Network Working Group J. Galvin
Request for Comments: 1352 Trusted Information Systems, Inc.
K. McCloghrie
Hughes LAN Systems, Inc.
J. Davin
MIT Laboratory for Computer Science
July 1992
SNMP Security Protocols
Status of this Memo
This document specifies an IAB standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "IAB
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
Table of Contents
1. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. IntrodUCtion . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1 Threats . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Goals and Constraints . . . . . . . . . . . . . . . . . . . 5
2.3 Security Services . . . . . . . . . . . . . . . . . . . . . 6
2.4 Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.1 Message Digest Algorithm . . . . . . . . . . . . . . . . . 7
2.4.2 Symmetric Encryption Algorithm . . . . . . . . . . . . . . 8
3. SNMP Party . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Digest Authentication Protocol . . . . . . . . . . . . . . . 11
4.1 Generating a Message . . . . . . . . . . . . . . . . . . . 14
4.2 Receiving a Message . . . . . . . . . . . . . . . . . . . . 15
5. Symmetric Privacy Protocol . . . . . . . . . . . . . . . . . 16
5.1 Generating a Message . . . . . . . . . . . . . . . . . . . 17
5.2 Receiving a Message . . . . . . . . . . . . . . . . . . . . 18
6. Clock and Secret Distribution . . . . . . . . . . . . . . . 19
6.1 Initial Configuration . . . . . . . . . . . . . . . . . . 20
6.2 Clock Distribution . . . . . . . . . . . . . . . . . . . . 22
6.3 Clock Synchronization . . . . . . . . . . . . . . . . . . . 24
6.4 Secret Distribution . . . . . . . . . . . . . . . . . . . . 26
6.5 Crash Recovery . . . . . . . . . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . 30
7.1 Recommended Practices . . . . . . . . . . . . . . . . . . . 30
7.2 Conformance . . . . . . . . . . . . . . . . . . . . . . . 33
7.3 Protocol Correctness . . . . . . . . . . . . . . . . . . . . 34
7.3.1 Clock Monotonicity Mechanism . . . . . . . . . . . . . . . 35
7.3.2 Data Integrity Mechanism . . . . . . . . . . . . . . . . . 36
7.3.3 Data Origin Authentication Mechanism . . . . . . . . . . . 36
7.3.4 Restricted Administration Mechanism . . . . . . . . . . . 36
7.3.5 Ordered Delivery Mechanism . . . . . . . . . . . . . . . 37
7.3.6 Message Timeliness Mechanism . . . . . . . . . . . . . . . 38
7.3.7 Selective Clock Acceleration Mechanism . . . . . . . . . . 38
7.3.8 Confidentiality Mechanism . . . . . . . . . . . . . . . . 39
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 41
1. Abstract
The Simple Network Management Protocol (SNMP) specification [1]
allows for the protection of network management operations by a
variety of security protocols. The SNMP administrative model
described in [2] provides a framework for securing SNMP network
management. In the context of that framework, this memo defines
protocols to support the following three security services:
o data integrity,
o data origin authentication, and
o data confidentiality.
Please send comments to the SNMP Security Developers mailing list
(snmp-sec-dev@tis.com).
2. Introduction
In the model described in [2], each SNMP party is, by definition,
associated with a single authentication protocol. The authentication
protocol provides a mechanism by which SNMP management communications
transmitted by the party may be reliably identified as having
originated from that party. The authentication protocol defined in
this memo also reliably determines that the message received is the
message that was sent.
Similarly, each SNMP party is, by definition, associated with a
single privacy protocol. The privacy protocol provides a mechanism by
which SNMP management communications transmitted to said party are
protected from disclosure. The privacy protocol in this memo
specifies that only authenticated messages may be protected from
disclosure.
These protocols are secure alternatives to the so-called "trivial"
protocol defined in [1].
USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE SECURE
NETWORK MANAGEMENT. THEREFORE, A NETWORK MANAGEMENT SYSTEM THAT
IMPLEMENTS ONLY THE TRIVIAL PROTOCOL IS NOT CONFORMANT TO THIS
SPECIFICATION.
The Digest Authentication Protocol is described in Section 4. It
provides a data integrity service by transmitting a message digest --
computed by the originator and verified by the recipient -- with each
SNMP message. The data origin authentication service is provided by
prefixing the message with a secret value known only to the
originator and recipient, prior to computing the digest. Thus, data
integrity is supported eXPlicitly while data origin authentication is
supported implicitly in the verification of the digest.
The Symmetric Privacy Protocol is described in Section 5. It protects
messages from disclosure by encrypting their contents according to a
secret cryptographic key known only to the originator and recipient.
The additional functionality afforded by this protocol is assumed to
justify its additional computational cost.
The Digest Authentication Protocol depends on the existence of
loosely synchronized clocks between the originator and recipient of a
message. The protocol specification makes no assumptions about the
strategy by which such clocks are synchronized. Section 6.3 presents
one strategy that is particularly suited to the demands of SNMP
network management.
Both protocols described here require the sharing of secret
information between the originator of a message and its recipient.
The protocol specifications assume the existence of the necessary
secrets. The selection of such secrets and their secure distribution
to appropriate parties may be accomplished by a variety of
strategies. Section 6.4 presents one such strategy that is
particularly suited to the demands of SNMP network management.
2.1 Threats
Several of the classical threats to network protocols are applicable
to the network management problem and therefore would be applicable
to any SNMP security protocol. Other threats are not applicable to
the network management problem. This section discusses principal
threats, secondary threats, and threats which are of lesser
importance.
The principal threats against which any SNMP security protocol should
provide protection are:
Modification of Information.
The SNMP protocol provides the means for management stations to
interrogate and to manipulate the value of objects in a managed
agent. The modification threat is the danger that some party may
alter in-transit messages generated by an authorized party in such
a way as to effect unauthorized management operations, including
falsifying the value of an object.
Masquerade.
The SNMP administrative model includes an Access control model.
Access control necessarily depends on knowledge of the origin of a
message. The masquerade threat is the danger that management
operations not authorized for some party may be attempted by that
party by assuming the identity of another party that has the
appropriate authorizations.
Two secondary threats are also identified. The security protocols
defined in this memo do provide protection against:
Message Stream Modification.
The SNMP protocol is based upon connectionless transport services.
The message stream modification threat is the danger that messages
may be arbitrarily re-ordered, delayed or replayed to effect
unauthorized management operations. This threat may arise either
by the work of a malicious attacker or by the natural operation of
a subnetwork service.
Disclosure.
The disclosure threat is the danger of eavesdropping on the
exchanges between managed agents and a management station.
Protecting against this threat is mandatory when the SNMP is used
to administer private parameters on which its security is based.
Protecting against the disclosure threat may also be required as a
matter of local policy.
There are at least two threats that a SNMP security protocol need not
protect against. The security protocols defined in this memo do not
provide protection against:
Denial of Service.
A SNMP security protocol need not attempt to address the broad
range of attacks by which service to authorized parties is denied.
Indeed, such denial-of-service attacks are in many cases
indistinguishable from the type of network failures with which any
viable network management protocol must cope as a matter of
course.
Traffic Analysis.
In addition, a SNMP security protocol need not attempt to address
traffic analysis attacks. Indeed, many traffic patterns are
predictable -- agents may be managed on a regular basis by a
relatively small number of management stations -- and therefore
there is no significant advantage afforded by protecting against
traffic analysis.
2.2 Goals and Constraints
Based on the foregoing account of threats in the SNMP network
management environment, the goals of a SNMP security protocol are
enumerated below.
1. The protocol should provide for verification that each
received SNMP message has not been modified during
its transmission through the network in such a way that
an unauthorized management operation might result.
2. The protocol should provide for verification of the
identity of the originator of each received SNMP
message.
3. The protocol should provide that the apparent time of
generation for each received SNMP message is recent.
4. The protocol should provide that the apparent time of
generation for each received SNMP message is
subsequent to that for all previously delivered messages
of similar origin.
