Network Working Group J. Myers
Request for Comments: 2222 Netscape Communications
Category: Standards Track October 1997
Simple Authentication and Security Layer (SASL)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1997). All Rights Reserved.
Table of Contents
1. Abstract .............................................. 2
2. Organization of this Document ......................... 2
2.1. How to Read This Document ............................. 2
2.2. Conventions Used in this Document ..................... 2
2.3. Examples .............................................. 3
3. IntrodUCtion and Overview ............................. 3
4. Profiling requirements ................................ 4
5. Specific issues ....................................... 5
5.1. Client sends data first ............................... 5
5.2. Server returns success with additional data ........... 5
5.3. Multiple authentications .............................. 5
6. Registration procedures ............................... 6
6.1. Comments on SASL mechanism registrations .............. 6
6.2. Location of Registered SASL Mechanism List ............ 6
6.3. Change Control ........................................ 7
6.4. Registration Template ................................. 7
7. Mechanism definitions ................................. 8
7.1. Kerberos version 4 mechanism .......................... 8
7.2. GSSAPI mechanism ...................................... 9
7.2.1 Client side of authentication protocol exchange ....... 9
7.2.2 Server side of authentication protocol exchange ....... 10
7.2.3 Security layer ........................................ 11
7.3. S/Key mechanism ....................................... 11
7.4. External mechanism .................................... 12
8. References ............................................ 13
9. Security Considerations ............................... 13
10. Author's Address ...................................... 14
Appendix A. Relation of SASL to Transport Security .......... 15
Full Copyright Statement .................................... 16
1. Abstract
This document describes a method for adding authentication support to
connection-based protocols. To use this specification, a protocol
includes a command for identifying and authenticating a user to a
server and for optionally negotiating protection of subsequent
protocol interactions. If its use is negotiated, a security layer is
inserted between the protocol and the connection. This document
describes how a protocol specifies such a command, defines several
mechanisms for use by the command, and defines the protocol used for
carrying a negotiated security layer over the connection.
2. Organization of this Document
2.1. How to Read This Document
This document is written to serve two different audiences, protocol
designers using this specification to support authentication in their
protocol, and implementors of clients or servers for those protocols
using this specification.
The sections "Introduction and Overview", "Profiling requirements",
and "Security Considerations" cover issues that protocol designers
need to understand and address in profiling this specification for
use in a specific protocol.
Implementors of a protocol using this specification need the
protocol-specific profiling information in addition to the
information in this document.
2.2. Conventions Used in this Document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key Words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [RFC2119].
2.3. Examples
Examples in this document are for the IMAP profile [RFC2060] of this
specification. The base64 encoding of challenges and responses, as
well as the "+ " preceding the responses are part of the IMAP4
profile, not part of the SASL specification itself.
3. Introduction and Overview
The Simple Authentication and Security Layer (SASL) is a method for
adding authentication support to connection-based protocols. To use
this specification, a protocol includes a command for identifying and
authenticating a user to a server and for optionally negotiating a
security layer for subsequent protocol interactions.
The command has a required argument identifying a SASL mechanism.
SASL mechanisms are named by strings, from 1 to 20 characters in
length, consisting of upper-case letters, digits, hyphens, and/or
underscores. SASL mechanism names must be registered with the IANA.
Procedures for registering new SASL mechanisms are given in the
section "Registration procedures"
If a server supports the requested mechanism, it initiates an
authentication protocol exchange. This consists of a series of
server challenges and client responses that are specific to the
requested mechanism. The challenges and responses are defined by the
mechanisms as binary tokens of arbitrary length. The protocol's
profile then specifies how these binary tokens are then encoded for
transfer over the connection.
After receiving the authentication command or any client response, a
server may issue a challenge, indicate failure, or indicate
completion. The protocol's profile specifies how the server
indicates which of the above it is doing.
After receiving a challenge, a client may issue a response or abort
the exchange. The protocol's profile specifies how the client
indicates which of the above it is doing.
During the authentication protocol exchange, the mechanism performs
authentication, transmits an authorization identity (frequently known
as a userid) from the client to server, and negotiates the use of a
mechanism-specific security layer. If the use of a security layer is
agreed upon, then the mechanism must also define or negotiate the
maximum cipher-text buffer size that each side is able to receive.
