue without interruption during new Photuris
exchanges.
1.4.1. Exchange LifeTimes
All retained exchange state of both parties has an associated
Exchange LifeTime (ELT), and is subject to periodic expiration. This
depends on the physical and logistical security of the machine, and
is typically in the range of 10 minutes to one day (default 30
minutes).
In addition, during a Photuris exchange, an Exchange TimeOut (ETO)
limits the wait for the exchange to complete. This timeout includes
the packet round trips, and the time for completing the
Identification Exchange calculations. The time is bounded by both
the maximum amount of calculation delay expected for the processing
power of an unknown peer, and the minimum user expectation for
results (default 30 seconds).
These Exchange LifeTimes and TimeOuts are implementation dependent
and are not disclosed in any Photuris message. The paranoid operator
will have a fairly short Exchange LifeTime, but it MUST NOT be less
than twice the ETO.
To prevent synchronization between Photuris exchanges, the
implementation SHOULD randomly vary each Exchange LifeTime within
twice the range of seconds that are required to calculate a new
Exchange-Value. For example, when the Responder uses a base ELT of
30 minutes, and takes 10 seconds to calculate the new Exchange-Value,
the equation might be (in milliseconds):
1790000 + urandom(20000)
The Exchange-Scheme, Exchange-Values, and resulting shared-secret MAY
be cached in short-term storage for the Exchange LifeTime. When
repetitive Photuris exchanges occur between the same parties, and the
Exchange-Values are discovered to be unchanged, the previously
calculated shared-secret can be used to rapidly generate new
session-keys.
1.4.2. SPI LifeTimes
Each SPI has an associated LifeTime, specified by the SPI owner
(receiver). This SPI LifeTime (SPILT) is usually related to the
speed of the link (typically 2 to 30 minutes), but it MUST NOT be
less than thrice the ETO.
The SPI can also be deleted by the SPI Owner using the SPI_Update.
Once the SPI has expired or been deleted, the parties cease using the
SPI.
To prevent synchronization between multiple Photuris exchanges, the
implementation SHOULD randomly vary each SPI LifeTime. For example,
when the Responder uses a base SPILT of 5 minutes, and 30 seconds for
the ETO, the equation might be (in milliseconds):
285000 + urandom(30000)
There is no requirement that a long LifeTime be accepted by the SPI
User. The SPI User might never use an established SPI, or cease
using the SPI at any time.
When more than one unexpired SPI is available to the SPI User for the
same function, a common implementation technique is to select the SPI
with the greatest remaining LifeTime. However, selecting randomly
among a large number of SPIs might provide some defense against
traffic analysis.
To prevent resurrection of deleted or expired SPIs, SPI Owners SHOULD
remember those SPIs, but mark them as unusable until the Photuris
exchange shared-secret used to create them also expires and purges
the associated state.
When the SPI Owner detects an incoming SPI that has recently expired,
but the associated exchange state has not yet been purged, the
implementation MAY accept the SPI. The length of time allowed is
highly dependent on clock drift and variable packet round trip time,
and is therefore implementation dependent.
1.5. Random Number Generation
The security of Photuris critically depends on the quality of the
secret random numbers generated by each party. A poor random number
generator at either party will compromise the shared-secret produced
by the algorithm.
Generating cryptographic quality random numbers on a general purpose
computer without hardware assistance is a very tricky problem. In
general, this requires using a cryptographic hashing function to
"distill" the entropy from a large number of semi-random external
events, such as the timing of key strokes. An Excellent discussion
can be found in [RFC-1750].
2. Protocol Details
The Initiator begins a Photuris exchange under several circumstances:
- The Initiator has a datagram that it wishes to send with
confidentiality, and has no current Photuris exchange state with
the IP Destination. This datagram is discarded, and a
Cookie_Request is sent instead.
- The Initiator has received the ICMP message [RFC-1812] Destination
Unreachable: Communication Administratively Prohibited (Type 3,
Code 13), and has no current Photuris exchange state with the ICMP
Source.
- The Initiator has received the ICMP message [RFC-2521] Security
Failures: Bad SPI (Type 40, Code 0), that matches current Photuris
exchange state with the ICMP Source.
- The Initiator has received the ICMP message [RFC-2521] Security
Failures: Need Authentication (Type 40, Code 4), and has no
current Photuris exchange state with the ICMP Source.
- The Initiator has received the ICMP message [RFC-2521] Security
Failures: Need Authorization (Type 40, Code 5), that matches
current Photuris exchange state with the ICMP Source.
When the event is an ICMP message, special care MUST be taken that
the ICMP message actually includes information that matches a
previously sent IP datagram. Otherwise, this could provide an
opportunity for a clogging attack, by stimulating a new Photuris
Exchange.
2.1. UDP
All Photuris messages use the User Datagram Protocol header [RFC-
768]. The Initiator sends to UDP Destination Port 468.
When replying to the Initiator, the Responder swaps the IP Source and
Destination, and the UDP Source and Destination Ports.
The UDP checksum MUST be correctly calculated when sent. When a
message is received with an incorrect UDP checksum, it is silently
discarded.
Implementation Notes:
It is expected that installation of Photuris will ensure that UDP
checksum calculations are enabled for the computer operating
system and later disabling by operators is prevented.
Internet Protocol version 4 [RFC-791] restricts the maximum
reassembled datagram to 576 bytes.
When processing datagrams containing variable size values, the
length must be checked against the overall datagram length. An
invalid size (too long or short) that causes a poorly coded
receiver to abort could be used as a denial of service attack.
2.2. Header Format
All of the messages have a format similar to the following, as
transmitted left to right in network order (most significant to least
significant):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message
+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes.
Responder-Cookie 16 bytes.
Message 1 byte. Each message type has a unique value.
Initial values are assigned as follows:
0 Cookie_Request
1 Cookie_Response
2 Value_Request
3 Value_Response
4 Identity_Request
5 Secret_Response (optional)
6 Secret_Request (optional)
7 Identity_Response
8 SPI_Needed
9 SPI_Update
10 Bad_Cookie
11 Resource_Limit
12 Verification_Failure
13 Message_Reject
Further details and differences are elaborated in the individual
messages.
2.3. Variable Precision Integers
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Size Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Size 2, 4, or 8 bytes. The number of significant bits
used in the Value field. Always transmitted most
significant byte first.
When the Size is zero, no Value field is present;
there are no significant bits. This means "missing"
or "null". It should not be confused with the value
zero, which includes an indication of the number of
significant bits.
When the most significant byte is in the range 0
through 254 (0xfe), the field is 2 bytes. Both
bytes are used to indicate the size of the Value
field, which ranges from 1 to 65,279 significant
bits (in 1 to 8,160 bytes).
When the most significant byte is 255 (0xff), the
field is 4 bytes. The remaining 3 bytes are added
to 65,280 to indicate the size of the Value field,
which is limited to 16,776,959 significant bits (in
2,097,120 bytes).
When the most significant 2 bytes are 65,535
(0xffff), the field is 8 bytes. The remaining 6
bytes are added to 16,776,960 to indicate the size
of the Value field.
Value 0 or more bytes. Always transmitted most
significant byte first.
The bits used are right justified within byte
boundaries; that is, any unused bits are in the most
significant byte. When there are no unused bits, or
unused bits are zero filled, the value is assumed to
be an unsigned positive integer.
When the leading unused bits are ones filled, the
number is assumed to be a two's-complement negative
integer. A negative integer will always have at
least one unused leading sign bit in the most
significant byte.
Shortened forms SHOULD NOT be used when the Value includes a number
of leading zero significant bits. The Size SHOULD indicate the
correct number of significant bits.
Implementation Notes:
Negative integers are not required to be supported, but are
included for completeness.
No more than 65,279 significant bits are required to be supported.
Other ranges are vastly too long for these UDP messages, but are
included for completeness.
2.4. Exchange-Schemes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Scheme Size
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Scheme 2 bytes. A unique value indicating the Exchange-
Scheme. See the "Basic Exchange-Schemes" for
details.
Size 2 bytes, ranging from 0 to 65,279. See "Variable
Precision Integer".
Value 0 or more bytes. See "Variable Precision Integer".
The Size MUST NOT be assumed to be constant for a particular Scheme.
Multiple kinds of the same Scheme with varying Sizes MAY be present
in any list of schemes.
However, only one of each Scheme and Size combination will be present
in any list of schemes.
2.5. Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Length Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 1 byte. A unique value indicating the kind of
attribute. See the "Basic Attributes" for details.
When the value is zero (padding), no Length field is
present (always zero).
Length 1 byte. The size of the Value(s) field in bytes.
When the Length is zero, no Value(s) field is
present.
Value(s) 0 or more bytes. See the "Basic Attributes" for
details.
The Length MUST NOT be assumed to be constant for a particular
Attribute. Multiple kinds of the same Attribute with varying Lengths
MAY be present in any list of attributes.
3. Cookie Exchange
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
3.0.1. Send Cookie_Request
The Initiator initializes local state, and generates a unique
"cookie". The Initiator-Cookie MUST be different in each new
Cookie_Request between the same parties. See "Cookie Generation" for
details.
- If any previous exchange between the peer IP nodes has not expired
in which this party was the Initiator, this Responder-Cookie is
set to the most recent Responder-Cookie, and this Counter is set
to the corresponding Counter.
For example, a new Virtual Private Network (VPN) tunnel is about
to be established to an existing partner. The Counter is the same
value received in the prior Cookie_Response, the Responder-Cookie
remains the same, and a new Initiator-Cookie is generated.
- If the new Cookie_Request is in response to a message of a
previous exchange in which this party was the Responder, this
Responder-Cookie is set to the previous Initiator-Cookie, and this
Counter is set to zero.
For example, a Bad_Cookie message was received from the previous
Initiator in response to SPI_Needed. The Responder-Cookie is
replaced with the Initiator-Cookie, and a new Initiator-Cookie is
generated. This provides bookkeeping to detect bogus Bad_Cookie
messages.
Also, can be used for bi-directional User, Transport, and Process
oriented keying. Such mechanisms are outside the scope of this
document.
- Otherwise, this Responder-Cookie and Counter are both set to zero.
By default, the Initiator operates in the same manner as when all
of its previous exchange state has expired. The Responder will
send a Resource_Limit when its own exchange state has not expired.