5. The protocol should provide, when necessary, that the
contents of each received SNMP message are protected
from disclosure.
In addition to the principal goal of supporting secure network
management, the design of any SNMP security protocol is also
influenced by the following constraints:
1. When the requirements of effective management in times
of network stress are inconsistent with those of security,
the former are preferred.
2. Neither the security protocol nor its underlying security
mechanisms should depend upon the ready availability
of other network services (e.g., Network Time Protocol
(NTP) or secret/key management protocols).
3. A security mechanism should entail no changes to the
basic SNMP network management philosophy.
2.3 Security Services
The security services necessary to support the goals of a SNMP
security protocol are as follows.
Data Integrity is the provision of the property that data
and data sequences have not been altered or destroyed
in an unauthorized manner.
Data Origin Authentication is the provision of the
property that the claimed origin of received data is
corroborated.
Data Confidentiality is the provision of the property that
information is not made available or disclosed to
unauthorized individuals, entities, or processes.
The protocols specified in this memo require both data
integrity and data origin authentication to be used at all
times. For these protocols, it is not possible to realize data
integrity without data origin authentication, nor is it possible
to realize data origin authentication without data integrity.
Further, there is no provision for data confidentiality without
both data integrity and data origin authentication.
2.4 Mechanisms
The security protocols defined in this memo employ several
types of mechanisms in order to realize the goals and security
services described above:
o In support of data integrity, a message digest algorithm
is required. A digest is calculated over an appropriate
portion of a SNMP message and included as part of the
message sent to the recipient.
o In support of data origin authentication and data
integrity, the portion of a SNMP message that is
digested is first prefixed with a secret value shared by
the originator of that message and its intended recipient.
o To protect against the threat of message reordering, a
timestamp value is included in each message generated.
A recipient evaluates the timestamp to determine if the
message is recent and it uses the timestamp to determine
if the message is ordered relative to other messages it
has received. In conjunction with other readily available
information (e.g., the request-id), the timestamp also
indicates whether or not the message is a replay of a
previous message. This protection against the threat of
message reordering implies no protection against
unauthorized deletion or suppression of messages.
o In support of data confidentiality, a symmetric
encryption algorithm is required. An appropriate
portion of the message is encrypted prior to being
transmitted to its recipient.
The security protocols in this memo are defined independently of the
particular choice of a message digest and encryption algorithm --
owing principally to the lack of a suitable metric by which to
evaluate the security of particular algorithm choices. However, in
the interests of completeness and in order to guarantee
interoperability, Sections 2.4.1 and 2.4.2 specify particular
choices, which are considered acceptably secure as of this writing.
In the future, this memo may be updated by the publication of a memo
specifying substitute or alternate choices of algorithms, i.e., a
replacement for or addition to the sections below.
2.4.1 Message Digest Algorithm
In support of data integrity, the use of the MD5 [3] message digest
algorithm is chosen. A 128-bit digest is calculated over the
designated portion of a SNMP message and included as part of the
message sent to the recipient.
An appendix of [3] contains a C Programming Language implementation
of the algorithm. This code was written with portability being the
principal objective. Implementors may wish to optimize the
implementation with respect to the characteristics of their hardware
and software platforms.
The use of this algorithm in conjunction with the Digest
Authentication Protocol (see Section 4) is identified by the ASN.1
object identifier value md5AuthProtocol, defined in [4].
For any SNMP party for which the authentication protocol is
md5AuthProtocol, the size of its private authentication key is 16
octets.
Within an authenticated management communication generated by such a
party, the size of the authDigest component of that communication
(see Section 4) is 16 octets.
2.4.2 Symmetric Encryption Algorithm
In support of data confidentiality, the use of the Data Encryption
Standard (DES) in the Cipher Block Chaining mode of operation is
chosen. The designated portion of a SNMP message is encrypted and
included as part of the message sent to the recipient.
Two organizations have published specifications defining the DES: the
National Institute of Standards and Technology (NIST) [5] and the
American National Standards Institute [6]. There is a companion
Modes of Operation specification for each definition (see [7] and
[8], respectively).
The NIST has published three additional documents that implementors
may find useful.
o There is a document with guidelines for implementing
and using the DES, including functional specifications
for the DES and its modes of operation [9].
o There is a specification of a validation test suite for the
DES [10]. The suite is designed to test all ASPects of the
DES and is useful for pinpointing specific problems.
o There is a specification of a maintenance test for the
DES [11]. The test utilizes a minimal amount of data
and processing to test all components of the DES. It
provides a simple yes-or-no indication of correct
operation and is useful to run as part of an initialization
step, e.g., when a computer reboots.
The use of this algorithm in conjunction with the Symmetric Privacy
Protocol (see Section 5) is identified by the ASN.1 object identifier
value desPrivProtocol, defined in [4].
For any SNMP party for which the privacy protocol is desPrivProtocol,
the size of the private privacy key is 16 octets, of which the first
8 octets are a DES key and the second 8 octets are a DES
Initialization Vector. The 64-bit DES key in the first 8 octets of
the private key is a 56 bit quantity used directly by the algorithm
plus 8 parity bits -- arranged so that one parity bit is the least
significant bit of each octet. The setting of the parity bits is
ignored.
The length of the octet sequence to be encrypted by the DES must be
an integral multiple of 8. When encrypting, the data should be padded
at the end as necessary; the actual pad value is insignificant.
If the length of the octet sequence to be decrypted is not an
integral multiple of 8 octets, the processing of the octet sequence
should be halted and an appropriate exception noted. Upon decrypting,
the padding should be ignored.
3. SNMP Party
Recall from [2] that a SNMP party is a conceptual, virtual execution
context whose operation is restricted (for security or other
purposes) to an administratively defined subset of all possible
operations of a particular SNMP protocol entity. A SNMP protocol
entity is an actual process which performs network management
operations by generating and/or responding to SNMP protocol messages
in the manner specified in [1]. Architecturally, every SNMP protocol
entity maintains a local database that represents all SNMP parties
known to it.
A SNMP party may be represented by an ASN.1 value with the following
syntax.
SnmpParty ::= SEQUENCE {
partyIdentity
OBJECT IDENTIFIER,
partyTDomain
OBJECT IDENTIFIER,
partyTAddr
OCTET STRING,
partyProxyFor
OBJECT IDENTIFIER,
partyMaxMessageSize
INTEGER,
partyAuthProtocol
OBJECT IDENTIFIER,
partyAuthClock
INTEGER,
partyAuthLastMsg
INTEGER,
partyAuthNonce
INTEGER,
partyAuthPrivate
OCTET STRING,
partyAuthPublic
OCTET STRING,
partyAuthLifetime
INTEGER,
partyPrivProtocol
OBJECT IDENTIFIER,
partyPrivPrivate
OCTET STRING,
partyPrivPublic
OCTET STRING
}
For each SnmpParty value that represents a SNMP party, the generic
significance of each of its components is defined in [2]. For each
SNMP party that supports the generation of messages using the Digest
Authentication Protocol, additional, special significance is
attributed to certain components of that party's representation:
o Its partyAuthProtocol component is called the
authentication protocol and identifies a combination of
the Digest Authentication Protocol with a particular
digest algorithm (such as that defined in Section 2.4.1).
This combined mechanism is used to authenticate the
origin and integrity of all messages generated by the
party.
o Its partyAuthClock component is called the
authentication clock and represents a notion of the
current time that is specific to the party.
o Its partyAuthLastMsg component is called the
last-timestamp and represents a notion of time
associated with the most recent, authentic protocol
message generated by the party.
o Its partyAuthNonce component is called the nonce
and represents a monotonically increasing integer
associated with the most recent, authentic protocol
message generated by the party. The nonce associated
with a particular message distinguishes it among all
others transmitted in the same unit time interval.
o Its partyAuthPrivate component is called the private
authentication key and represents any secret value
needed to support the Digest Authentication Protocol
and associated digest algorithm.
o Its partyAuthPublic component is called the public
authentication key and represents any public value that
may be needed to support the authentication protocol.