The transmitted authorization identity may be different than the
identity in the client's authentication credentials. This permits
agents such as proxy servers to authenticate using their own
credentials, yet request the Access privileges of the identity for
which they are proxying. With any mechanism, transmitting an
authorization identity of the empty string directs the server to
derive an authorization identity from the client's authentication
credentials.
If use of a security layer is negotiated, it is applied to all
subsequent data sent over the connection. The security layer takes
effect immediately following the last response of the authentication
exchange for data sent by the client and the completion indication
for data sent by the server. Once the security layer is in effect,
the protocol stream is processed by the security layer into buffers
of cipher-text. Each buffer is transferred over the connection as a
stream of octets prepended with a four octet field in network byte
order that represents the length of the following buffer. The length
of the cipher-text buffer must be no larger than the maximum size
that was defined or negotiated by the other side.
4. Profiling requirements
In order to use this specification, a protocol definition must supply
the following information:
1. A service name, to be selected from the IANA registry of "service"
elements for the GSSAPI host-based service name form [RFC2078].
2. A definition of the command to initiate the authentication
protocol exchange. This command must have as a parameter the
mechanism name being selected by the client.
The command SHOULD have an optional parameter giving an initial
response. This optional parameter allows the client to avoid a
round trip when using a mechanism which is defined to have the
client send data first. When this initial response is sent by the
client and the selected mechanism is defined to have the server
start with an initial challenge, the command fails. See section
5.1 of this document for further information.
3. A definition of the method by which the authentication protocol
exchange is carried out, including how the challenges and
responses are encoded, how the server indicates completion or
failure of the exchange, how the client aborts an exchange, and
how the exchange method interacts with any line length limits in
the protocol.
4. Identification of the octet where any negotiated security layer
starts to take effect, in both directions.
5. A specification of how the authorization identity passed from the
client to the server is to be interpreted.
5. Specific issues
5.1. Client sends data first
Some mechanisms specify that the first data sent in the
authentication protocol exchange is from the client to the server.
If a protocol's profile permits the command which initiates an
authentication protocol exchange to contain an initial client
response, this parameter SHOULD be used with such mechanisms.
If the initial client response parameter is not given, or if a
protocol's profile does not permit the command which initiates an
authentication protocol exchange to contain an initial client
response, then the server issues a challenge with no data. The
client's response to this challenge is then used as the initial
client response. (The server then proceeds to send the next
challenge, indicates completion, or indicates failure.)
5.2. Server returns success with additional data
Some mechanisms may specify that server challenge data be sent to the
client along with an indication of successful completion of the
exchange. This data would, for example, authenticate the server to
the client.
If a protocol's profile does not permit this server challenge to be
returned with a success indication, then the server issues the server
challenge without an indication of successful completion. The client
then responds with no data. After receiving this empty response, the
server then indicates successful completion.
5.3. Multiple authentications
Unless otherwise stated by the protocol's profile, only one
successful SASL negotiation may occur in a protocol session. In this
case, once an authentication protocol exchange has successfully
completed, further attempts to initiate an authentication protocol
exchange fail.
In the case that a profile eXPlicitly permits multiple successful
SASL negotiations to occur, then in no case may multiple security
layers be simultaneously in effect. If a security layer is in effect
and a subsequent SASL negotiation selects no security layer, the
original security layer remains in effect. If a security layer is in
effect and a subsequent SASL negotiation selects a second security
layer, then the second security layer replaces the first.
6. Registration procedures
Registration of a SASL mechanism is done by filling in the template
in section 6.4 and sending it in to iana@isi.edu. IANA has the right
to reject obviously bogus registrations, but will perform no review
of clams made in the registration form.
There is no naming convention for SASL mechanisms; any name that
conforms to the syntax of a SASL mechanism name can be registered.
While the registration procedures do not require it, authors of SASL
mechanisms are encouraged to seek community review and comment
whenever that is feasible. Authors may seek community review by
posting a specification of their proposed mechanism as an internet-
draft. SASL mechanisms intended for widespread use should be
standardized through the normal IETF process, when appropriate.
6.1. Comments on SASL mechanism registrations
Comments on registered SASL mechanisms should first be sent to the
"owner" of the mechanism. Submitters of comments may, after a
reasonable attempt to contact the owner, request IANA to attach their
comment to the SASL mechanism registration itself. If IANA approves
of this the comment will be made accessible in conjunction with the
SASL mechanism registration itself.