The Initiator also starts a retransmission timer. If no valid
Cookie_Response arrives within the time limit, the same
Cookie_Request is retransmitted for the remaining number of
Retransmissions. The Initiator-Cookie value MUST be the same in each
such retransmission to the same IP Destination and UDP Port.
When Retransmissions have been exceeded, if a Resource_Limit message
has been received during the exchange, the Initiator SHOULD begin the
Photuris exchange again by sending a new Cookie_Request with updated
values.
3.0.2. Receive Cookie_Request
On receipt of a Cookie_Request, the Responder determines whether
there are sufficient resources to begin another Photuris exchange.
- When too many SPI values are already in use for this particular
peer, or too many concurrent exchanges are in progress, or some
other resource limit is reached, a Resource_Limit message is sent.
- When any previous exchange initiated by this particular peer has
not exceeded the Exchange TimeOut, and the Responder-Cookie does
not specify one of these previous exchanges, a Resource_Limit
message is sent.
Otherwise, the Responder returns a Cookie_Response.
Note that the Responder creates no additional state at this time.
3.0.3. Send Cookie_Response
The IP Source for the Initiator is examined. If any previous
exchange between the peer IP nodes has not expired, the response
Counter is set to the most recent exchange Counter plus one (allowing
for out of order retransmissions). Otherwise, the response Counter
is set to the request Counter plus one.
If (through rollover of the Counter) the new Counter value is zero
(modulo 256), the value is set to one.
If this new Counter value matches some previous exchange initiated by
this particular peer that has not yet exceeded the Exchange TimeOut,
the Counter is incremented again, until a unique Counter value is
reached.
Nota Bene:
No more than 254 concurrent exchanges between the same two peers
are supported.
The Responder generates a unique cookie. The Responder-Cookie value
in each successive response SHOULD be different. See "Cookie
Generation" for details.
The Exchange-Schemes available between the peers are listed in the
Offered-Schemes.
3.0.4. Receive Cookie_Response
The Initiator validates the Initiator-Cookie, and the Offered-
Schemes.
- When an invalid/expired Initiator-Cookie is detected, the message
is silently discarded.
- When the variable length Offered-Schemes do not match the UDP
Length, or all Offered-Schemes are obviously defective and/or
insufficient for the purposes intended, the message is silently
discarded; the implementation SHOULD log the occurance, and notify
an operator as appropriate.
- Once a valid message has been received, later Cookie_Responses
with matching Initiator-Cookies are also silently discarded, until
a new Cookie_Request is sent.
When the message is valid, an Exchange-Scheme is chosen from the list
of Offered-Schemes.
This Scheme-Choice may affect the next Photuris message sent. By
default, the next Photuris message is a Value_Request.
Implementation Notes:
Only the Initiator-Cookie is used to identify the exchange. The
Counter and Responder-Cookie will both be different from the
Cookie_Request.
Various proposals for extensions utilize the Scheme-Choice to
indicate a different message sequence. Such mechanisms are
outside the scope of this document.
3.1. Cookie_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Counter
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. A randomized value that identifies the
exchange. The value MUST NOT be zero. See "Cookie
Generation" for details.
Responder-Cookie 16 bytes. Identifies a specific previous exchange.
Copied from a previous Cookie_Response.
When zero, no previous exchange is specified.
When non-zero, and the Counter is zero, contains the
Initiator-Cookie of a previous exchange. The
specified party is requested to be the Responder in
this exchange, to retain previous party pairings.
When non-zero, and the Counter is also non-zero,
contains the Responder-Cookie of a previous
exchange. The specified party is requested to be
the Responder in this exchange, to retain previous
party pairings.
Message 0
Counter 1 byte. Indicates the number of previous exchanges.
When zero, the Responder-Cookie indicates the
Initiator of a previous exchange, or no previous
exchange is specified.
When non-zero, the Responder-Cookie indicates the
Responder to a previous exchange. This value is set
to the Counter from the corresponding
Cookie_Response or from a Resource_Limit.
3.2. Cookie_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Counter Offered-Schemes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. Copied from the Cookie_Request.
Responder-Cookie 16 bytes. A randomized value that identifies the
exchange. The value MUST NOT be zero. See "Cookie
Generation" for details.
Message 1
Counter 1 byte. Indicates the number of the current
exchange. Must be greater than zero.
Offered-Schemes 4 or more bytes. A list of one or more Exchange-
Schemes supported by the Responder, ordered from
most to least preferable. See the "Basic Exchange-
Schemes" for details.
Only one Scheme (#2) is required to be supported,
and SHOULD be present in every Offered-Schemes list.
More than one of each kind of Scheme may be offered,
but each is distinguished by its Size. The end of
the list is indicated by the UDP Length.
3.3. Cookie Generation
The exact technique by which a Photuris party generates a cookie is
implementation dependent. The method chosen must satisfy some basic
requirements:
1. The cookie MUST depend on the specific parties. This prevents an
attacker from oBTaining a cookie using a real IP address and UDP
port, and then using it to swamp the victim with requests from
randomly chosen IP addresses or ports.
2. It MUST NOT be possible for anyone other than the issuing entity
to generate cookies that will be accepted by that entity. This
implies that the issuing entity will use local secret information
in the generation and subsequent verification of a cookie. It
must not be possible to deduce this secret information from any
particular cookie.
3. The cookie generation and verification methods MUST be fast to
thwart attacks intended to sabotage CPU resources.
A recommended technique is to use a cryptographic hashing function
(such as MD5).
An incoming cookie can be verified at any time by regenerating it
locally from values contained in the incoming datagram and the local
secret random value.
3.3.1. Initiator Cookie
The Initiator secret value that affects its cookie SHOULD change for
each new Photuris exchange, and is thereafter internally cached on a
per Responder basis. This provides improved synchronization and
protection against replay attacks.
An alternative is to cache the cookie instead of the secret value.
Incoming cookies can be compared directly without the computational
cost of regeneration.
It is recommended that the cookie be calculated over the secret
value, the IP Source and Destination addresses, and the UDP Source
and Destination ports.
Implementation Notes:
Although the recommendation includes the UDP Source port, this is
very implementation specific. For example, it might not be
included when the value is constant.
However, it is important that the implementation protect mutually
suspicious users of the same machine from generating the same
cookie.
3.3.2. Responder Cookie
The Responder secret value that affects its cookies MAY remain the
same for many different Initiators. However, this secret SHOULD be
changed periodically to limit the time for use of its cookies
(typically each 60 seconds).
The Responder-Cookie SHOULD include the Initiator-Cookie. The
Responder-Cookie MUST include the Counter (that is returned in the
Cookie_Response). This provides improved synchronization and
protection against replay attacks.
It is recommended that the cookie be calculated over the secret
value, the IP Source and Destination addresses, its own UDP
Destination port, the Counter, the Initiator-Cookie, and the
currently Offered-Schemes.
The cookie is not cached per Initiator to avoid saving state during
the initial Cookie Exchange. On receipt of a Value_Request
(described later), the Responder regenerates its cookie for
validation.
Once the Value_Response is sent (also described later), both
Initiator and Responder cookies are cached to identify the exchange.
Implementation Notes:
Although the recommendation does not include the UDP Source port,
this is very implementation specific. It might be successfully
included in some variants.
However, it is important that the UDP Source port not be included
when matching existing Photuris exchanges for determining the
appropriate Counter.
The recommendation includes the Offered-Schemes to detect a
dynamic change of scheme value between the Cookie_Response and
Value_Response.
Some mechanism MAY be needed to detect a dynamic change of pre-
calculated Responder Exchange-Value between the Value_Response and
Identity_Response. For example, change the secret value to render
the cookie invalid, or explicitly mark the Photuris exchange state
as expired.
4. Value Exchange
Initiator Responder
========= =========
Value_Request ->
pick scheme
offer value
offer attributes
<- Value_Response
offer value
offer attributes
[generate shared-secret from exchanged values]
4.0.1. Send Value_Request
The Initiator generates an appropriate Exchange-Value for the
Scheme-Choice. This Exchange-Value may be pre-calculated and used
for multiple Responders.
The IP Destination for the Responder is examined, and the attributes
available between the parties are listed in the Offered-Attributes.
The Initiator also starts a retransmission timer. If no valid
Value_Response arrives within the time limit, the same Value_Request
is retransmitted for the remaining number of Retransmissions.
When Retransmissions have been exceeded, if a Bad_Cookie or
Resource_Limit message has been received during the exchange, the
Initiator SHOULD begin the Photuris exchange again by sending a new
Cookie_Request.
4.0.2. Receive Value_Request
The Responder validates the Responder-Cookie, the Counter, the
Scheme-Choice, the Exchange-Value, and the Offered-Attributes.
- When an invalid/expired Responder-Cookie is detected, a Bad_Cookie
message is sent.
- When too many SPI values are already in use for this particular
peer, or too many concurrent exchanges are in progress, or some
other resource limit is reached, a Resource_Limit message is sent.
- When an invalid Scheme-Choice is detected, or the Exchange-Value
is obviously defective, or the variable length Offered-Attributes
do not match the UDP Length, the message is silently discarded;
the implementation SHOULD log the occurance, and notify an
operator as appropriate.
When the message is valid, the Responder sets its Exchange timer to
the Exchange TimeOut, and returns a Value_Response.
The Responder keeps a copy of the incoming Value_Request cookie pair,
and its Value_Response. If a duplicate Value_Request is received, it
merely resends its previous Value_Response, and takes no further
action.
4.0.3. Send Value_Response
The Responder generates an appropriate Exchange-Value for the
Scheme-Choice. This Exchange-Value may be pre-calculated and used
for multiple Initiators.
The IP Source for the Initiator is examined, and the attributes
available between the parties are listed in the Offered-Attributes.
Implementation Notes:
At this time, the Responder begins calculation of the shared-
secret. Calculation of the shared-secret is executed in parallel
to minimize delay.
This may take a substantial amount of time. The implementor
should ensure that retransmission is not blocked by this
calculation. This is not usually a problem, as retransmission
timeouts typically exceed calculation time.
4.0.4. Receive Value_Response
The Initiator validates the pair of Cookies, the Exchange-Value, and
the Offered-Attributes.
- When an invalid/expired cookie is detected, the message is
silently discarded.
- When the Exchange-Value is obviously defective, or the variable
length Offered-Attributes do not match the UDP Length, the message
is silently discarded; the implementation SHOULD log the
occurance, and notify an operator as appropriate.