This component is not significant except as suggested in
Section 6.4.
o Its partyAuthLifetime component is called the
lifetime and represents an administrative upper bound
on acceptable delivery delay for protocol messages
generated by the party.
For each SNMP party that supports the receipt of messages via the
Symmetric Privacy Protocol, additional, special significance is
attributed to certain components of that party's representation:
o Its partyPrivProtocol component is called the privacy
protocol and identifies a combination of the Symmetric
Privacy Protocol with a particular encryption algorithm
(such as that defined in Section 2.4.2). This combined
mechanism is used to protect from disclosure all protocol
messages received by the party.
o Its partyPrivPrivate component is called the private
privacy key and represents any secret value needed to
support the Symmetric Privacy Protocol and associated
encryption algorithm.
o Its partyPrivPublic component is called the public
privacy key and represents any public value that may be
needed to support the privacy protocol. This component
is not significant except as suggested in Section 6.4.
4. Digest Authentication Protocol
This section describes the Digest Authentication Protocol. It
provides both for verifying the integrity of a received message
(i.e., the message received is the message sent) and for verifying
the origin of a message (i.e., the reliable identification of the
originator). The integrity of the message is protected by computing a
digest over an appropriate portion of a message. The digest is
computed by the originator of the message, transmitted with the
message, and verified by the recipient of the message.
A secret value known only to the originator and recipient of the
message is prefixed to the message prior to the digest computation.
Thus, the origin of the message is known implicitly with the
verification of the digest.
Recall from [2] that a SNMP management communication is represented
by an ASN.1 value with the following syntax.
SnmpMgmtCom ::= [1] IMPLICIT SEQUENCE {
dstParty
OBJECT IDENTIFIER,
srcParty
OBJECT IDENTIFIER,
pdu PDUs
}
For each SnmpMgmtCom value that represents a SNMP management
communication, the following statements are true:
o Its dstParty component is called the destination and
identifies the SNMP party to which the communication
is directed.
o Its srcParty component is called the source and
identifies the SNMP party from which the
communication is originated.
o Its pdu component has the form and significance
attributed to it in [1].
Recall from [2] that a SNMP authenticated management communication is
represented by an ASN.1 value with the following syntax.
SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE {
authInfo
ANY, - defined by authentication protocol
authData
SnmpMgmtCom
}
For each SnmpAuthMsg value that represents a SNMP authenticated
management communication, the following statements are true:
o Its authInfo component is called the authentication
information and represents information required in
support of the authentication protocol used by the
SNMP party originating the message. The detailed
significance of the authentication information is specific
to the authentication protocol in use; it has no effect on
the application semantics of the communication other
than its use by the authentication protocol in
determining whether the communication is authentic or
not.
o Its authData component is called the authentication
data and represents a SNMP management
communication.
In support of the Digest Authentication Protocol, an authInfo
component is of type AuthInformation:
AuthInformation ::= [1] IMPLICIT SEQUENCE {
authTimestamp
INTEGER (0..2147483647),
authNonce
INTEGER (0..2147483647),
authDigest
OCTET STRING
}
For each AuthInformation value that represents authentication
information, the following statements are true:
o Its authTimestamp component is called the
authentication timestamp and represents the time of the
generation of the message according to the
partyAuthClock of the SNMP party that originated
it. Note that the granularity of the authentication
timestamp is 1 second.
o Its authNonce component is called the authentication
nonce and represents a non-negative integer value
evaluated according to the authTimestamp value. In
order not to limit transmission frequency of management
communications to the granularity of the authentication
timestamp, the authentication nonce is provided to
differentiate between multiple messages sent with the
same value of authTimestamp. The authentication
nonce is a monotonically increasing sequence number,
that is reset for each new authentication timestamp
value.
o Its authDigest component is called the authentication
digest and represents the digest computed over an
appropriate portion of the message, where the message is
temporarily prefixed with a secret value for the purposes
of computing the digest.
4.1 Generating a Message
This section describes the behavior of a SNMP protocol entity when it
acts as a SNMP party for which the authentication protocol is
administratively specified as the Digest Authentication Protocol.
Insofar as the behavior of a SNMP protocol entity when transmitting
protocol messages is defined generically in [2], only those aspects
of that behavior that are specific to the Digest Authentication
Protocol are described below. In particular, this section describes
the encapsulation of a SNMP management communication into a SNMP
authenticated management communication.
According to [2], a SnmpAuthMsg value is constructed during Step 3 of
generic processing. In particular, it states the authInfo component
is constructed according to the authentication protocol identified
for the SNMP party originating the message. When the relevant
authentication protocol is the Digest Authentication Protocol, the
procedure performed by a SNMP protocol entity whenever a management
communication is to be transmitted by a SNMP party is as follows.
1. The local database is consulted to determine the
authentication clock, last-timestamp, nonce, and private
authentication key (extracted, for example, according to
the conventions defined in Section 2.4.1) of the SNMP
party originating the message.
2. The authTimestamp component is set to the retrieved
authentication clock value.
3. If the last-timestamp is equal to the authentication
clock, the nonce is incremented. Otherwise the nonce is
set to zero. The authNonce component is set to the
nonce value. In the local database, the originating
SNMP party's nonce and last-timestamp are set to the
nonce value and the authentication clock, respectively.
4. The authentication digest is temporarily set to the
private authentication key. The SnmpAuthMsg value
is serialized according to the conventions of [12] and [1].
A digest is computed over the octet sequence
representing that serialized value using, for example, the
algorithm specified in Section 2.4.1. The authDigest
component is set to the computed digest value.
As set forth in [2], the SnmpAuthMsg value is then encapsulated
according to the appropriate privacy protocol into a SnmpPrivMsg
value. This latter value is then serialized and transmitted to the
receiving SNMP party.
4.2 Receiving a Message
This section describes the behavior of a SNMP protocol entity upon
receipt of a protocol message from a SNMP party for which the
authentication protocol is administratively specified as the Digest
Authentication Protocol. Insofar as the behavior of a SNMP protocol
entity when receiving protocol messages is defined generically in
[2], only those aspects of that behavior that are specific to the
Digest Authentication Protocol are described below.
According to [2], a SnmpAuthMsg value is evaluated during Step 9 of
generic processing. In particular, it states the SnmpAuthMsg value is
evaluated according to the authentication protocol identified for the
SNMP party that originated the message. When the relevant
authentication protocol is the Digest Authentication Protocol, the
procedure performed by a SNMP protocol entity whenever a management
communication is received by a SNMP party is as follows.
1. If the ASN.1 type of the authInfo component is not
AuthInformation, the message is evaluated as
unauthentic. Otherwise, the authTimestamp,
authNonce, and authDigest components are
extracted from the SnmpAuthMsg value.
2. The local database is consulted to determine the
authentication clock, last-timestamp, nonce, private
authentication key (extracted, for example, according to
the conventions defined in Section 2.4.1), and lifetime of
the SNMP party that originated the message.
3. If the authTimestamp component plus the lifetime is
less than the authentication clock, the message is
evaluated as unauthentic.
4. If the authTimestamp component is less than the
last-timestamp recorded for the originating party in the
local database, the message is evaluated as unauthentic.
5. If the authTimestamp component is equal to the
last-timestamp and if the authNonce component is less
than or equal to the nonce, the message is evaluated as
unauthentic.
6. The authDigest component is extracted and
temporarily recorded.
7. A new SnmpAuthMsg value is constructed such that
its authDigest component is set to the private
authentication key and its other components are set to
the value of the corresponding components in the
received SnmpAuthMsg value. This new
SnmpAuthMsg value is serialized according to the
conventions of [12] and [1]. A digest is computed over
the octet sequence representing that serialized value
using, for example, the algorithm specified in
Section 2.4.1.
8. If the computed digest value is not equal to the
previously recorded digest value, the message is
evaluated as unauthentic.