6.2. Location of Registered SASL Mechanism List
SASL mechanism registrations will be posted in the anonymous FTP
Directory "ftp://ftp.isi.edu/in-notes/iana/assignments/sasl-
mechanisms/" and all registered SASL mechanisms will be listed in the
periodically issued "Assigned Numbers" RFC[currently STD 2, RFC
1700]. The SASL mechanism description and other supporting material
may also be published as an Informational RFCby sending it to "rfc-
editor@isi.edu" (please follow the instructions to RFCauthors [RFC
2223]).
6.3. Change Control
Once a SASL mechanism registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a SASL mechanism may pass responsibility for the SASL
mechanism to another person or agency by informing IANA; this can be
done without discussion or review.
The IESG may reassign responsibility for a SASL mechanism. The most
common case of this will be to enable changes to be made to
mechanisms where the author of the registration has died, moved out
of contact or is otherwise unable to make changes that are important
to the community.
SASL mechanism registrations may not be deleted; mechanisms which are
no longer believed appropriate for use can be declared OBSOLETE by a
change to their "intended use" field; such SASL mechanisms will be
clearly marked in the lists published by IANA.
The IESG is considered to be the owner of all SASL mechanisms which
are on the IETF standards track.
6.4. Registration Template
To: iana@iana.org
Subject: Registration of SASL mechanism X
SASL mechanism name:
Security considerations:
Published specification (optional, recommended):
Person & email address to contact for further information:
Intended usage:
(One of COMMON, LIMITED USE or OBSOLETE)
Author/Change controller:
(Any other information that the author deems interesting may be
added below this line.)
7. Mechanism definitions
The following mechanisms are hereby defined.
7.1. Kerberos version 4 mechanism
The mechanism name associated with Kerberos version 4 is
"KERBEROS_V4".
The first challenge consists of a random 32-bit number in network
byte order. The client responds with a Kerberos ticket and an
authenticator for the principal "service.hostname@realm", where
"service" is the service name specified in the protocol's profile,
"hostname" is the first component of the host name of the server with
all letters in lower case, and where "realm" is the Kerberos realm of
the server. The encrypted checksum field included within the
Kerberos authenticator contains the server provided challenge in
network byte order.
Upon decrypting and verifying the ticket and authenticator, the
server verifies that the contained checksum field equals the original
server provided random 32-bit number. Should the verification be
successful, the server must add one to the checksum and construct 8
octets of data, with the first four octets containing the incremented
checksum in network byte order, the fifth octet containing a bit-mask
specifying the security layers supported by the server, and the sixth
through eighth octets containing, in network byte order, the maximum
cipher-text buffer size the server is able to receive. The server
must encrypt using DES ECB mode the 8 octets of data in the session
key and issue that encrypted data in a second challenge. The client
considers the server authenticated if the first four octets of the
un-encrypted data is equal to one plus the checksum it previously
sent.
The client must construct data with the first four octets containing
the original server-issued checksum in network byte order, the fifth
octet containing the bit-mask specifying the selected security layer,
the sixth through eighth octets containing in network byte order the
maximum cipher-text buffer size the client is able to receive, and
the following octets containing the authorization identity. The
client must then append from one to eight zero-valued octets so that
the length of the data is a multiple of eight octets. The client must
then encrypt using DES PCBC mode the data with the session key and
respond with the encrypted data. The server decrypts the data and
verifies the contained checksum. The server must verify that the
principal identified in the Kerberos ticket is authorized to connect
as that authorization identity. After this verification, the
authentication process is complete.
The security layers and their corresponding bit-masks are as follows:
1 No security layer
2 Integrity (krb_mk_safe) protection
4 Privacy (krb_mk_priv) protection
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
EXAMPLE: The following are two Kerberos version 4 login scenarios to
the IMAP4 protocol (note that the line breaks in the sample
authenticators are for editorial clarity and are not in real
authenticators)
S: * OK IMAP4 Server
C: A001 AUTHENTICATE KERBEROS_V4
S: + AmFYig==
C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
+nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
S: + or//EoAADZI=
C: DiAF5A4gA+oOIALuBkAAmw==
S: A001 OK Kerberos V4 authentication successful
S: * OK IMAP4 Server
C: A001 AUTHENTICATE KERBEROS_V4
S: + gcfgCA==
C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
+nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
S: A001 NO Kerberos V4 authentication failed
7.2. GSSAPI mechanism
The mechanism name associated with all mechanisms employing the
GSSAPI [RFC2078] is "GSSAPI".