- Once a valid message has been received, later Value_Responses with
both matching cookies are also silently discarded, until a new
Cookie_Request is sent.
When the message is valid, the Initiator begins its parallel
computation of the shared-secret.
When the Initiator completes computation, it sends an
Identity_Request to the Responder.
4.1. Value_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Counter Scheme-Choice
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Exchange-Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 bytes. Copied from the Cookie_Response.
Responder-Cookie 16 bytes. Copied from the Cookie_Response.
Message 2
Counter 1 byte. Copied from the Cookie_Response.
Scheme-Choice 2 bytes. A value selected by the Initiator from the
list of Offered-Schemes in the Cookie_Response.
Only the Scheme is specified; the Size will match
the Initiator-Exchange-Value, and the Value(s) are
implicit.
Initiator-Exchange-Value
Variable Precision Integer. Provided by the
Initiator for calculating a shared-secret between
the parties. The Value format is indicated by the
Scheme-Choice.
The field may be any integral number of bytes in
length, as indicated by its Size field. It does not
require any particular alignment. The 32-bit
alignment shown is for convenience in the
illustration.
Initiator-Offered-Attributes
4 or more bytes. A list of Security Parameter
attributes supported by the Initiator.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.2. Value_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Exchange-Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Responder-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 bytes. Copied from the Value_Request.
Responder-Cookie 16 bytes. Copied from the Value_Request.
Message 3
Reserved 3 bytes. For future use; MUST be set to zero when
transmitted, and MUST be ignored when received.
Responder-Exchange-Value
Variable Precision Integer. Provided by the
Responder for calculating a shared-secret between
the parties. The Value format is indicated by the
current Scheme-Choice specified in the
Value_Request.
The field may be any integral number of bytes in
length, as indicated by its Size field. It does not
require any particular alignment. The 32-bit
alignment shown is for convenience in the
illustration.
Responder-Offered-Attributes
4 or more bytes. A list of Security Parameter
attributes supported by the Responder.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.3. Offered Attribute List
This list includes those attributes supported by the party that are
available to the other party. The attribute formats are specified in
the "Basic Attributes".
The list is composed of two or three sections: Identification-
Attributes, Authentication-Attributes, and (optional) Encapsulation-
Attributes. Within each section, the attributes are ordered from
most to least preferable.
The first section of the list includes methods of identification. An
Identity-Choice is selected from this list.
The second section of the list begins with "AH-Attributes" (#1). It
includes methods of authentication, and other operational types.
The third section of the list begins with "ESP-Attributes" (#2). It
includes methods of authentication, compression, encryption, and
other operational types. When no Encapsulation-Attributes are
offered, the "ESP-Attributes" attribute itself is omitted from the
list.
Attribute-Choices are selected from the latter two sections of the
list.
Support is required for the "MD5-IPMAC" (#5) attribute for both
"Symmetric Identification" and "Authentication" and they SHOULD be
present in every Offered-Attributes list.
Implementation Notes:
For example,
"MD5-IPMAC" (Symmetric Identification),
"AH-Attributes",
"MD5-IPMAC" (Authentication).
Since the offer is made by the prospective SPI User (sender),
order of preference likely reflects the capabilities and
engineering tradeoffs of a particular implementation.
However, the critical processing bottlenecks are frequently in the
receiver. The SPI Owner (receiver) may express its needs by
choosing a less preferable attribute.
The order may also be affected by operational policy and requested
services for an application. Such considerations are outside the
scope of this document.
The list may be divided into additional sections. These sections
will always follow the ESP-Attributes section, and are
indistinguishable from unrecognized attributes.
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions.
5. Identification Exchange
Initiator Responder
========= =========
Identity_Request ->
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
<- Identity_Response
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
[make SPI session-keys in each direction]
The exchange of messages is ordered, although the formats and
meanings of the messages are identical in each direction. The
messages are easily distinguished by the parties themselves, by
examining the Message and Identification fields.
Implementation Notes:
The amount of time for the calculation may be dependent on the
value of particular bits in secret values used in generating the
shared-secret or identity verification. To prevent analysis of
these secret bits by recording the time for calculation, sending
of the Identity_Messages SHOULD be delayed until the time expected
for the longest calculation. This will be different for different
processor speeds, different algorithms, and different length
variables. Therefore, the method for estimating time is
implementation dependent.
Any authenticated and/or encrypted user datagrams received before
the completion of identity verification can be placed on a queue
pending completion of this step. If verification succeeds, the
queue is processed as though the datagrams had arrived subsequent
to the verification. If verification fails, the queue is
discarded.
5.0.1. Send Identity_Request
The Initiator chooses an appropriate Identification, the SPI and
SPILT, a set of Attributes for the SPI, calculates the Verification,
and masks the message using the Privacy-Method indicated by the
current Scheme-Choice.
The Initiator also starts a retransmission timer. If no valid
Identity_Response arrives within the time limit, its previous
Identity_Request is retransmitted for the remaining number of
Retransmissions.
When Retransmissions have been exceeded, if a Bad_Cookie message has
been received during the exchange, the Initiator SHOULD begin the
Photuris exchange again by sending a new Cookie_Request.
5.0.2. Receive Identity_Request
The Responder validates the pair of Cookies, the Padding, the
Identification, the Verification, and the Attribute-Choices.
- When an invalid/expired cookie is detected, a Bad_Cookie message
is sent.
- After unmasking, when invalid Padding is detected, the variable
length Attribute-Choices do not match the UDP Length, or an
attribute was not in the Offered-Attributes, the message is
silently discarded.
- When an invalid Identification is detected, or the message
verification fails, a Verification_Failure message is sent.
- Whenever such a problem is detected, the Security Association is
not established; the implementation SHOULD log the occurance, and
notify an operator as appropriate.
When the message is valid, the Responder sets its Exchange timer to
the Exchange LifeTime (if this has not already been done for a
previous exchange). When its parallel computation of the shared-
secret is complete, the Responder returns an Identity_Response.
The Responder keeps a copy of the incoming Identity_Request values,
and its Identity_Response. If a duplicate Identity_Request is
received, it merely resends its previous Identity_Response, and takes
no further action.
5.0.3. Send Identity_Response
The Responder chooses an appropriate Identification, the SPI and
SPILT, a set of Attributes for the SPI, calculates the Verification,
and masks the message using the Privacy-Method indicated by the
current Scheme-Choice.
The Responder calculates the SPI session-keys in both directions.
At this time, the Responder begins the authentication and/or
encryption of user datagrams.
5.0.4. Receive Identity_Response
The Initiator validates the pair of Cookies, the Padding, the
Identification, the Verification, and the Attribute-Choices.
- When an invalid/expired cookie is detected, the message is
silently discarded.
- After unmasking, when invalid Padding is detected, the variable
length Attribute-Choices do not match the UDP Length, or an
attribute was not in the Offered-Attributes, the message is
silently discarded.
- When an invalid Identification is detected, or the message
verification fails, a Verification_Failure message is sent.
- Whenever such a problem is detected, the Security Association is
not established; the implementation SHOULD log the occurance, and
notify an operator as appropriate.
- Once a valid message has been received, later Identity_Responses
with both matching cookies are also silently discarded, until a
new Cookie_Request is sent.
When the message is valid, the Initiator sets its Exchange timer to
the Exchange LifeTime (if this has not already been done for a
previous exchange).
The Initiator calculates the SPI session-keys in both directions.
At this time, the Initiator begins the authentication and/or
encryption of user datagrams.
5.1. Identity_Messages
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message LifeTime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security-Parameters-Index
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Identity-Choice
+ + + + + + + + + + + + + + + + + +
~ Identification ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Verification ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute-Choices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. Copied from the Value_Request.
Responder-Cookie 16 bytes. Copied from the Value_Request.
Message 4 (Request) or 7 (Response)
LifeTime 3 bytes. The number of seconds remaining before the
indicated SPI expires.
When the SPI is zero, this field MUST be filled with
a random non-zero value.
Security-Parameters-Index (SPI)
4 bytes. The SPI to be used for incoming
communications.
When zero, indicates that no SPI is created in this
direction.
Identity-Choice 2 or more bytes. An identity attribute is selected
from the list of Offered-Attributes sent by the
peer, and is used to calculate the Verification.
The field may be any integral number of bytes in
length, as indicated by its Length field. It does
not require any particular alignment. The 16-bit
alignment shown is for convenience in the
illustration.
Identification Variable Precision Integer, or alternative format
indicated by the Identity-Choice. See the "Basic
Attributes" for details.
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Verification Variable Precision Integer, or alternative format
indicated by the Identity-Choice. The calculation
of the value is described in "Identity
Verification".
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Attribute-Choices
0 or more bytes. When the SPI is non-zero, a list
of attributes selected from the list of Offered-
Attributes supported by the peer.
The contents and usage of this list are further
described in "Attribute Choices List". The end of
the list is indicated by the UDP Length after
removing the Padding (UDP Length - last Padding
value).
Padding 8 to 255 bytes. This field is filled up to at least
a 128 byte boundary, measured from the beginning of
the message. The number of pad bytes are chosen
randomly.
In addition, when a Privacy-Method indicated by the
current Scheme-Choice requires the plaintext to be a
multiple of some number of bytes (the block size of
a block cipher), this field is adjusted as necessary
to the size required by the algorithm.
Self-Describing-Padding begins with the value 1.
Each byte contains the index of that byte. Thus,
the final pad byte indicates the number of pad bytes
to remove. For example, when the unpadded message
length is 120 bytes, the padding values might be 1,
2, 3, 4, 5, 6, 7, and 8.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
5.2. Attribute Choices List
This list specifies the attributes of the SPI. The attribute formats
are specified in the "Basic Attributes".
The list is composed of one or two sections: Authentication-
Attributes, and/or Encapsulation-Attributes.
When sending from the SPI User to the SPI Owner, the attributes are
processed in the order listed. For example,
"ESP-Attributes",
"Deflate" (Compression),
"XOR" (Encryption),
"DES-CBC" (Encryption),
"XOR" (Encryption),
"AH-Attributes",
"AH-Sequence",
"MD5-IPMAC" (Authentication),
would result in ESP with compression and triple encryption (inside),
and then AH authentication with sequence numbers (outside) of the ESP
payload.