9. The message is evaluated as authentic.
10. The last-timestamp and nonce values locally recorded
for the originating SNMP party are set to the
authTimestamp value and the authNonce value,
respectively.
11. The authentication clock value locally recorded for the
originating SNMP party is advanced to the
authTimestamp value if this latter exceeds the
recorded value.
If the SnmpAuthMsg value is evaluated as unauthentic, an
authentication failure is noted and the received message is discarded
without further processing. Otherwise, processing of the received
message continues as specified in [2].
5. Symmetric Privacy Protocol
This section describes the Symmetric Privacy Protocol. It provides
for protection from disclosure of a received message. An appropriate
portion of the message is encrypted according to a secret key known
only to the originator and recipient of the message.
This protocol assumes the underlying mechanism is a symmetric
encryption algorithm. In addition, the message to be encrypted must
be protected according to the conventions of the Digest
Authentication Protocol.
Recall from [2] that a SNMP private management communication is
represented by an ASN.1 value with the following syntax.
SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE {
privDst
OBJECT IDENTIFIER,
privData
[1] IMPLICIT OCTET STRING
}
For each SnmpPrivMsg value that represents a SNMP private management
communication, the following statements are true:
o Its privDst component is called the privacy destination
and identifies the SNMP party to which the
communication is directed.
o Its privData component is called the privacy data and
represents the (possibly encrypted) serialization
(according to the conventions of [12] and [1]) of a SNMP
authenticated management communication.
5.1 Generating a Message
This section describes the behavior of a SNMP protocol entity when it
communicates with a SNMP party for which the privacy protocol is
administratively specified as the Symmetric Privacy Protocol. Insofar
as the behavior of a SNMP protocol entity when transmitting a
protocol message is defined generically in [2], only those aspects of
that behavior that are specific to the Symmetric Privacy Protocol are
described below. In particular, this section describes the
encapsulation of a SNMP authenticated management communication into a
SNMP private management communication.
According to [2], a SnmpPrivMsg value is constructed during Step 5 of
generic processing. In particular, it states the privData component
is constructed according to the privacy protocol identified for the
SNMP party receiving the message. When the relevant privacy protocol
is the Symmetric Privacy Protocol, the procedure performed by a SNMP
protocol entity whenever a management communication is to be
transmitted by a SNMP party is as follows.
1. If the SnmpAuthMsg value is not authenticated
according to the conventions of the Digest
Authentication Protocol, the generation of the private
management communication fails according to a local
procedure, without further processing.
2. The local database is consulted to determine the private
privacy key of the SNMP party receiving the message
(represented, for example, according to the conventions
defined in Section 2.4.2).
3. The SnmpAuthMsg value is serialized according to the
conventions of [12] and [1].
4. The octet sequence representing the serialized
SnmpAuthMsg value is encrypted using, for example,
the algorithm specified in Section 2.4.2 and the
extracted private privacy key.
5. The privData component is set to the encrypted value.
As set forth in [2], the SnmpPrivMsg value is then serialized
and transmitted to the receiving SNMP party.
5.2 Receiving a Message
This section describes the behavior of a SNMP protocol entity when it
acts as a SNMP party for which the privacy protocol is
administratively specified as the Symmetric Privacy Protocol. Insofar
as the behavior of a SNMP protocol entity when receiving a protocol
message is defined generically in [2], only those aspects of that
behavior that are specific to the Symmetric Privacy Protocol are
described below.
According to [2], the privData component of a received SnmpPrivMsg
value is evaluated during Step 4 of generic processing. In
particular, it states the privData component is evaluated according
to the privacy protocol identified for the SNMP party receiving the
message. When the relevant privacy protocol is the Symmetric Privacy
Protocol, the procedure performed by a SNMP protocol entity whenever
a management communication is received by a SNMP party is as follows.
1. The local database is consulted to determine the private
privacy key of the SNMP party receiving the message
(represented, for example, according to the conventions
defined in Section 2.4.2).
2. The contents octets of the privData component are
decrypted using, for example, the algorithm specified in
Section 2.4.2 and the extracted private privacy key.
Processing of the received message continues as specified in [2].
6. Clock and Secret Distribution
The protocols described in Sections 4 and 5 assume the existence of
loosely synchronized clocks and shared secret values. Three
requirements constrain the strategy by which clock values and secrets
are distributed.
o If the value of an authentication clock is decreased, the
last-timestamp and private authentication key must be
changed concurrently.
When the value of an authentication clock is decreased,
messages that have been sent with a timestamp value
between the value of the authentication clock and its
new value may be replayed. Changing the private
authentication key obviates this threat. However,
changing the authentication clock and the private
authentication key is not sufficient to ensure proper
operation. If the last-timestamp is not reduced similarly
to the authentication clock, no message will be
considered authentic until the value of the authentication
clock exceeds the value of the last-timestamp.
o The private authentication key and private privacy key
must be known only to the parties requiring knowledge
of them.
Protecting the secrets from disclosure is critical to the
security of the protocols. In particular, if the secrets are
distributed via a network, the secrets must be protected
with a protocol that supports confidentiality, e.g., the
Symmetric Privacy Protocol. Further, knowledge of the
secrets must be as restricted as possible within an
implementation. In particular, although the secrets may
be known to one or more persons during the initial
configuration of a device, the secrets should be changed
immediately after configuration such that their actual
value is known only to the software. A management
station has the additional responsibility of recovering the
state of all parties whenever it boots, and it may address
this responsibility by recording the secrets on a
long-term storage device. Access to information on this
device must be as restricted as is practically possible.
o There must exist at least one SNMP protocol entity that
assumes the role of a responsible management station.
This management station is responsible for ensuring that
all authentication clocks are synchronized and for
changing the secret values when necessary. Although
more than one management station may share this
responsibility, their coordination is essential to the
secure management of the network. The mechanism by
which multiple management stations ensure that no
more than one of them attempts to synchronize the
clocks or update the secrets at any one time is a local
implementation issue.
A responsible management station may either support
clock synchronization and secret distribution as separate
functions, or combine them into a single functional unit.
The first section below specifies the procedures by which a SNMP
protocol entity is initially configured. The next two sections
describe one strategy for distributing clock values and one for
determining a synchronized clock value among SNMP parties supporting
the Digest Authentication Protocol. For SNMP parties supporting the
Symmetric Privacy Protocol, the next section describes a strategy for
distributing secret values. The last section specifies the procedures
by which a SNMP protocol entity recovers from a "crash."
6.1 Initial Configuration
This section describes the initial configuration of a SNMP protocol
entity that supports the Digest Authentication Protocol or both the
Digest Authentication Protocol and the Symmetric Privacy Protocol.
When a network device is first installed, its initial, secure
configuration must be done manually, i.e., a person must physically
visit the device and enter the initial secret values for at least its
first secure SNMP party. This requirement suggests that the person
will have knowledge of the initial secret values.
In general, the security of a system is enhanced as the number of
entities that know a secret is reduced. Requiring a person to
physically visit a device every time a SNMP party is configured not
only exposes the secrets unnecessarily but is administratively
prohibitive. In particular, when MD5 is used, the initial
authentication secret is 128 bits long and when DES is used an
additional 128 bits are needed -- 64 bits each for the key and
initialization vector. Clearly, these values will need to be recorded
on a medium in order to be transported between a responsible
management station and a managed agent. The recommended procedure is
to configure a small set of initial SNMP parties for each SNMP
protocol entity, one pair of which may be used initially to configure
all other SNMP parties.
In fact, there is a minimal, useful set of SNMP parties that could be
configured between each responsible management station and managed
agent. This minimal set includes one of each of the following for
both the responsible management station and the managed agent:
o a SNMP party for which the authentication protocol and
privacy protocol are the values noAuth and noPriv,
respectively,
o a SNMP party for which the authentication protocol
identifies the mechanism defined in Section 2.4.1 and its
privacy protocol is the value noPriv, and
o a SNMP party for which the authentication protocol and
privacy protocol identify the mechanisms defined in
Section 2.4.1 and Section 2.4.2, respectively.