7.2.1 Client side of authentication protocol exchange
The client calls GSS_Init_sec_context, passing in 0 for
input_context_handle (initially) and a targ_name equal to output_name
from GSS_Import_Name called with input_name_type of
GSS_C_NT_HOSTBASED_SERVICE and input_name_string of
"service@hostname" where "service" is the service name specified in
the protocol's profile, and "hostname" is the fully qualified host
name of the server. The client then responds with the resulting
output_token. If GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED,
then the client should expect the server to issue a token in a
subsequent challenge. The client must pass the token to another call
to GSS_Init_sec_context, repeating the actions in this paragraph.
When GSS_Init_sec_context returns GSS_S_COMPLETE, the client takes
the following actions: If the last call to GSS_Init_sec_context
returned an output_token, then the client responds with the
output_token, otherwise the client responds with no data. The client
should then expect the server to issue a token in a subsequent
challenge. The client passes this token to GSS_Unwrap and interprets
the first octet of resulting cleartext as a bit-mask specifying the
security layers supported by the server and the second through fourth
octets as the maximum size output_message to send to the server. The
client then constructs data, with the first octet containing the
bit-mask specifying the selected security layer, the second through
fourth octets containing in network byte order the maximum size
output_message the client is able to receive, and the remaining
octets containing the authorization identity. The client passes the
data to GSS_Wrap with conf_flag set to FALSE, and responds with the
generated output_message. The client can then consider the server
authenticated.
7.2.2 Server side of authentication protocol exchange
The server passes the initial client response to
GSS_Accept_sec_context as input_token, setting input_context_handle
to 0 (initially). If GSS_Accept_sec_context returns
GSS_S_CONTINUE_NEEDED, the server returns the generated output_token
to the client in challenge and passes the resulting response to
another call to GSS_Accept_sec_context, repeating the actions in this
paragraph.
When GSS_Accept_sec_context returns GSS_S_COMPLETE, the client takes
the following actions: If the last call to GSS_Accept_sec_context
returned an output_token, the server returns it to the client in a
challenge and expects a reply from the client with no data. Whether
or not an output_token was returned (and after receipt of any
response from the client to such an output_token), the server then
constructs 4 octets of data, with the first octet containing a bit-
mask specifying the security layers supported by the server and the
second through fourth octets containing in network byte order the
maximum size output_token the server is able to receive. The server
must then pass the plaintext to GSS_Wrap with conf_flag set to FALSE
and issue the generated output_message to the client in a challenge.
The server must then pass the resulting response to GSS_Unwrap and
interpret the first octet of resulting cleartext as the bit-mask for
the selected security layer, the second through fourth octets as the
maximum size output_message to send to the client, and the remaining
octets as the authorization identity. The server must verify that
the src_name is authorized to authenticate as the authorization
identity. After these verifications, the authentication process is
complete.
7.2.3 Security layer
The security layers and their corresponding bit-masks are as follows:
1 No security layer
2 Integrity protection.
Sender calls GSS_Wrap with conf_flag set to FALSE
4 Privacy protection.
Sender calls GSS_Wrap with conf_flag set to TRUE
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
7.3. S/Key mechanism
The mechanism name associated with S/Key [RFC1760] using the MD4
digest algorithm is "SKEY".
The client sends an initial response with the authorization identity.
The server then issues a challenge which contains the decimal
sequence number followed by a single space and the seed string for
the indicated authorization identity. The client responds with the
one-time-password, as either a 64-bit value in network byte order or
encoded in the "six English words" format.
The server must verify the one-time-password. After this
verification, the authentication process is complete.
S/Key authentication does not provide for any security layers.
EXAMPLE: The following are two S/Key login scenarios in the IMAP4
protocol.
S: * OK IMAP4 Server
C: A001 AUTHENTICATE SKEY
S: +
C: bW9yZ2Fu
S: + OTUgUWE1ODMwOA==
C: Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: A001 OK S/Key authentication successful
S: * OK IMAP4 Server
C: A001 AUTHENTICATE SKEY
S: +
C: c21pdGg=
S: + OTUgUWE1ODMwOA==
C: BsAY3g4gBNo=
S: A001 NO S/Key authentication failed
The following is an S/Key login scenario in an IMAP4-like protocol
which has an optional "initial response" argument to the AUTHENTICATE
command.