The SPI Owner will naturally process the datagram in the reverse
order.
This ordering also affects the order of key generation. Both SPI
Owner and SPI User generate the keys in the order listed.
Implementation Notes:
When choices are made from the list of Offered-Attributes, it is
not required that any Security Association include every kind of
offered attribute in any single SPI, or that a separate SPI be
created for every offered attribute.
Some kinds of attributes may be included more than once in a
single SPI. The set of allowable combinations of attributes are
dependent on implementation and operational policy. Such
considerations are outside the scope of this document.
The list may be divided into additional sections. This can occur
only when both parties recognize the affected attributes.
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions.
5.3. Shared-Secret
A shared-secret is used in a number of calculations. Regardless of
the internal representation of the shared-secret, when used in
calculations it is in the same form as the Value part of a Variable
Precision Integer:
- most significant byte first.
- bits used are right justified within byte boundaries.
- any unused bits are in the most significant byte.
- unused bits are zero filled.
The shared-secret does not include a Size field.
5.4. Identity Verification
These messages are authenticated using the Identity-Choice. The
Verification value is calculated prior to masking (and optional
encryption), and verified after unmasking (and optional decryption).
The Identity-Choice authentication function is supplied with two
input values:
- the sender (SPI Owner) verification-key,
- the data to be verified (as a concatenated sequence of bytes).
The resulting output value is stored in the Verification field.
The Identity-Choice verification data consists of the following
concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI fields,
+ the Identity-Choice and Identification,
+ the SPI User Identity Verification (response only),
+ the Attribute-Choices following the Verification field,
+ the Padding,
+ the SPI Owner TBV,
+ the SPI Owner Exchange-Value,
+ the SPI Owner Offered-Attributes,
+ the SPI User TBV,
+ the SPI User Exchange-Value,
+ the SPI User Offered-Attributes,
+ the Responder Offered-Schemes.
The TBV (Three Byte Value) consists of the Counter and Scheme-Choice
fields from the Value_Request, or the Reserved field from the
Value_Response, immediately preceding the associated Exchange-Value.
Note that the order of the Exchange-Value and Offered-Attributes
fields is different in each direction, and the Identification and SPI
fields are also likely to be different in each direction. Note also
that the SPI User Identity Verification (from the Identity_Request)
is present only in the Identity_Response.
If the verification fails, the users are notified, and a
Verification_Failure message is sent, without adding any SPI. On
success, normal operation begins with the authentication and/or
encryption of user datagrams.
Implementation Notes:
This is distinct from any authentication method specified for the
SPI.
The exact details of the Identification and verification-key
included in the Verification calculation are dependent on the
Identity-Choice, as described in the "Basic Attributes".
Each party may wish to keep their own trusted databases, such as
the Pretty Good Privacy (PGP) web of trust, and accept only those
identities found there. Failure to find the Identification in
either an internal or external database results in the same
Verification_Failure message as failure of the verification
computation.
The Exchange-Value data includes both the Size and Value fields.
The Offered-Attributes and Attribute-Choices data includes the
Attribute, Length and Value fields.
5.5. Privacy-Key Computation
Identification Exchange messages are masked using the Privacy-Method
indicated by the current Scheme-Choice. Masking begins with the next
field after the SPI, and continues to the end of the data indicated
by the UDP Length, including the Padding.
The Scheme-Choice specified Key-Generation-Function is used to create
a special privacy-key for each message. This function is calculated
over the following concatenated values:
+ the SPI Owner Exchange-Value,
+ the SPI User Exchange-Value,
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI (or Reserved) fields,
+ the computed shared-secret.
Since the order of the Exchange-Value fields is different in each
direction, and the Message, LifeTime and SPI fields are also
different in each direction, the resulting privacy-key will usually
be different in each direction.
When a larger number of keying-bits are needed than are available
from one iteration of the specified Key-Generation-Function, more
keying-bits are generated by duplicating the trailing shared-secret,
and recalculating the function. That is, the first iteration will
have one trailing copy of the shared-secret, the second iteration
will have two trailing copies of the shared-secret, and so forth.
Implementation Notes:
This is distinct from any encryption method specified for the SPI.
The length of the Padding, and other details, are dependent on the
Privacy-Method. See the "Basic Privacy-Method" list for details.
To avoid keeping the Exchange-Values in memory after the initial
verification, it is often possible to pre-compute the function
over the initial bytes of the concatenated data values for each
direction, and append the trailing copies of the shared-secret.
The Exchange-Value data includes both the Size and Value fields.
5.6. Session-Key Computation
Each SPI has one or more session-keys. These keys are generated
based on the attributes of the SPI. See the "Basic Attributes" for
details.
The Scheme-Choice specified Key-Generation-Function is used to create
the SPI session-key for that particular attribute. This function is
calculated over the following concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the SPI Owner generation-key,
+ the SPI User generation-key,
+ the message Verification field,
+ the computed shared-secret.
Since the order of the generation-keys is different in each
direction, and the Verification field is also likely to be different
in each direction, the resulting session-key will usually be
different in each direction.
When a larger number of keying-bits are needed than are available
from one iteration of the specified Key-Generation-Function, more
keying-bits are generated by duplicating the trailing shared-secret,
and recalculating the function. That is, the first iteration will
have one trailing copy of the shared-secret, the second iteration
will have two trailing copies of the shared-secret, and so forth.
Implementation Notes:
This is distinct from any privacy-key generated for the Photuris
exchange. Different initialization data is used, and iterations
are maintained separately.
The exact details of the Verification field and generation-keys
that are included in the session-key calculation are dependent on
the Identity-Choices, as described in the "Basic Attributes".
To avoid keeping the generation-keys in memory after the initial
verification, it is often possible to pre-compute the function
over the initial bytes of the concatenated data values for each
direction, and append the trailing copies of the shared-secret.
When both authentication and encryption attributes are used for
the same SPI, there may be multiple session-keys associated with
the same SPI. These session-keys are generated in the order of
the Attribute-Choices list.
6. SPI Messages
SPI User SPI Owner
======== =========
SPI_Needed ->
list SPI attribute(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
<- SPI_Update
make SPI
pick SPI attribute(s)
make SPI session-key(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
The exchange of messages is not related to the Initiator and
Responder. Instead, either party may send one of these messages at
any time. The messages are easily distinguished by the parties.
6.0.1. Send SPI_Needed
At any time after completion of the Identification Exchange, either
party can send SPI_Needed. This message is sent when a prospective
SPI User needs particular attributes for a datagram (such as
confidentiality), and no current SPI has those attributes.
The prospective SPI User selects from the intersection of attributes
that both parties have previously offered, calculates the
Verification, and masks the message using the Privacy-Method
indicated by the current Scheme-Choice.
6.0.2. Receive SPI_Needed
The potential SPI Owner validates the pair of Cookies, the Padding,
the Verification, and the Attributes-Needed.
- When an invalid/expired cookie is detected, a Bad_Cookie message
is sent.
- When too many SPI values are already in use for this particular
peer, or some other resource limit is reached, a Resource_Limit
message is sent.
- After unmasking, when invalid Padding is detected, the variable
length Attributes-Needed do not match the UDP Length, or an
attribute was not in the Offered-Attributes, the message is
silently discarded.
- When the message verification fails, a Verification_Failure
message is sent.
- Whenever such a problem is detected, the SPI is not established;
the implementation SHOULD log the occurance, and notify an
operator as appropriate.
When the message is valid, the party SHOULD send SPI_Update with the
necessary attributes.
If an existing SPI has those attributes, that SPI is returned in the
SPI_Update with the remaining SPILT.
6.0.3. Send SPI_Update
At any time after completion of the Identification Exchange, either
party can send SPI_Update. This message has effect in only one
direction, from the SPI Owner to the SPI User.
The SPI Owner chooses the SPI and SPILT, a set of Attributes for the
SPI, calculates the Verification, and masks the message using the
Privacy-Method indicated by the current Scheme-Choice.
6.0.4. Receive SPI_Update
The prospective SPI User validates the pair of Cookies, the Padding,
the Verification, and the Attributes-Needed.
- When an invalid/expired cookie is detected, a Bad_Cookie message
is sent.
- After unmasking, when invalid Padding is detected, the variable
length Attribute-Choices do not match the UDP Length, an attribute
was not in the Offered-Attributes, or the message modifies an
existing SPI, the message is silently discarded.
- When the message verification fails, a Verification_Failure
message is sent.
- Whenever such a problem is detected, the SPI is not established;
the implementation SHOULD log the occurance, and notify an
operator as appropriate.
When the message is valid, further actions are dependent on the value
of the LifeTime field, as described later.
6.0.5. Automated SPI_Updates
Each SPI requires replacement under several circumstances:
- the volume of data processed (inhibiting probability
cryptanalysis),
- exhaustion of available anti-replay Sequence Numbers,
- and expiration of the LifeTime.
In general, a determination is made upon receipt of a datagram. If
the transform specific processing finds that refreshment is needed,
an automated SPI_Update is triggered.
In addition, automated SPI_Updates allow rapid SPI refreshment for
high bandwidth applications in a high delay environment. The update
messages flow in the opposite direction from the primary traffic,
conserving bandwidth and avoiding service interruption.
When creating each SPI, the implementation MAY optionally set an
Update TimeOut (UTO); by default, to half the value of the LifeTime
(SPILT/2). This time is highly dynamic, and adjustable to provide an
automated SPI_Update long before transform specific processing. If
no new Photuris exchange occurs within the time limit, and the
current exchange state has not expired, an automated SPI_Update is
sent.
6.1. SPI_Needed
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Reserved-LT
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved-SPI
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
~ Verification ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attributes-Needed ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. Copied from the Value_Request.
Responder-Cookie 16 bytes. Copied from the Value_Request.
Message 8
Reserved-LT 3 bytes. For future use; MUST be filled with a
random non-zero value when transmitted, and MUST be
ignored when received.
Reserved-SPI 4 bytes. For future use; MUST be set to zero when
transmitted, and MUST be ignored when received.
Verification Variable Precision Integer, or other format
indicated by the current Scheme-Choice. The
calculation of the value is described in "Validity
Verification".
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Attributes-Needed
4 or more bytes. A list of two or more attributes,
selected from the list of Offered-Attributes
supported by the peer.
The contents and usage of this list are as
previously described in "Attribute Choices List".