The last of these SNMP parties in both the responsible management
station and the managed agent could be used to configure all other
SNMP parties. It is the only suitable party for this purpose because
it is the only party that supports data confidentiality, which is
necessary in order to protect the distributed secrets from disclosure
to unauthorized entities.
Configuring one pair of SNMP parties to be used to configure all
other parties has the advantage of exposing only one pair of secrets
-- the secrets used to configure the minimal, useful set identified
above. To limit this exposure, the responsible management station
should change these values as its first operation upon completion of
the initial configuration. In this way, secrets are known only to the
peers requiring knowledge of them in order to communicate.
The Management Information Base (MIB) document [4] supporting these
security protocols specifies 6 initial party identities and initial
values, which, by convention, are assigned to the parties and their
associated parameters.
All 6 parties should be configured in each new managed agent and its
responsible management station. The responsible management station
should be configured first, since the management station can be used
to generate the initial secrets and provide them to a person, on a
suitable medium, for distribution to the managed agent. The following
sequence of steps describes the initial configuration of a managed
agent and its responsible management station.
1. Determine the initial values for each of the attributes of
the SNMP party to be configured. Some of these values
may be computed by the responsible management
station, some may be specified in the MIB document,
and some may be administratively determined.
2. Configure the parties in the responsible management
station, according to the set of initial values. If the
management station is computing some initial values to
be entered into the agent, an appropriate medium must
be present to record the values.
3. Configure the parties in the managed agent, according to
the set of initial values.
4. The responsible management station must synchronize
the authentication clock values for each party it shares
with each managed agent. Section 6.3 specifies one
strategy by which this could be accomplished.
5. The responsible management station should change the
secret values manually configured to ensure the actual
values are known only to the peers requiring knowledge
of them in order to communicate. To do this, the
management station generates new secrets for each party
to be reconfigured and distributes those secrets with a
strategy that uses a protocol that protects them from
disclosure, e.g., Symmetric Privacy Protocol (see
Section 6.4). Upon receiving positive acknowledgement
that the new values have been distributed, the
management station should update its local database
with the new values.
If the managed agent does not support a protocol that protects
messages from disclosure, then automatic maintenance and
configuration of parties is not possible, i.e., the last step above
is not possible. The secrets can only be changed by a physical visit
to the device.
If there are other SNMP protocol entities requiring knowledge of the
secrets, the responsible management station must distribute the
information upon completion of the initial configuration. The
mechanism used must protect the secrets from disclosure to
unauthorized entities. The Symmetric Privacy Protocol, for example,
is an acceptable mechanism.
6.2 Clock Distribution
A responsible management station must ensure that the authentication
clock value for each SNMP party for which it is responsible
o is loosely synchronized among all the local databases in
which it appears,
o is reset, as indicated below, upon reaching its maximal
value, and
o is non-decreasing, except as indicated below.
The skew among the clock values must be accounted for in the lifetime
value, in addition to the expected communication delivery delay.
A skewed authentication clock may be detected by a number of
strategies, including knowledge of the accuracy of the system clock,
unauthenticated queries of the party database, and recognition of
authentication failures originated by the party.
Whenever clock skew is detected, and whenever the SNMP entities at
both the responsible management station and the relevant managed
agent support an appropriate privacy protocol (e.g., the Symmetric
Privacy Protocol), a straightforward strategy for the correction of
clock skew is simultaneous alteration of authentication clock and
private key for the relevant SNMP party. If the request to alter the
key and clock for a particular party originates from that same party,
then, prior to transmitting that request, the local notion of the
authentication clock is artificially advanced to assure acceptance of
the request as authentic.
More generally, however, since an authentication clock value need not
be protected from disclosure, it is not necessary that a managed
agent support a privacy protocol in order for a responsible
management station to correct skewed clock values. The procedure for
correcting clock skew in the general case is presented in Section
6.3.
In addition to correcting skewed notions of authentication clocks,
every SNMP entity must react correctly as an authentication clock
approaches its maximal value. If the authentication clock for a
particular SNMP party ever reaches the maximal time value, the clock
must halt at that value. (The value of interest may be the maximum
less lifetime. When authenticating a message, its authentication
timestamp is added to lifetime and compared to the authentication
clock. A SNMP protocol entity must guarantee that the sum is never
greater than the maximal time value.) In this state, the only
authenticated request a management station should generate for this
party is one that alters the value of at least its authentication
clock and private authentication key. In order to reset these values,
the responsible management station may set the authentication
timestamp in the message to the maximal time value. In this case, the
nonce value may be used to distinguish multiple messages.
The value of the authentication clock for a particular SNMP party
must never be altered such that its new value is less than its old
value, unless its last-timestamp and private authentication key are
also altered at the same time.
6.3 Clock Synchronization
Unless the secrets are changed at the same time, the correct way to
synchronize clocks is to advance the slower clock to be equal to the
faster clock. Suppose that party agentParty is realized by the SNMP
entity in a managed agent; suppose that party mgrParty is realized by
the SNMP entity in the corresponding responsible management station.
For any pair of parties, there are four possible conditions of the
authentication clocks that could require correction:
1. The management station's notion of the value of the
authentication clock for agentParty exceeds the agent's
notion.
2. The management station's notion of the value of the
authentication clock for mgrParty exceeds the agent's
notion.
3. The agent's notion of the value of the authentication
clock for agentParty exceeds the management station's
notion.
4. The agent's notion of the value of the authentication
clock for mgrParty exceeds the management station's
notion.
The selective clock acceleration mechanism intrinsic to the protocol
corrects conditions 2 and 3 as part of the normal processing of an
authentic message. Therefore, the clock adjustment procedure below
does not provide for any adjustments in those cases. Rather, the
following sequence of steps specifies how the clocks may be
synchronized when condition 1, condition 4, or both of those
conditions are manifest.
1. The responsible management station saves its existing
notions of the authentication clocks for the two parties
agentParty and mgrParty.
2. The responsible management station retrieves the
authentication clock values for both agentParty and
mgrParty from the agent. This retrieval must be an
unauthenticated request, since the management station
does not know if the clocks are synchronized. If the
request fails, the clocks cannot be synchronized, and the
clock adjustment procedure is aborted without further
processing.
3. If the management station's notion of the authentication
clock for agentParty exceeds the notion just retrieved
from the agent by more than the amount of the
communications delay between the two protocol entities,
then condition 1 is manifest. The recommended estimate
of communication delay in this context is one half of the
lifetime value recorded for agentParty.
4. If the notion of the authentication clock for mgrParty
just retrieved from the agent exceeds the management
station's notion, then condition 4 is manifest, and the
responsible management station advances its notion of
the authentication clock for mgrParty to match the
agent's notion.
5. If condition 1 is manifest, then the responsible
management station sends an authenticated
management operation to the agent that advances the
agent's notion of the authentication clock for
agentParty to be equal to the management station's
notion. If this management operation fails, then the
management station restores its previously saved notions
of the clock values, and the clock adjustment procedure
is aborted without further processing.
6. The responsible management station retrieves the
authentication clock values for both agentParty and
mgrParty from the agent. This retrieval must be an
authenticated request, in order that the management
station may verify that the clock values are properly
synchronized. If this authenticated query fails, then the
management station restores its previously saved notions
of the clock values, and the clock adjustment procedure
is aborted without further processing. Otherwise, clock
synchronization has been successfully realized.
It is important to note step 4 above must be completed before
attempting step 5. Otherwise, the agent may evaluate the request in
step 5 as unauthentic. Similarly, step 5 above must be completed
before attempting step 6. Otherwise, the management station may
evaluate the query response in step 6 as unauthentic.
Administrative advancement of a clock as described above does not
introduce any new vulnerabilities, since the value of the clock is
intended to increase with the passage of time. A potential
operational problem is the rejection of management operations that
are authenticated using a previous value of the relevant party clock.
