One document matched: draft-ietf-sasl-rfc2222bis-08.txt
Differences from draft-ietf-sasl-rfc2222bis-07.txt
Network Working Group A. Melnikov
Internet Draft Editor
Document: draft-ietf-sasl-rfc2222bis-08.txt June 2004
Obsoletes: RFC 2222 Expires in six months
Simple Authentication and Security Layer (SASL)
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet Drafts are working documents of the Internet Engineering
Task Force (IETF), its Areas, and its Working Groups. Note that
other groups may also distribute working documents as Internet
Drafts. Internet Drafts are draft documents valid for a maximum of
six months. Internet Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is not appropriate to use
Internet Drafts as reference material or to cite them other than as
``work in progress''.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
A revised version of this draft document will be submitted to the RFC
editor as a Standards Track RFC for the Internet Community.
Discussion and suggestions for improvement are requested.
Distribution of this draft is unlimited.
When published as an RFC this document will obsolete RFC 2222.
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Abstract
The Simple Authentication and Security Layer (SASL) is a framework
for providing authentication and data security services in
connection-oriented protocols via replaceable mechanisms. It
provides structured interface between protocols and mechanisms. The
resulting framework allows new protocols to reuse existing mechanisms
and allows old protocols to make use of new mechanisms. The
framework also provides a protocol for securing subsequent protocol
exchanges within a data security layer.
This document describes how a SASL mechanism is structured, describes
how protocols add support for SASL, and defines the protocol for
carrying a data security layer over a connection. Additionally, this
document defines one SASL mechanism, the EXTERNAL mechanism.
1. 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" [KEYWORDS].
Character names in this document use the notation for code points and
names from the Unicode Standard [Unicode]. For example, the letter
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
This document uses terms "integrity protection" and "confidentiality
protection". The former refers to a security layer (see Section
"Introduction" below for the definition) designed to provide "data
integrity service" as defined in [Sec-Glossary]. Confidentiality
protection is a security layer that provides "data confidentiality
service" as defined in [Sec-Glossary]. The term "confidentiality
protection" usually implies "integrity protection". Security layers
may offer other kinds of security services.
2. Introduction
The Simple Authentication and Security Layer (SASL) is a framework
for providing authentication and data security services in
connection-oriented protocols via replaceable mechanisms. SASL
provides a structured interface between protocols and mechanisms.
SASL also provides a protocol for securing subsequent protocol
exchanges within a data security layer.
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SASL design is intended to allow new protocols to reuse existing
mechanisms without requiring redesign of the mechanisms and allows
existing protocols to make use of new mechanisms without redesign of
protocols.
The SASL is conceptually a framework which provides a layer between
protocols and mechanisms, as illustrated in the following diagram.
SMTP Protocol LDAP Protocol Other Protocols
Profile Profile . . .
\----- | -----/
\ | /
SASL framework
/ | \
/----- | -----\
DIGEST-MD5 EXTERNAL Other Mechanisms
SASL mechanism SASL mechanism . . .
It is through the interfaces of this layer that the framework allows
any protocol to be utilized with any mechanism. While the layer does
generally hide the particulars of protocols from mechanisms, the
layer does not generally hide the particulars of mechanisms from
protocols. For example, different mechanisms require different
information to operate, some of them use password based
authentication, other make use of Kerberos tickets, certificates,
etc. Also, in order to perform authorization step server
implementations have to implement mapping from a mechanism specific
authentication identity format to a protocol specific format.
It is noted that it is possible to design and implement this
framework in ways which do abstract away particulars of similar
mechanisms. Such implementation could also be designed to be shared
by multiple implementations of various protocols.
As illustrated above, the SASL framework interfaces with both
protocols and mechanisms.
To use SASL, 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. If the use of a
security layer is negotiated, that security layer is inserted between
the protocol and the connection. Section 4 ("Protocol profile
requirements") profiles the requirements that a protocol
specification must fulfill to make use of SASL.
A SASL mechanism is a series of server challenges and client
responses specific to the mechanism. Each SASL mechanism are
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identity by a registered name. Section 5. ("Mechanism profile
guidelines") profiles the requirements that a mechanism specification
must fulfill to define a SASL mechanism.
This document is written to serve several different audiences:
o) protocol designers using this specification to support
authentication in their protocol
o) mechanism designers that define new SASL mechanisms
o) implementors of clients or servers for those protocols using this
specification.
The sections "Authentication Mechanisms", "Protocol Profile
Requirements", "Specific Issues", and "Security Considerations" cover
issues that protocol designers need to understand and address in
profiling this specification for use in a specific protocol.
The sections "Authentication Mechanisms", "Mechanism Profile
Requirements", "Security Considerations" and "Registration procedure"
cover issues that mechanism designers need to understand and address
in designing new SASL mechanisms.