The end of the list is indicated by the UDP Length
after removing the Padding (UDP Length - last
Padding value).
Padding 8 or more bytes. The message is padded in the same
fashion specified for Identification Exchange
messages.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
6.2. SPI_Update
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message LifeTime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security-Parameters-Index
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
~ Verification ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute-Choices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. Copied from the Value_Request.
Responder-Cookie 16 bytes. Copied from the Value_Request.
Message 9
LifeTime 3 bytes. The number of seconds remaining before the
indicated SPI expires. The value zero indicates
deletion of the indicated SPI.
Security-Parameters-Index (SPI)
4 bytes. The SPI to be used for incoming
communications.
This may be a new SPI value (for creation), or an
existing SPI value (for deletion). The value zero
indicates special processing.
Verification Variable Precision Integer, or other format
indicated by the current Scheme-Choice. The
calculation of the value is described in "Validity
Verification".
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Attribute-Choices
0 or more bytes. When the SPI and SPILT are non-
zero, a list of attributes selected from the list of
Offered-Attributes supported by the peer.
The contents and usage of this list are as
previously described in "Attribute Choices List".
The end of the list is indicated by the UDP Length
after removing the Padding (UDP Length - last
Padding value).
Padding 8 or more bytes. The message is padded in the same
fashion specified for Identification Exchange
messages.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
6.2.1. Creation
When the LifeTime is non-zero, and the SPI is also non-zero, the
SPI_Update can be used to create a new SPI. When the SPI is zero,
the SPI_Update is silently discarded.
The new session-keys are calculated in the same fashion as the
Identity_Messages. Since the SPI value is always different than any
previous SPI during the Exchange LifeTime of the shared-secret, the
resulting session-keys will necessarily be different from all others
used in the same direction.
No retransmission timer is necessary. Success is indicated by the
peer use of the new SPI.
Should all creation attempts fail, eventually the peer will find that
all existing SPIs have expired, and will begin the Photuris exchange
again by sending a new Cookie_Request. When appropriate, this
Cookie_Request MAY include a Responder-Cookie to retain previous
party pairings.
6.2.2. Deletion
When the LifeTime is zero, the SPI_Update can be used to delete a
single existing SPI. When the SPI is also zero, the SPI_Update will
delete all existing SPIs related to this Security Association, and
mark the Photuris exchange state as expired. This is especially
useful when the application that needed them terminates.
No retransmission timer is necessary. This message is advisory, to
reduce the number of ICMP Security Failures messages.
Should any deletion attempts fail, the peer will learn that the
deleted SPIs are invalid through the normal ICMP Security Failures
messages, and will initiate a Photuris exchange by sending a new
Cookie_Request.
6.2.3. Modification
The SPI_Update cannot be used to modify existing SPIs, such as
lengthen an existing SPI LifeTime, resurrect an expired SPI, or
add/remove an Attribute-Choice.
On receipt, such an otherwise valid message is silently discarded.
6.3. Validity Verification
These messages are authenticated using the Validity-Method indicated
by the current Scheme-Choice. The Verification value is calculated
prior to masking (and optional encryption), and verified after
unmasking (and optional decryption).
The Validity-Method authentication function is supplied with two
input values:
- the sender (SPI Owner) verification-key,
- the data to be verified (as a concatenated sequence of bytes).
The resulting output value is stored in the Verification field.
The Validity-Method verification data consists of the following
concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI (or Reserved) fields,
+ the SPI Owner Identity Verification,
+ the SPI User Identity Verification,
+ the Attribute-Choices following the Verification field,
+ the Padding.
Note that the order of the Identity Verification fields (from the
Identity_Messages) is different in each direction, and the Message,
LifeTime and SPI fields are also likely to be different in each
direction.
If the verification fails, the users are notified, and a
Verification_Failure message is sent, without adding or deleting any
SPIs. On success, normal operation begins with the authentication
and/or encryption of user datagrams.
Implementation Notes:
This is distinct from any authentication method specified for the
SPI.
The Identity Verification data includes both the Size and Value
fields. The Attribute-Choices data includes the Attribute, Length
and Value fields.
7. Error Messages
These messages are issued in response to Photuris state loss or other
problems. A message has effect in only one direction. No
retransmission timer is necessary.
These messages are not masked.
The receiver checks the Cookies for validity. Special care MUST be
taken that the Cookie pair in the Error Message actually match a pair
currently in use, and that the protocol is currently in a state where
such an Error Message might be expected. Otherwise, these messages
could provide an opportunity for a denial of service attack. Invalid
messages are silently discarded.
7.1. Bad_Cookie
For the format of the 33 byte message, see "Header Format". There
are no additional fields.
Initiator-Cookie 16 bytes. Copied from the offending message.
Responder-Cookie 16 bytes. Copied from the offending message.
Message 10
This error message is sent when a Value_Request, Identity_Request,
SPI_Needed, or SPI_Update is received, and the receiver specific
Cookie is invalid or the associated exchange state has expired.
During the Photuris exchange, when this error message is received, it
has no immediate effect on the operation of the protocol phases.
Later, when Retransmissions have been exceeded, and this error
message has been received, the Initiator SHOULD begin the Photuris
exchange again by sending a new Cookie_Request with the Responder-
Cookie and Counter updated appropriately.
When this error message is received in response to SPI_Needed, the
exchange state SHOULD NOT be marked as expired, but the party SHOULD
initiate a Photuris exchange by sending a new Cookie_Request.
When this error message is received in response to SPI_Update, the
exchange state SHOULD NOT be marked as expired, and no further action
is taken. A new exchange will be initiated later when needed by the
peer to send authenticated and/or encrypted data.
Existing SPIs are not deleted. They expire normally, and are purged
sometime later.
7.2. Resource_Limit
For the format of the 34 byte message, see "Cookie_Request". There
are no additional fields.
Initiator-Cookie 16 bytes. Copied from the offending message.
Responder-Cookie 16 bytes. Copied from the offending message.
Special processing is applied to a Cookie_Request.
When the offending message Responder-Cookie and
Counter were both zero, and an existing exchange has
not yet been purged, this field is replaced with the
Responder-Cookie from the existing exchange.
Message 11
Counter 1 byte. Copied from the offending message.
When zero, the Responder-Cookie indicates the
Initiator of a previous exchange, or no previous
exchange is specified.
When non-zero, the Responder-Cookie indicates the
Responder to a previous exchange. This value is set
to the Counter from the corresponding
Cookie_Response.
This error message is sent when a Cookie_Request, Value_Request or
SPI_Needed is received, and too many SPI values are already in use
for that peer, or some other Photuris resource is unavailable.
During the Photuris exchange, when this error message is received in
response to a Cookie_Request or Value_Request, the implementation
SHOULD double the retransmission timeout (as usual) for sending
another Cookie_Request or Value_Request. Otherwise, it has no
immediate effect on the operation of the protocol phases. Later,
when Retransmissions have been exceeded, and this error message has
been received, the Initiator SHOULD begin the Photuris exchange again
by sending a new Cookie_Request with the Responder-Cookie and Counter
updated appropriately.
When this error message is received in response to SPI_Needed, the
implementation SHOULD NOT send another SPI_Needed until one of the
existing SPIs associated with this exchange is deleted or has
expired.
7.3. Verification_Failure
For the format of the 33 byte message, see "Header Format". There
are no additional fields.
Initiator-Cookie 16 bytes. Copied from the offending message.
Responder-Cookie 16 bytes. Copied from the offending message.
Message 12
This error message is sent when an Identity_Message, SPI_Needed or
SPI_Update is received, and verification fails.
When this error message is received, the implementation SHOULD log
the occurance, and notify an operator as appropriate. However,
receipt has no effect on the operation of the protocol.
7.4. Message_Reject
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Initiator-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Responder-Cookie ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Bad-Message Offset
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes. Copied from the offending message.
Responder-Cookie 16 bytes. Copied from the offending message.
Message 13
Bad-Message 1 byte. Indicates the Message number of the
offending message.
Offset 2 bytes. The number of bytes from the beginning of
the offending message where the unrecognized field
starts. The minimum value is 32.
This error message is sent when an optional Message type is received
that is not supported, or an optional format of a supported Message
is not recognized.
When this error message is received, the implementation SHOULD log
the occurance, and notify an operator as appropriate. However,
receipt has no effect on the operation of the protocol.
8. Public Value Exchanges
Photuris is based in principle on public-key cryptography,
specifically Diffie-Hellman key exchange. Exchange of public D-H
Exchange-Values based on private-secret values results in a mutual
shared-secret between the parties. This shared-secret can be used on
its own, or to generate a series of session-keys for authentication
and encryption of subsequent traffic.
This document assumes familiarity with the Diffie-Hellman public-key
algorithm. A good description can be found in [Schneier95].
8.1. Modular Exponentiation Groups
The original Diffie-Hellman technique [DH76] specified modular
exponentiation. A public-value is generated using a generator (g),
raised to a private-secret exponent (x), modulo a prime (p):
(g**x) mod p.
When these public-values are exchanged between parties, the parties
can calculate a shared-secret value between themselves:
(g**xy) mod p.
The generator (g) and modulus (p) are established by the Scheme-
Choice (see the "Basic Exchange-Schemes" for details). They are
offered in the Cookie_Response, and one pair is chosen in the
Value_Request.
The private exponents (x) and (y) are kept secret by the parties.
Only the public-value result of the modular exponentiation with (x)
or (y) is sent as the Initiator and Responder Exchange-Value.
These public-values are represented in single Variable Precision
Integers. The Size of these Exchange-Values will match the Size of
the modulus (p).
8.2. Moduli Selection
Each implementation proposes one or more moduli in its Offered-
Schemes. Every implementation MUST support up to 1024-bit moduli.
For any particular Photuris node, these moduli need not change for
significant periods of time; likely days or weeks. A background
process can periodically generate new moduli.
For 512-bit moduli, current estimates would provide 64
(pessimistic) bit-equivalents of cryptographic strength.
For 1024-bit moduli, current estimates would range from 80
(pessimistic) through 98 (optimistic) bit-equivalents of
cryptographic strength.
These estimates are used when choosing moduli that are appropriate
for the desired Security Parameter attributes.
8.2.1. Bootstrap Moduli
Each implementation is likely to use a fixed modulus during its
bootstrap, until it can generate another modulus in the background.