This possibility may be avoided if a management station suppresses
generation of management traffic between relevant parties while this
clock adjustment procedure is in progress.
6.4 Secret Distribution
This section describes one strategy by which a SNMP protocol entity
that supports both the Digest Authentication Protocol and the
Symmetric Privacy Protocol can change the secrets for a particular
SNMP party.
The frequency with which the secrets of a SNMP party should be
changed is a local administrative issue. However, the more frequently
a secret is used, the more frequently it should be changed. At a
minimum, the secrets must be changed whenever the associated
authentication clock approaches its maximal value (see Section 7).
Note that, owing to both administrative and automatic advances of the
authentication clock described in this memo, the authentication clock
for a SNMP party may well approach its maximal value sooner than
might otherwise be expected.
The following sequence of steps specifies how a responsible
management station alters a secret value (i.e., the private
authentication key or the private privacy key) for a particular SNMP
party.
1. The responsible management station generates a new
secret value.
2. The responsible management station encapsulates a
SNMP Set request in a SNMP private management
communication with at least the following properties.
o Its source supports the Digest Authentication
Protocol and the Symmetric Privacy Protocol.
o Its destination supports the Symmetric Privacy
Protocol and the Digest Authentication Protocol.
3. The SNMP private management communication is
transmitted to its destination.
4. Upon receiving the request, the recipient processes the
message according to [1] and [2].
5. The recipient encapsulates a SNMP Set response in a
SNMP private management communication with at least
the following properties.
o Its source supports the Digest Authentication
Protocol and the Symmetric Privacy Protocol.
o Its destination supports the Symmetric Privacy
Protocol and the Digest Authentication Protocol.
6. The SNMP private management communication is
transmitted to its destination.
7. Upon receiving the response, the responsible
management station updates its local database with the
new value.
If the responsible management station does not receive a response to
its request, there are two possible causes.
o The request may not have been delivered to the
destination.
o The response may not have been delivered to the
originator of the request.
In order to distinguish the two possible error conditions, a
responsible management station could check the destination to see if
the change has occurred. Unfortunately, since the secret values are
unreadable, this is not directly possible.
The recommended strategy for verifying key changes is to set the
public value corresponding to the secret being changed to a
recognizable, novel value: that is, alter the public authentication
key value for the relevant party when changing its private
authentication key, or alter its public privacy key value when
changing its private privacy key. In this way, the responsible
management station may retrieve the public value when a response is
not received, and verify whether or not the change has taken place.
(This strategy is available since the public values are not used by
the protocols defined in this memo. If this strategy is employed,
then the public values are significant in this context. Of course,
protocols using the public values may make use of this strategy
directly.)
One other scenario worthy of mention is using a SNMP party to change
its own secrets. In this case, the destination will change its local
database prior to generating a response. Thus, the response will be
constructed according to the new value. However, the responsible
management station will not update its local database until after the
response is received. This suggests the responsible management
station may receive a response which will be evaluated as
unauthentic, unless the correct secret is used. The responsible
management station may either account for this scenario as a special
case, or use an alteration of the relevant public values (as
described above) to verify the key change.
Note, during the period of time after the request has been sent and
before the response is received, the management station must keep
track of both the old and new secret values. Since the delay may be
the result of a network failure, the management station must be
prepared to retain both values for an extended period of time,
including across reboots.
6.5 Crash Recovery
This section describes the requirements for SNMP protocol entities in
connection with recovery from system crashes or other service
interruptions.
For each SNMP party in the local database for a particular SNMP
protocol entity, its identity, authentication clock, private
authentication key, and private privacy key must enjoy non-volatile,
incorruptible representations. If possible, lifetime should also
enjoy a non-volatile, incorruptible representation. If said protocol
entity supports other security protocols or algorithms in addition to
the two defined in this memo, then the authentication protocol and
the privacy protocol for each party also require non-volatile,
incorruptible representation.
The authentication clock of a SNMP party is a critical component of
the overall security of the protocols. The inclusion of a reliable
representation of a clock in a SNMP protocol entity enhances overall
security. A reliable clock representation continues to increase
according to the passage of time, even when the local SNMP protocol
entity -- due to power loss or other system failure -- may not be
operating. An example of a reliable clock representation is that
provided by battery-powered clock-calendar devices incorporated into
some contemporary systems. It is assumed that management stations
always support reliable clock representations, where clock adjustment
by a human operator during crash recovery may contribute to that
reliability.
If a managed agent crashes and does not reboot in time for its
responsible management station to prevent its authentication clock
from reaching its maximal value, upon reboot the clock must be halted
at its maximal value. The procedures specified in Section 6.3 would
then apply.
If a managed network element supports a reliable clock
representation, recovering from a crash requires few special actions.
Upon recovery, those attributes of each SNMP party that do not enjoy
non-volatile or reliable representation are initialized as follows.
o If the private authentication key is not the OCTET
STRING of zero length, the authentication protocol is
set to identify use of the Digest Authentication Protocol
in conjunction with the algorithm specified in
Section 2.4.1.
o The last-timestamp is initialized to the value of the
authentication clock.
o The nonce is initialized to zero.
o If the lifetime is not retained, it should be initialized to
zero.
o If the private privacy key is not the OCTET STRING
of zero length, the privacy protocol is set to identify use
of the Symmetric Privacy Protocol in conjunction with
the algorithm specified in Section 2.4.2.
Upon detecting that a managed agent has rebooted, a responsible
management station must reset all other party attributes, including
the lifetime if it was not retained. In order to reset the lifetime,
the responsible management station should set the authentication
timestamp in the message to the sum of the authentication clock and
desired lifetime. This is an artificial advancement of the
authentication timestamp in order to guarantee the message will be
authentic when received by the recipient.
If, alternatively, a managed network element does not support a
reliable clock representation, then those attributes of each SNMP
party that do not enjoy non-volatile representation are initialized
as follows.
o If the private authentication key is not the OCTET
STRING of zero length, the authentication protocol is
set to identify use of the Digest Authentication Protocol
in conjunction with the algorithm specified in
Section 2.4.1.
o The authentication clock is initialized to the maximal
time value.
o The last-timestamp is initialized to the maximal time
value.
o The nonce is initialized to zero.
o If the lifetime is not retained, it should be initialized to
zero.
o If the private privacy key is not the OCTET STRING
of zero length, the privacy protocol is set to identify use
of the Symmetric Privacy Protocol in conjunction with
the algorithm specified in Section 2.4.2.
The only authenticated request a management station should generate
for a party in this initial state is one that alters the value of at
least its authentication clock, private authentication key, and
lifetime (if that was not retained). In order to reset these values,
the responsible management station must set the authentication
timestamp in the message to the maximal time value. The nonce value
may be used to distinguish multiple messages.
7. Security Considerations
This section highlights security considerations relevant to the
protocols and procedures defined in this memo. Practices that
contribute to secure, effective operation of the mechanisms defined
here are described first. Constraints on implementation behavior that
are necessary to the security of the system are presented next.
Finally, an informal account of the contribution of each mechanism of
the protocols to the required goals is presented.
7.1 Recommended Practices
This section describes practices that contribute to the secure,
effective operation of the mechanisms defined in this memo.
o A management station should discard SNMP responses
for which neither the request-id component nor the
represented management information corresponds to any
currently outstanding request.
Although it would be typical for a management station
to do this as a matter of course, in the context of these
security protocols it is significant owing to the possibility
of message duplication (malicious or otherwise).
o A management station should not interpret an agent's
lack of response to an authenticated SNMP management
communication as a conclusive indication of agent or
network failure.
It is possible for authentication failure traps to be lost or
suppressed as a result of authentication clock skew or
inconsistent notions of shared secrets. In order either to
facilitate administration of such SNMP parties or to
provide for continued management in times of network
stress, a management station implementation may
provide for arbitrary, artificial advancement of the
timestamp or selection of shared secrets on locally
generated messages.
o The lifetime value for a SNMP party should be chosen
(by the local administration) to be as small as possible,
given the accuracy of clock devices available, relevant
round-trip communications delays, and the frequency
with which a responsible management station will be
able to verify all clock values.