S: * OK IMAP4-Like Server
C: A001 AUTHENTICATE SKEY bW9yZ2Fu
S: + OTUgUWE1ODMwOA==
C: Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: A001 OK S/Key authentication successful
7.4. External mechanism
The mechanism name associated with external authentication is
"EXTERNAL".
The client sends an initial response with the authorization identity.
The server uses information, external to SASL, to determine whether
the client is authorized to authenticate as the authorization
identity. If the client is so authorized, the server indicates
successful completion of the authentication exchange; otherwise the
server indicates failure.
The system providing this external information may be, for example,
IPsec or TLS.
If the client sends the empty string as the authorization identity
(thus requesting the authorization identity be derived from the
client's authentication credentials), the authorization identity is
to be derived from authentication credentials which exist in the
system which is providing the external authentication.
8. References
[RFC2060] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC2060, December 1996.
[RFC2078] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC2078, January 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, March 1997.
[RFC2223] Postel, J., and J. Reynolds, "Instructions to RFC
Authors", RFC2223, October 1997.
[RFC1760] Haller, N., "The S/Key One-Time Password System", RFC
1760, February 1995.
[RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC1700, October 1994.
9. Security Considerations
Security issues are discussed throughout this memo.
The mechanisms that support integrity protection are designed such
that the negotiation of the security layer and authorization identity
is integrity protected. When the client selects a security layer
with at least integrity protection, this protects against an active
attacker hijacking the connection and modifying the authentication
exchange to negotiate a plaintext connection.
When a server or client supports multiple authentication mechanisms,
each of which has a different security strength, it is possible for
an active attacker to cause a party to use the least secure mechanism
supported. To protect against this sort of attack, a client or
server which supports mechanisms of different strengths should have a
configurable minimum strength that it will use. It is not sufficient
for this minimum strength check to only be on the server, since an
active attacker can change which mechanisms the client sees as being
supported, causing the client to send authentication credentials for
its weakest supported mechanism.
The client's selection of a SASL mechanism is done in the clear and
may be modified by an active attacker. It is important for any new
SASL mechanisms to be designed such that an active attacker cannot
oBTain an authentication with weaker security properties by modifying
the SASL mechanism name and/or the challenges and responses.
Any protocol interactions prior to authentication are performed in
the clear and may be modified by an active attacker. In the case
where a client selects integrity protection, it is important that any
security-sensitive protocol negotiations be performed after
authentication is complete. Protocols should be designed such that
negotiations performed prior to authentication should be either
ignored or revalidated once authentication is complete.
10. Author's Address
John G. Myers
Netscape Communications
501 E. Middlefield Road
Mail Stop MV-029
Mountain View, CA 94043-4042
EMail: jgmyers@netscape.com
Appendix A. Relation of SASL to Transport Security
Questions have been raised about the relationship between SASL and
various services (such as IPsec and TLS) which provide a secured
connection.
Two of the key features of SASL are:
1. The separation of the authorization identity from the identity in
the client's credentials. This permits agents such as proxy
servers to authenticate using their own credentials, yet request
the access privileges of the identity for which they are proxying.
2. Upon successful completion of an authentication exchange, the
server knows the authorization identity the client wishes to use.
This allows servers to move to a "user is authenticated" state in
the protocol.
These features are extremely important to some application protocols,
yet Transport Security services do not always provide them. To
define SASL mechanisms based on these services would be a very messy
task, as the framing of these services would be redundant with the
framing of SASL and some method of providing these important SASL
features would have to be devised.
Sometimes it is desired to enable within an existing connection the
use of a security service which does not fit the SASL model. (TLS is
an example of such a service.) This can be done by adding a command,
for example "STARTTLS", to the protocol. Such a command is outside
the scope of SASL, and should be different from the command which
starts a SASL authentication protocol exchange.
In certain situations, it is reasonable to use SASL underneath one of
these Transport Security services. The transport service would
secure the connection, either service would authenticate the client,
and SASL would negotiate the authorization identity. The SASL
negotiation would be what moves the protocol from "unauthenticated"
to "authenticated" state. The "EXTERNAL" SASL mechanism is
explicitly intended to handle the case where the transport service
secures the connection and authenticates the client and SASL
negotiates the authorization identity.
When using SASL underneath a sufficiently strong Transport Security
service, a SASL security layer would most likely be redundant. The
client and server would thus probably want to negotiate off the use
of a SASL security layer.
Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights Reserved.
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