As the bootstrap modulus will be widely distributed, and reused
whenever the machine reinitializes, it SHOULD be a "safe" prime (p =
2q+1) to provide the greatest long-term protection.
Implementors are encouraged to generate their own bootstrap moduli,
and to change bootstrap moduli in successive implementation releases.
8.2.2. Learning Moduli
As Photuris exchanges are initiated, new moduli will be learned from
the Responder Offered-Schemes. The Initiator MAY cache these moduli
for its own use.
Before offering any learned modulus, the implementation MUST perform
at least one iteration of probable primality verification. In this
fashion, many processors will perform verification in parallel as
moduli are passed around.
When primality verification failures are found, the failed moduli
SHOULD be retained for some (implementation dependent) period of
time, to avoid re-learning and re-testing after subsequent exchanges.
8.3. Generator Selection
The generator (g) should be chosen such that the private-secret
exponents will generate all possible public-values, evenly
distributed throughout the range of the modulus (p), without cycling
through a smaller subset. Such a generator is called a "primitive
root" (which is trivial to find when p is "safe").
Only one generator (2) is required to be supported.
Implementation Notes:
One useful technique is to select the generator, and then limit
the modulus selection sieve to primes with that generator:
2 when p (mod 24) = 11.
3 when p (mod 12) = 5.
5 when p (mod 10) = 3 or 7.
The required generator (2) improves efficiency in multiplication
performance. It is usable even when it is not a primitive root,
as it still covers half of the space of possible residues.
8.4. Exponent Selection
Each implementation generates a separate random private-secret
exponent for each different modulus. Then, a D-H Exchange-Value is
calculated for the given modulus, generator, and exponent.
This specification recommends that the exponent length be at least
twice the desired cryptographic strength of the longest session-key
needed by the strongest offered-attribute.
Based on the estimates in "Moduli Selection" (above):
For 512-bit moduli, exponent lengths of 128 bits (or more) are
recommended.
For 1024-bit moduli, exponent lengths of 160 to 256 bits (or more)
are recommended.
Although the same exponent and Exchange-Value may be used with
several parties whenever the same modulus and generator are used, the
exponent SHOULD be changed at random intervals. A background process
can periodically destroy the old values, generate a new random
private-secret exponent, and recalculate the Exchange-Value.
Implementation Notes:
The size of the exponent is entirely implementation dependent, is
unknown to the other party, and can be easily changed.
Since these operations involve several time-consuming modular
exponentiations, moving them to the "background" substantially
improves the apparent execution speed of the Photuris protocol.
It also reduces CPU loading sufficiently to allow a single
public/private key-pair to be used in several closely spaced
Photuris executions, when creating Security Associations with
several different nodes over a short period of time.
Other pre-computation suggestions are described in [BGMW93, LL94,
Rooij94].
8.5. Defective Exchange Values
Some exponents do not qualify as secret. The exponent 0 will
generate the Exchange-Value 1, and the exponent 1 will generate the
Exchange-Value g. Small exponents will be easily visible and SHOULD
be avoided where:
g**x < p.
Depending on the structure of the moduli, certain exponents can be
used for sub-group confinement attacks. For "safe" primes (p =
2q+1), these exponents are p-1 and (p-1)/2, which will generate the
Exchange-Values 1 and p-1 respectively.
When an implementation chooses a random exponent, the resulting
Exchange-Value is examined. If the Exchange-Value is represented in
less than half the number of significant bits in the modulus, then a
new random exponent MUST be chosen.
For 512-bit moduli, Exchange-Values of 2**256 or greater are
required.
For 1024-bit moduli, Exchange-Values of 2**512 or greater are
required.
In addition, if the resulting Exchange-Value is p-1, then a new
random exponent MUST be chosen.
Upon receipt of an Exchange-Value that fails to meet the
requirements, the Value Exchange message is silently discarded.
Implementation Notes:
Avoidance of small exponents can be assured by setting at least
one bit in the most significant half of the exponent.
9. Basic Exchange-Schemes
Initial values are assigned as follows:
(0) Reserved.
(1) Reserved.
(2) Implementation Required. Any modulus (p) with a recommended
generator (g) of 2. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
An Exchange-Scheme Size of zero is invalid.
Key-Generation-Function "MD5 Hash"
Privacy-Method "Simple Masking"
Validity-Method "MD5-IPMAC Check"
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(3) Exchange-Schemes 3 to 255 are intended for future well-known
published schemes.
(256) Exchange-Schemes 256 to 32767 are intended for vendor-specific
unpublished schemes. Implementors wishing a number MUST
request the number from the authors.
(32768)
Exchange-Schemes 32768 to 65535 are available for cooperating
parties to indicate private schemes, regardless of vendor
implementation. These numbers are not reserved, and are
subject to duplication. Other criteria, such as the IP Source
and Destination of the Cookie_Request, are used to
differentiate the particular Exchange-Schemes available.
10. Basic Key-Generation-Function
10.1. MD5 Hash
MD5 [RFC-1321] is used as a pseudo-random-function for generating the
key(s). The key(s) begin with the most significant bits of the hash.
MD5 is iterated as needed to generate the requisite length of key
material.
When an individual key does not use all 128-bits of the last hash,
any remaining unused (least significant) bits of the last hash are
discarded. When combined with other uses of key generation for the
same purpose, the next key will begin with a new hash iteration.
11. Basic Privacy-Method
11.1. Simple Masking
As described in "Privacy-Key Computation", sufficient privacy-key
material is generated to match the message length, beginning with the
next field after the SPI, and including the Padding. The message is
masked by XOR with the privacy-key.
12. Basic Validity-Method
12.1. MD5-IPMAC Check
As described in "Validity Verification", the Verification field value
is the MD5 [RFC-1321] hash over the concatenation of
MD5( key, keyfill, data, datafill, key, md5fill )
where the key is the computed verification-key.
The keyfill and datafill use the same pad-with-length technique
defined for md5fill. This padding and length is implicit, and does
not appear in the datagram.
The resulting Verification field is a 128-bit Variable Precision
Integer (18 bytes including Size). When used in calculations, the
Verification data includes both the Size and Value fields.
13. Basic Attributes
Implementors wishing a number MUST request the number from the
authors. Initial values are assigned as follows:
Use Type
- 0* padding
- 1* AH-Attributes
- 2+ ESP-Attributes
AEI 5* MD5-IPMAC
AEIX 255+ Organizational
A AH Attribute-Choice
E ESP Attribute-Choice
I Identity-Choice
X dependent on list location
+ feature must be recognized even when not supported
* feature must be supported (mandatory)
Other attributes are specified in companion documents.
13.1. Padding
+-+-+-+-+-+-+-+-+
Attribute
+-+-+-+-+-+-+-+-+
Attribute 0
Each attribute may have value fields that are multiple bytes. To
facilitate processing efficiency, these fields are aligned on
integral modulo 8 byte (64-bit) boundaries.
Padding is accomplished by insertion of 1 to 7 Attribute 0 padding
bytes before the attribute that needs alignment.
No padding is used after the final attribute in a list.
13.2. AH-Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 1
Length 0
When a list of Attributes is specified, this Attribute begins the
section of the list which applies to the Authentication Header (AH).
13.3. ESP-Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Length PayloadType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 2
Length 1
PayloadType 1 byte. Indicates the contents of the ESP Transform
Data field, using the IP Next Header (Protocol)
value. Up-to-date values of the IP Next Header
(Protocol) are specified in the most recent
"Assigned Numbers" [RFC-1700].
For example, when encrypting an entire IP datagram,
this field will contain the value 4, indicating IP-
in-IP encapsulation.
When a list of Attributes is specified, this Attribute begins the
section of the list which applies to the Encapsulating Security
Payload (ESP).
When listed as an Offered-Attribute, the PayloadType is set to 255.
When selected as an Attribute-Choice, the PayloadType is set to the
actual value to be used.
13.4. MD5-IPMAC
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 5
Length 0
13.4.1. Symmetric Identification
When selected as an Identity-Choice, the immediately following
Identification field contains an unstructured Variable Precision
Integer. Valid Identifications and symmetric secret-keys are
preconfigured by the parties.
There is no required format or content for the Identification value.
The value may be a number or string of any kind. See "Use of
Identification and Secrets" for details.
The symmetric secret-key (as specified) is selected based on the
contents of the Identification field. All implementations MUST
support at least 62 bytes. The selected symmetric secret-key SHOULD
provide at least 64-bits of cryptographic strength.
As described in "Identity Verification", the Verification field value
is the MD5 [RFC-1321] hash over the concatenation of:
MD5( key, keyfill, data, datafill, key, md5fill )
where the key is the computed verification-key.
The keyfill and datafill use the same pad-with-length technique
defined for md5fill. This padding and length is implicit, and does
not appear in the datagram.
The resulting Verification field is a 128-bit Variable Precision
Integer (18 bytes including Size). When used in calculations, the
Verification data includes both the Size and Value fields.
For both "Identity Verification" and "Validity Verification", the
verification-key is the MD5 [RFC-1321] hash of the following
concatenated values:
+ the symmetric secret-key,
+ the computed shared-secret.
For "Session-Key Computation", the symmetric secret-key is used
directly as the generation-key.
Regardless of the internal representation of the symmetric secret-
key, when used in calculations it is in the same form as the Value
part of a Variable Precision Integer:
- most significant byte first.
- bits used are right justified within byte boundaries.
- any unused bits are in the most significant byte.
- unused bits are zero filled.
The symmetric secret-key does not include a Size field.
13.4.2. Authentication
May be selected as an AH or ESP Attribute-Choice, pursuant to [RFC-
1828] et sequitur. The selected Exchange-Scheme SHOULD provide at
least 64-bits of cryptographic strength.
As described in "Session-Key Computation", the most significant 384-
bits (48 bytes) of the Key-Generation-Function iterations are used
for the key.
Profile:
When negotiated with Photuris, the transform differs slightly from
[RFC-1828].
The form of the authenticated message is:
MD5( key, keyfill, datagram, datafill, key, md5fill )
where the key is the SPI session-key.
The additional datafill protects against the (impractical) attack
described in [PO96]. The keyfill and datafill use the same pad-
with-length technique defined for md5fill. This padding and
length is implicit, and does not appear in the datagram.
13.5. Organizational
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute Length OUI
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Kind Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 255
Length >= 4
When the Length is four, no Value(s) field is
present.