A large lifetime increases the vulnerability to malicious
delays of SNMP messages. The implementation of a
management station may, when explicitly authorized,
provide for dynamic adjustment of the lifetime in order
to accommodate changing network conditions.
o When sending state altering messages to a managed
agent, a management station should delay sending
successive messages to the managed agent until a
positive acknowledgement is received for the previous
message or until the previous message expires.
When using the noAuth protocol, no message ordering
is imposed by the SNMP. Messages may be received in
any order relative to their time of generation and each
will be processed in the ordered received. In contrast,
the security protocols guarantee that received messages
are ordered insofar as each received message must have
been sent subsequent to the sending of a previously
received message.
When an authenticated message is sent to a managed
agent, it will be valid for a period of time that does not
exceed lifetime under normal circumstances. During the
period of time this message is valid, if the management
station sends another authenticated message to the
managed agent that is received and processed prior to
the first message, the first message will be considered
unauthentic when it is received by the managed agent.
Indeed, a management station must cope with the loss
and re-ordering of messages resulting from anomalies in
the network as a matter of course. A management
station implementation may choose to prevent the loss
of messages resulting from re-ordering when using the
security protocols defined in this memo by delaying
sending successive messages.
o The frequency with which the secrets of a SNMP party
should be changed is indirectly related to the frequency
of their use.
Protecting the secrets from disclosure is critical to the
overall security of the protocols. Frequent use of a secret
provides a continued source of data that may be useful
to a cryptanalyst in exploiting known or perceived
weaknesses in an algorithm. Frequent changes to the
secret avoid this vulnerability.
Changing a secret after each use is is generally regarded
as the most secure practice, but a significant amount of
overhead may be associated with that approach.
Note, too, in a local environment the threat of disclosure
may be insignificant, and as such the changing of secrets
may be less frequent. However, when public data
networks are the communication paths, more caution is
prudent.
o In order to foster the greatest degree of security, a
management station implementation must support
constrained, pairwise sharing of secrets among SNMP
entities as its default mode of operation.
Owing to the use of symmetric cryptography in the
protocols defined here, the secrets associated with a
particular SNMP party must be known to all other
SNMP parties with which that party may wish to
communicate. As the number of locations at which
secrets are known and used increases, the likelihood of
their disclosure also increases, as does the potential
impact of that disclosure. Moreover, if the set of SNMP
protocol entities with knowledge of a particular secret
numbers more than two, data origin cannot be reliably
authenticated because it is impossible to determine with
any assurance which entity of that set may be the
originator of a particular SNMP message. Thus, the
greatest degree of security is afforded by configurations
in which the secrets for each SNMP party are known to
at most two protocol entities.
7.2 Conformance
A SNMP protocol entity implementation that claims conformance to this
memo must satisfy the following requirements:
1. It must implement the noAuth and noPriv protocols
whose object identifiers are defined in [4].
noAuth This protocol signifies that messages generated
by a party using it are not protected as to origin or
integrity. It is required to ensure that a party's
authentication clock is always accessible.
noPriv This protocol signifies that messages received
by a party using it are not protected from
disclosure. It is required to ensure that a party's
authentication clock is always accessible.
2. It must implement the Digest Authentication Protocol in
conjunction with the algorithm defined in Section 2.4.1.
3. It must include in its local database at least one SNMP
party with the following parameters set as follows:
o partyAuthProtocol is set to noAuth and
o partyPrivProtocol is set to noPriv.
This party must have a MIB view [2] specified that
includes at least the authentication clock of all other
parties. Alternatively, the authentication clocks of the
other parties may be partitioned among several similarly
configured parties according to a local implementation
convention.
4. For each SNMP party about which it maintains
information in a local database, an implementation must
satisfy the following requirements:
(a) It must not allow a party's parameters to be set to
a value inconsistent with its expected syntax. In
particular, Section 2.4 specifies constraints for the
chosen mechanisms.
(b) It must, to the maximal extent possible, prohibit
read-access to the private authentication key and
private encryption key under all circumstances
except as required to generate and/or validate
SNMP messages with respect to that party. This
prohibition includes prevention of read-access by
the entity's human operators.
(c) It must allow the party's authentication clock to be
publicly accessible. The correct operation of the
Digest Authentication Protocol requires that it be
possible to determine this value at all times in
order to guarantee that skewed authentication
clocks can be resynchronized.
(d) It must prohibit alterations to its record of the
authentication clock for that party independently of
alterations to its record of the private
authentication key (unless the clock alteration is an
advancement).
(e) It must never allow its record of the authentication
clock for that party to be incremented beyond the
maximal time value and so "roll-over" to zero.
(f) It must never increase its record of the lifetime for
that party except as may be explicitly authorized
(via imperative command or securely represented
configuration information) by the responsible
network administrator.
(g) In the event that the non-volatile, incorruptible
representations of a party's parameters (in
particular, either the private authentication key or
private encryption key) are lost or destroyed, it
must alter its record of these quantities to random
values so subsequent interaction with that party
requires manual redistribution of new secrets and
other parameters.
5. If it selects new value(s) for a party's secret(s), it must
avoid bad or obvious choices for said secret(s). Choices
to be avoided are boundary values (such as all-zeros)
and predictable values (such as the same value as
previously or selecting from a predetermined set).
7.3 Protocol Correctness
The correctness of these SNMP security protocols with respect to the
stated goals depends on the following assumptions:
1. The chosen message digest algorithm satisfies its design
criteria. In particular, it must be computationally
infeasible to discover two messages that share the same
digest value.
2. It is computationally infeasible to determine the secret
used in calculating a digest on the concatenation of the
secret and a message when both the digest and the
message are known.
3. The chosen symmetric encryption algorithm satisfies its
design criteria. In particular, it must be computationally
infeasible to determine the cleartext message from the
ciphertext message without knowledge of the key used in
the transformation.
4. Local notions of a party's authentication clock while it is
associated with a specific private key value are
monotonically non-decreasing (i.e., they never run
backwards) in the absence of administrative
manipulations.
5. The secrets for a particular SNMP party are known only
to authorized SNMP protocol entities.
6. Local notions of the authentication clock for a particular
SNMP party are never altered such that the
authentication clock's new value is less than the current
value without also altering the private authentication
key.
For each mechanism of the protocol, an informal account of its
contribution to the required goals is presented below. Pseudocode
fragments are provided where appropriate to exemplify possible
implementations; they are intended to be self-explanatory.
7.3.1 Clock Monotonicity Mechanism
By pairing each sequence of a clock's values with a unique key, the
protocols partially realize goals 3 and 4, and the conjunction of
this property with assumption 6 above is sufficient for the claim
that, with respect to a specific private key value, all local notions
of a party's authentication clock are, in general, non-decreasing
with time.
7.3.2 Data Integrity Mechanism
The protocols require computation of a message digest computed over
the SNMP message prepended by the secret for the relevant party. By
virtue of this mechanism and assumptions 1 and 2, the protocols
realize goal 1.
Normally, the inclusion of the message digest value with the digested
message would not be sufficient to guarantee data integrity, since
the digest value can be modified in addition to the message while it
is enroute. However, since not all of the digested message is
included in the transmission to the destination, it is not possible
to substitute both a message and a digest value while enroute to a
destination.
Strictly speaking, the specified strategy for data integrity does not
detect a SNMP message modification which appends extraneous material
to the end of such messages. However, owing to the representation of
SNMP messages as ASN.1 values, such modifications cannot --
consistent with goal 1 -- result in unauthorized management
operations.
The data integrity mechanism specified in this memo protects only
against unauthorized modification of individual SNMP messages. A more
general data integrity service that affords protection against the
threat of message stream modification is not realized by this
mechanism, although limited protection against reordering, delay, and
duplication of messages within a message stream are provided by other
mechanisms of the protocol.