OUI 3 bytes. The vendor's Organizationally Unique
Identifier, assigned by IEEE 802 or IANA (see [RFC-
1700] for contact details). The bits within the
byte are in canonical order, and the most
significant byte is transmitted first.
Kind 1 byte. Indicates a sub-type for the OUI. There is
no standardization for this field. Each OUI
implements its own values.
Value(s) 0 or more bytes. The details are implementation
specific.
Some implementors might not need nor want to publish their
proprietary algorithms and attributes. This OUI mechanism is
available to specify these without encumbering the authors with
proprietary number requests.
A. Automaton
An example automaton is provided to illustrate the operation of the
protocol. It is incomplete and non-deterministic; many of the
Good/Bad semantic decisions are policy-based or too difficult to
represent in tabular form. Where conflicts appear between this
example and the text, the text takes precedence.
The finite-state automaton is defined by events, actions and state
transitions. Events include reception of external commands such as
expiration of a timer, and reception of datagrams from a peer.
Actions include the starting of timers and transmission of datagrams
to the peer.
Events
DU13 = Communication Administratively Prohibited
SF0 = Bad SPI
SF4 = Need Authentication
SF5 = Need Authorization
WC = Want Confidentiality
RCQ+ = Receive Cookie_Request (Good)
RCQ- = Receive Cookie_Request (Bad)
RCR+ = Receive Cookie_Response (Good)
RCR- = Receive Cookie_Response (Bad)
RVQ+ = Receive Value_Request (Good)
RVQ- = Receive Value_Request (Bad)
RVR+ = Receive Value_Response (Good)
RVR- = Receive Value_Response (Bad)
RIQ+ = Receive Identity_Request (Good)
RIQ- = Receive Identity_Request (Bad)
RIR+ = Receive Identity_Response (Good)
RIR- = Receive Identity_Response (Bad)
RUN+ = Receive SPI_Needed (Good)
RUN- = Receive SPI_Needed (Bad)
RUM+ = Receive SPI_Update (Good)
RUM- = Receive SPI_Update (Bad)
RBC = Receive Bad Cookie
RRL = Receive Resource Limit
RVF = Receive Verification Failure
RMR = Receive Message Reject
TO+ = Timeout with counter > 0
TO- = Timeout with counter expired
UTO = Update TimeOut
XTO = Exchange TimeOut
Actions
scq = Send Cookie_Request
scr = Send Cookie_Response
svq = Send Value_Request
svr = Send Value_Response
siq = Send Identity_Request
sir = Send Identity_Response
sum = Send SPI_Update
se* = Send error message (see text)
sbc = Send Bad Cookie
srl = Send Resource Limit
svf = Send Verification Failure
brto = BacKOFf Retransmission TimeOut
buto = Backoff Update TimeOut
rto = Set Retransmission TimeOut
uto = Set Update TimeOut
xto = Set Exchange TimeOut
log = log operator message
A.1. State Transition Table
States are indicated horizontally, and events are read vertically.
State transitions and actions are represented in the form
action/new-state. Multiple actions are separated by commas, and may
continue on succeeding lines as space requires; multiple actions may
be implemented in any convenient order. The state may be followed by
a letter, which indicates an explanatory footnote. The dash ('-')
indicates an illegal transition.
Initiator
0 1 2 3 4
Initial Cookie CookieBad Value ValueBad
------+--------------------------------------------------
DU13 rto,scq/1 rto,scq/1 rto,scq/1 3 4
SF0 rto,scq/1 1 2 3 4
SF4 rto,scq/1 1 2 3 4
SF5 rto,scq/1 1 2 3 4
WC rto,scq/1 1 2 3 4
RCR+ - rto,svq/3 rto,svq/3 3 4
RCR- 0 1 2 3 4
RVR+ - - - rto,siq/5 rto,siq/5
RVR- 0 1 2 3 4
RIR+ - - - - -
RIR- 0 1 2 3 4
RUN+ - - - - -
RUN- sbc/0 sbc/1 sbc/2 sbc/3 sbc/4
RUM+ - - - - -
RUM- sbc/0 sbc/1 sbc/2 sbc/3 sbc/4
RBC - - - 4 4
RRL - brto/2 brto/2 brto/4 brto/4
RVF - - - - -
RMR - - - - -
TO+ - scq/1 scq/2 svq/3 svq/4
TO- - 0 scq/1 0 scq/1
UTO - - - - -
XTO - 0 0 0 0
Initiator
5 6 8
Identity IdentityBad Update
------+-----------------------------
DU13 5 6 8
SF0 5 6 rto,scq/1
SF4 5 6 rto,scq/1
SF5 5 6 rto,scq/1
WC 5 6 sun/8
RCR+ 5 6 8
RCR- 5 6 8
RVR+ 5 6 8
RVR- 5 6 8
RIR+ uto/8 uto/8 8
RIR- svf/5 svf/6 8
RUN+ - - sum/8
RUN- sbc/5 sbc/6 se*/8
RUM+ - - 8
RUM- sbc/5 sbc/6 se*/8
RBC 6 6 rto,scq/1
RRL 5 6 buto/8
RVF log/5 log/6 log/8
RMR log/5 log/6 log/8
TO+ sim/5 sim/6 -
TO- 0 scq/1 -
UTO - - sum/8
XTO 0 0 0
Responder
0 7 8
Initial Ready Update
------+-----------------------------
WC - 7 sun/8
RCQ+ scr/0 scr/7 scr/8
RCQ- srl/0 srl/7 srl/8
RVQ+ xto,svr/7 svr/7 svr/8
RVQ- sbc/0 sbc/7 sbc/8
RIQ+ - uto,sir/8 sir/8
RIQ- sbc/0 se*/7 se*/8
RUN+ - - sum/8
RUN- sbc/0 sbc/7 se*/8
RUM+ - - 8
RUM- sbc/0 sbc/7 se*/8
RBC - 7 rto,scq/1
RRL - - buto/8
RVF - - log/8
RMR - - log/8
UTO - - sum/8
XTO - 0 0
A.2. States
Following is a more detailed description of each automaton state.
The "Bad" version of a state is to indicate that the Bad_Cookie or
Resource_Limit message has been received.
A.2.1. Initial
The Initial state is fictional, in that there is no state between the
parties.
A.2.2. Cookie
In the Cookie state, the Initiator has sent a Cookie_Request, and is
waiting for a Cookie_Response. Both the Restart and Exchange timers
are running.
Note that the Responder has no Cookie state.
A.2.3. Value
In the Value state, the Initiator has sent its Exchange-Value, and is
waiting for an Identity_Message. Both the Restart and Exchange
timers are running.
A.2.4. Identity
In the Identity state, the Initiator has sent an Identity_Request,
and is waiting for an Identity_Response in reply. Both the Restart
and Exchange timers are running.
A.2.5. Ready
In the Ready state, the Responder has sent its Exchange-Value, and is
waiting for an Identity_Message. The Exchange timer is running.
A.2.6. Update
In the Update state, each party has concluded the Photuris exchange,
and is unilaterally updating expiring SPIs until the Exchange
LifeTime expires. Both the Update and Exchange timers are running.
B. Use of Identification and Secrets
Implementation of the base protocol requires support for operator
configuration of participant identities and associated symmetric
secret-keys.
The form of the Identification and Secret fields is not constrained
to be a readable string. In addition to a simpler quoted string
configuration, an implementation MUST allow configuration of an
arbitrary stream of bytes.
B.1. Identification
Typically, the Identification is a user name, a site name, a Fully
Qualified Domain Name, or an email address which contains a user name
and a domain name. Examples include:
user
node.site.
user@node.site.
rcmd@node.site.
"Mundane Name" <user@node.site>
There is no requirement that the domain name match any of the
particular IP addresses in use by the parties.
B.2. Group Identity With Group Secret
A simple configuration approach could use a single Identity and
Secret, distributed to all the participants in the trusted group.
This might be appropriate between routers under a single
administration comprising a Virtual Private Network over the
Internet.
Nota Bene:
The passwords used in these examples do not meet the "MD5-IPMAC
Symmetric Identification" recommendation for at least 64-bits of
cryptographic strength.
The administrator configures each router with the same username and
password:
identity local "Tiny VPN 1995 November" "abracadabra"
identity remote "Tiny VPN 1995 November" "abracadabra"
When the Initiator sends its Identity_Request, the SPI Owner
Identification field is "Tiny VPN 1995 November" and the SPI Owner
secret-key is "abracadabra".
When the Responder sends its Identity_Response, the SPI Owner
Identification field is "Tiny VPN 1995 November" and the SPI Owner
secret-key is "abracadabra". The SPI User Identification is "Tiny
VPN 1995 November" (taken from the request), and the SPI User
secret-key is "abracadabra".
Note that even in the face of implementations with very poor random
number generation yielding the same random numbers for both parties
at every step, and with this completely identical configuration, the
addition of the SPI User Verification field in the response
calculation is highly likely to produce a different Verification
value (see "Identity Verification"). In turn, the different
Verification values affect the calculation of SPI session-keys that
are highly likely to be different in each direction (see "Session-Key
Computation").
B.3. Multiple Identities With Group Secrets
A more robust configuration approach could use a separate Identity
and Secret for each party, distributed to the participants in the
trusted group. This might be appropriate for authenticated firewall
traversal.
An administrator has one or more networks, and a number of mobile
users. It is desirable to restrict Access to authorized external
users. The example boundary router is 10.0.0.1.
The administrator gives each user a different username and password,
together with a group username and password for the router.
The administrator configures (in part):
identity local "199511@router.site" "FalDaRah"
identity remote "Happy_Wanderer@router.site" "FalDaRee"
Each mobile user adds commands to tunnel and authenticate.
route addprivate 10.0.0.0/8 tunnel 10.0.0.1
secure 10.0.0.1 authenticate-only
identity local "Happy_Wanderer@router.site" "FalDaRee"
identity remote "199511@router.site" "FalDaRah"
identity remote "199512@router.site" "FalDaHaHaHaHaHaHa"
When the mobile Initiator sends its Identity_Request, the SPI Owner
Identification field is "Happy_Wanderer@router.site" and the SPI
Owner secret-key is "FalDaRee".
When the firewall Responder sends its Identity_Response, the SPI
Owner Identification field is "199511@router.site" and the SPI Owner
secret-key is "FalDaRah". The SPI User Identification field is
"Happy_Wanderer@router.site" (taken from the request), and the SPI
User secret-key is "FalDaRee".