7.3.3 Data Origin Authentication Mechanism
The data integrity mechanism requires the use of a secret value known
only to communicating parties. By virtue of this mechanism and
assumptions 1 and 2, the protocols explicitly prevent unauthorized
modification of messages. Data origin authentication is implicit if
the message digest value can be verified. That is, the protocols
realize goal 2.
7.3.4 Restricted Administration Mechanism
This memo requires that implementations preclude administrative
alterations of the authentication clock for a particular party
independently from its private authentication key (unless that clock
alteration is an advancement). An example of an efficient
implementation of this restriction is provided in a pseudocode
fragment below. This pseudocode fragment meets the requirements of
assumption 6.
Pseudocode Fragment. Observe that the requirement is not for
simultaneous alteration but to preclude independent alteration. This
latter requirement is fairly easily realized in a way that is
consistent with the defined semantics of the SNMP Set operation.
Void partySetKey (party, newKeyValue)
{
if (party->clockAltered) {
party->clockAltered = FALSE;
party->keyAltered = FALSE;
party->keyInUse = newKeyValue;
party->clockInUse = party->clockCache;
}
else {
party->keyAltered = TRUE;
party->keyCache = newKeyValue;
}
}
Void partySetClock (party, newClockValue)
{
if (party->keyAltered) {
party->keyAltered = FALSE;
party->clockAltered = FALSE;
party->clockInUse = newClockValue;
party->keyInUse = party->keyCache;
}
else {
party->clockAltered = TRUE;
party->clockCache = newClockValue;
}
}
7.3.5 Ordered Delivery Mechanism
The definition of the Digest Authentication Protocol requires that,
if the timestamp value on a received message does not exceed the
timestamp of the most recent validated message locally delivered from
the originating party, then that message is not delivered. Otherwise,
the record of the timestamp for the most recent locally delivered
validated message is updated.
if (msgIsValidated) {
if (timestampOfReceivedMsg >
party->timestampOfLastDeliveredMsg) {
party->timestampOfLastDeliveredMsg =
timestampOfReceivedMsg;
}
else {
msgIsValidated = FALSE;
}
}
Although not explicitly represented in the pseudocode above, in the
Digest Authentication Protocol, the ordered delivery mechanism must
ensure that, when the authentication timestamp of the received
message is equal to the last-timestamp, received messages continue to
be delivered as long as their nonce values are monotonically
increasing. By virtue of this mechanism, the protocols realize goal
4.
7.3.6 Message Timeliness Mechanism
The definition of the SNMP security protocols requires that, if the
authentication timestamp value on a received message -- augmented by
an administratively chosen lifetime value -- is less than the local
notion of the clock for the originating SNMP party, the message is
not delivered.
if (timestampOfReceivedMsg +
party->administrativeLifetime <=
party->localNotionOfClock) {
msgIsValidated = FALSE;
}
By virtue of this mechanism, the protocols realize goal 3. In cases
in which the local notions of a particular SNMP party clock are
moderately well-synchronized, the timeliness mechanism effectively
limits the age of validly delivered messages. Thus, if an attacker
diverts all validated messages for replay much later, the delay
introduced by this attack is limited to a period that is proportional
to the skew among local notions of the party clock.
7.3.7 Selective Clock Acceleration Mechanism
The definition of the SNMP security protocols requires that, if the
timestamp value on a received, validated message exceeds the local
notion of the clock for the originating party, then that notion is
adjusted forward to correspond to said timestamp value. This
mechanism is neither strictly necessary nor sufficient to the
security of the protocol; rather, it fosters the clock
synchronization on which valid message delivery depends -- thereby
enhancing the effectiveness of the protocol in a management context.
if (msgIsValidated) {
if (timestampOfReceivedMsg >
party->localNotionOfClock) {
party->localNotionOfClock =
timestampOfReceivedMsg;
}
}
The effect of this mechanism is to synchronize local notions of the
party clock more closely in the case where a sender's notion is more
advanced than a receiver's. In the opposite case, this mechanism has
no effect on local notions of the party clock and either the received
message is validly delivered or not according to other mechanisms of
the protocol.
Operation of this mechanism does not, in general, improve the
probability of validated delivery for messages generated by party
participants whose local notion of the party clock is relatively less
advanced. In this case, queries from a management station may not be
validly delivered and the management station needs to react
appropriately (e.g., by administratively resynchronizing local
notions of the clock in conjunction with a key change). In contrast,
the delivery of SNMP trap messages generated by an agent that suffers
from a less advanced notion of a party clock is more problematic, for
an agent may lack the capacity to recognize and react to security
failures that prevent delivery of its messages. Thus, the inherently
unreliable character of trap messages is likely to be compounded by
attempts to provide for their validated delivery.
7.3.8 Confidentiality Mechanism
The protocols require the use of a symmetric encryption algorithm
when the data confidentiality service is required. By virtue of this
mechanism and assumption 3, the protocols realize goal 5.
8. Acknowledgements
The authors would like to thank the members of the SNMP Security
Working Group of the IETF for their patience and comments. Special
thanks go to Jeff Case who provided the first implementation of the
protocols. Dave Balenson, John Linn, Dan Nessett, and all the members
of the Privacy and Security Research Group provided many valuable and
detailed comments.
9. References
[1] Case, J., M. Fedor, M. Schoffstall, and J. Davin, The Simple
Network Management Protocol", RFC1157, University of Tennessee
at Knoxville, Performance Systems International, Performance
Systems International, and the MIT Laboratory for Computer
Science, May 1990. (Obsoletes RFC1098.)
[2] Davin, J., Galvin, J., and K. McCloghrie, "SNMP Administrative
Model", RFC1351, MIT Laboratory for Computer Science, Trusted
Information Systems, Inc., Hughes LAN Systems, Inc., July 1992.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, MIT
Laboratory for Computer Science, April 1992.
[4] McCloghrie, K., Davin, J., and J. Galvin, "Definitions of Managed
Objects for Administration of SNMP Parties", RFC1353, Hughes LAN
Systems, Inc., MIT Laboratory for Computer Science, Trusted
Information Systems, Inc., July 1992.
[5] FIPS Publication 46-1, "Data Encryption Standard", National
Institute of Standards and Technology, Federal Information
Processing Standard (FIPS); Supersedes FIPS Publication 46,
January 15, 1977; Reaffirmed January 22, 1988.
[6] ANSI X3.92-1981, "Data Encryption Algorithm", American National
Standards Institute, December 30, 1980.
[7] FIPS Publication 81, "DES Modes of Operation", National Institute
of Standards and Technology, December 2, 1980, Federal
Information Processing Standard (FIPS).
[8] ANSI X3.106-1983, "Data Encryption Algorithm - Modes of
Operation", American National Standards Institute, May 16, 1983.
[9] FIPS Publication 74, "Guidelines for Implementing and Using the
NBS Data Encryption Standard", National Institute of Standards
and Technology, April 1, 1981. Federal Information Processing
Standard (FIPS).
[10] Special Publication 500-20, "Validating the Correctness of
Hardware Implementations of the NBS Data Encryption Standard",
National Institute of Standards and Technology.
[11] Special Publication 500-61, "Maintenance Testing for the Data
Encryption Standard", National Institute of Standards and
Technology, August 1980.
[12] Information Processing -- Open Systems Interconnection --
Specification of Basic Encoding Rules for Abstract Syntax
Notation One (ASN.1), International Organization for
Standardization/International Electrotechnical Institute, 1987,
International Standard 8825.
10. Authors' Addresses
James M. Galvin
Trusted Information Systems, Inc.
3060 Washington Road, Route 97
Glenwood, MD 21738
Phone: (301) 854-6889
EMail: galvin@tis.com
Keith McCloghrie
Hughes LAN Systems, Inc.
1225 Charleston Road
Mountain View, CA 94043
Phone: (415) 966-7934
EMail: kzm@hls.com
James R. Davin
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139
Phone: (617) 253-6020
EMail: jrd@ptt.lcs.mit.edu