In this example, the mobile user is already prepared for a monthly
password changeover, where the router might identify itself as
"199512@router.site".
B.4. Multiple Identities With Multiple Secrets
Greater security might be achieved through configuration of a pair of
secrets between each party. As before, one secret is used for
initial contact to any member of the group, but another secret is
used between specific parties. Compromise of one secret or pair of
secrets does not affect any other member of the group. This might be
appropriate between the routers forming a boundary between
cooperating Virtual Private Networks that establish local policy for
each VPN member access.
One administrator configures:
identity local "Apple" "all for one"
identity local "Apple-Baker" "Apple to Baker" "Baker"
identity remote "Baker" "one for all"
identity remote "Baker-Apple" "Baker to Apple"
Another configures:
identity local "Baker" "one for all"
identity local "Baker-Apple" "Baker to Apple" "Apple"
identity remote "Apple" "all for one"
identity remote "Apple-Baker" "Apple to Baker"
When the Initiator sends its Identity_Request, the SPI Owner
Identification field is "Apple" and the SPI Owner secret-key is "all
for one".
When the Responder sends its Identity_Response, finding that the
special pairing exists for "Apple" (in this example, indicated by a
third field), the SPI Owner Identification field is "Baker-Apple" and
the SPI Owner secret-key is "Baker to Apple". The SPI User
Identification is "Apple" (taken from the request), and the SPI User
secret-key is "all for one".
Operational Considerations
The specification provides only a few configurable parameters, with
defaults that should satisfy most situations.
Retransmissions
Default: 3.
Initial Retransmission TimeOut (IRTO)
Default: 5 seconds.
Exchange TimeOut (ETO)
Default: 30 seconds. Minimum: Retransmissions * IRTO.
Exchange LifeTime (ELT)
Default: 30 minutes. Minimum: 2 * ETO.
SPI LifeTime (SPILT)
Default: 5 minutes. Minimum: 3 * ETO.
Each party configures a list of known identities and symmetric
secret-keys.
In addition, each party configures local policy that determines what
access (if any) is granted to the holder of a particular identity.
For example, the party might allow anonymous FTP, but prohibit
Telnet. Such considerations are outside the scope of this document.
Security Considerations
Photuris was based on currently available tools, by experienced
network protocol designers with an interest in cryptography, rather
than by cryptographers with an interest in network protocols. This
specification is intended to be readily implementable without
requiring an extensive background in cryptology.
Therefore, only minimal background cryptologic discussion and
rationale is included in this document. Although some review has
been provided by the general cryptologic community, it is anticipated
that design decisions and tradeoffs will be thoroughly analysed in
subsequent dissertations and debated for many years to come.
Cryptologic details are reserved for separate documents that may be
more readily and timely updated with new analysis.
History
The initial specification of Photuris, now called version 1 (December
1994 to March 1995), was based on a short list of design
requirements, and simple experimental code by Phil Karn. Only one
modular exponentiation form was used, with a single byte index of
pre-specified group parameters. The transform attributes were
selected during the public value exchange. Party privacy was
protected in the identification signature exchange with standard ESP
transforms.
Upon submission for review by the IP Security Working Group, a large
number of features were demanded. A mere 254 future group choices
were not deemed enough; it was expanded to two bytes (and renamed
schemes), and was expanded again to carry variable parameters. The
transform attributes were made variable length to accomodate optional
parameters. Every other possible parameter was made negotiable.
Some participants were unable to switch modes on the UDP sockets to
use standard ESP transforms for only some messages, and party privacy
was integrated into the protocol. The message headers were
reorganized, and selection of transform attributes was delayed until
the identification exchange. An additional update message phase was
added.
Version 2 (July 1995 to December 1995) specification stability was
achieved in November 1995 by moving most parameters into separate
documents for later discussion, and leaving only a few mandatory
features in the base specification. Within a month, multiple
interoperable implementations were produced.
Unfortunately, in a fit of demagoguery, the IP Security Working Group
decided in a straw poll to remove party privacy protection, and the
Working Group chair terminated the meeting without allowing further
discussion. Because the identification exchange messages required
privacy to function correctly, the messages were reorganized again.
Party privacy and other optional schemes were split into a separate
document.
The implementors established a separate discussion group. Version 3
(April 1996 to June 1997) enjoyed a long period of specification
stability and multiple implementations on half a dozen platforms.
Meanwhile, the IP Security Working Group has developed a competing
specification with large numbers of negotiable parameters. Also, the
PPP Extensions Working Group has deployed link security transforms.
Version 4 (July 1997 onward) attempts to maintain a semblance of
interface compatibility with these other efforts. Minor changes are
specified in transform padding format and key generation. More than
one value is permitted per scheme, giving greater latitude in choice
for future extensions. The opportunity is taken to return party
privacy to the base document, and make small semantic changes in
automated updates and error recovery. All ESP transform attributes
are moved to separate documents, to (hopefully) avoid future
incompatible changes to the base document.
Acknowledgements
Thou shalt make no law restricting the size of integers that may
be multiplied together, nor the number of times that an integer
may be multiplied by itself, nor the modulus by which an integer
may be reduced. [Prime Commandment]
Phil Karn was principally responsible for the design of the protocol
phases, particularly the "cookie" anti-clogging defense, developed
the initial testing implementation, and provided much of the design
rationale text (now removed to a separate document).
William Simpson was responsible for the packet formats and
attributes, additional message types, editing and formatting. All
such mistakes are his responsibility.
This protocol was later discovered to have many elements in common
with the Station-To-Station authentication protocol [DOW92].
Angelos Keromytis developed the first completely independent
implementation (circa October 1995). Also, he suggested the cookie
exchange rate limitation counter.
Paul C van Oorschot suggested signing both the public exponents and
the shared-secret, to provide an authentication-only version of
identity verification. Also, he provided text regarding moduli,
generator, and exponent selection (now removed to a separate
document).
Hilarie Orman suggested adding secret "nonces" to session-key
generation for asymmetric public/private-key identity methods (now
removed to a separate document), and provided extensive review of the
protocol details.
Bart Preneel and Paul C van Oorschot in [PO96] recommended padding
between the data and trailing key when hashing for authentication.
Niels Provos developed another independent implementation (circa May
1997), ported to AIX, Linux, OpenBSD, and Solaris. Also, he made
suggestions regarding automated update, and listing multiple moduli
per scheme.
Bill Sommerfeld suggested including the authentication symmetric
secret-keys in the session-key generation, and using the Cookie
values on successive exchanges to provide bi-directional user-
oriented keying (now removed to a separate document).
Oliver Spatscheck developed the second independent implementation
(circa December 1995) for the Xkernel.
International interoperability testing between early implementors
provided the impetus for many of the implementation notes herein, and
numerous refinements in the semantics of the protocol messages.
Randall Atkinson, Steven Bellovin, Wataru Hamada, James Hughes, Brian
LaMacchia, Cheryl Madson, Lewis McCarthy, Perry Metzger, Bob Quinn,
Ron Rivest, Rich Schroeppel, and Norman Shulman provided useful
critiques of earlier versions of this document.
Special thanks to the Center for Information Technology Integration
(CITI) for providing computing resources.
References
[BGMW93] E. Brickell, D. Gordon, K. McCurley, and D. Wilson, "Fast
Exponentiation with Precomputation (Extended Abstract)",
Advances in Cryptology -- Eurocrypt '92, Lecture Notes in
Computer Science 658 (1993), Springer-Verlag, 200-207.
Also U.S. Patent #5,299,262, E.F. Brickell, D.M. Gordon,
K.S. McCurley, "Method for exponentiating in
cryptographic systems", 29 Mar 1994.
[DH76] Diffie, W., and Hellman, H.E., "New Directions in
Cryptography", IEEE Transactions on Information Theory, v
IT-22 n 6 pp 644-654, November 1976.
[DOW92] Whitfield Diffie, Paul C van Oorshot, and Michael J
Wiener, "Authentication and Authenticated Key Exchanges",
Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
Academic Publishers, 1992.
[Firefly] "Photuris" is the latin name for the firefly. "Firefly"
is in turn the name for the USA National Security
Administration's (classified) key exchange protocol for
the STU-III secure telephone. Informed speculation has
it that Firefly is based on very similar design
principles.
[LL94] Lim, C.H., Lee, P.J., "More flexible exponentiation with
precomputation", Advances in Cryptology -- Crypto '94,
Lecture Notes in Computer Science 839 (1994), Springer-
Verlag, pages 95-107.
[Prime Commandment]
A derivation of an apocryphal quote from the usenet list
sci.crypt.
[PO96] Bart Preneel, and Paul C van Oorshot, "On the security of
two MAC algorithms", Advances in Cryptology -- Eurocrypt
'96, Lecture Notes in Computer Science 1070 (May 1996),
Springer-Verlag, pages 19-32.
[RFC-768] Postel, J., "User Datagram Protocol", STD 6,
USC/Information Sciences Institute, August 1980.
[RFC-791] Postel, J., "Internet Protocol", STD 5, USC/Information
Sciences Institute, September 1981.
[RFC-1321] Rivest, R., "The MD5 Message-Digest Algorithm", MIT
Laboratory for Computer Science, April 1992.
[RFC-1700] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
USC/Information Sciences Institute, October 1994.
[RFC-1812] Baker, F., Editor, "Requirements for IP Version 4
Routers", Cisco Systems, June 1995.
[RFC-1828] Metzger, P., Simpson, W., "IP Authentication using Keyed
MD5", July 1995.
[RFC-1829] Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
Transform", July 1995.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, Harvard University, March
1997.
[RFC-2521] Karn, P., and Simpson, W., "ICMP Security Failures
Messages", March 1999.
[Rooij94] P. de Rooij, "Efficient exponentiation using
precomputation and vector addition chains", Advances in
Cryptology -- Eurocrypt '94, Lecture Notes in Computer
Science, Springer-Verlag, pages 403-415.
[Schneier95]
Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
Contacts
Comments about this document should be discussed on the
photuris@adk.gr mailing list.
Questions about this document can also be directed to:
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com
karn@unix.ka9q.ampr.org (preferred)
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
Full Copyright Statement
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and William Allen Simpson (1994-1999). All Rights Reserved.
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