The sections "Authentication Mechanisms", "Protocol profile
requirements", "Specific issues" and "Security Considerations" cover
issues that implementors of a protocol that uses SASL framework need
to understand. The implementors will also need to understand a
specification of a profile specific to the protocol, as well as
aspects of mechanism specifications they intend to use (regardless of
whether they are implementing the mechanisms themselves or using an
existing implementation) to understand, for instance, the mechanism
specific authentication identity forms, the offered services, and
security and other considerations.
3. Authentication mechanisms
SASL mechanisms are named by strings, from 1 to 20 characters in
length, consisting of ASCII [ASCII] upper-case letters, digits,
hyphens, and/or underscores. Names of SASL mechanisms or families of
mechanisms must be registered with the Internet Assigned Numbers
Authority (IANA) as described in section 8.2.
The "sasl-mech" ABNF production below defines the syntax of a SASL
mechanism name. This uses the Augmented Backus-Naur Form (ABNF)
notation as specified in [ABNF].
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sasl-mech = 1*20mech-char
mech-char = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE
; mech-char is restricted to "A"-"Z", "0"-"9", "-",
; and "_" from ASCII character set.
UPPER-ALPHA = %x41-5A
; "A"-"Z"
DIGIT = %x30-39
; "0"-"9"
HYPHEN = %x2D
; "-"
UNDERSCORE = %x5F
; "_"
3.1. Authentication protocol exchange
A SASL mechanism is responsible for conducting an authentication
protocol exchange. This consists of a series of server challenges
and client responses, the contents of which are specific to and
defined by the mechanism. To the protocol, the challenges and
responses are opaque 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 an authentication command or any client response, a
server mechanism may issue a challenge, indicate failure, or indicate
completion. The server mechanism may return additional data with a
completion indication. The protocol's profile specifies how each of
these is then represented over the connection.
After receiving a challenge, a client mechanism may issue a response
or abort the exchange. The protocol's profile specifies how each of
these are then represented over the connection.
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 security layer buffer size that each side is able to receive.
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3.2. Authorization and authentication identities
SASL authentication deals with two identities: the authentication
identity, derived from the client's authentication credentials; and
the authorization identity, which is the result of SASL processing
and is used by the server as the primary identity for making access
policy decisions.
The processing model is as follows. A server, upon completion of the
authentication mechanism, uses the results produced by the
authentication mechanism, the client-provided authorization identity
value (which may be the empty string), and local policy information
to derive an authorization identity. The authorization identity is
made available to further server processing for use in making access
policy decisions. The provision of additional client attributes that
may affect access policy is not covered by this specification.
The authorization identity may be an empty (zero length) string. In
this case, the server derives an authorization identity from the
client's authentication identity.
A mechanism which is incapable of transmitting an authorization
identity must be treated as if it always transmits an authorization
identity of an empty string.
Any normalization of the authentication identity is defined by a
particular SASL mechanism, the protocol profile doesn't influence it.
The mechanism MUST preserve Unicode codepoint when transferring
authorization identity (e.g. the mechanism cann't apply any form of
normalization).
3.2.1. Authorization identities and proxy authentication
If the authorization identity transmitted during the authentication
protocol exchange is not the empty string, this is typically referred
to as "proxy authentication". This feature 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.
The server makes an implementation defined policy decision as to
whether the authentication identity is permitted to have the access
privileges of the authorization identity and whether the
authorization identity is permitted to receive service. If it is
not, the server indicates failure of the authentication protocol
exchange.
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As a client might not have the same information as the server,
clients SHOULD NOT derive authorization identities from
authentication identities. Instead, clients SHOULD provide no (or
empty) authorization identity when the user has not provided an
authorization identity.
The server SHOULD verify that a received authorization identity is in
the correct form. Protocol profiles whose authorization identities
are simple user names (e.g. IMAP [RFC 3501]) SHOULD use "SASLprep"
profile [SASLprep] of the "stringprep" algorithm [StringPrep] to
prepare these names for matching. The profiles MAY use a stringprep
profile that is more strict than "SASLprep". If the preparation of
the authorization identity fails or results in an empty string, the
server MUST fail the authentication exchange. The only exception to
this rule is when the received authorization identity is already the
empty string.
3.2.2. Authorization Identity Format
An authorization identity is a string of zero or more Unicode
[Unicode] coded characters. The NUL <U+0000> character is not
permitted in authorization identities.
The character encoding scheme used for transmitting an authorization
identity over the protocol is specified in each authentication
mechanism. All IETF-defined mechanisms MUST, and all other
mechanisms SHOULD, use UTF-8 [UTF-8]. (See [CHARSET-POLICY] for IETF
policy regarding character sets and encoding schemes.)
Mechanisms are expected to be capable of carrying the entire Unicode
repertoire (with the exception of the NUL character). An
authorization identity of the empty string and an absent
authorization identity MUST be treated as equivalent. A mechanism
which provides an optional field for an authorization identity,
SHOULD NOT allow that field, when present, to be empty. The meaning
of the empty string as an authorization identity is described in the
previous section.
3.3. Security layers
If use of a security layer is negotiated by the authentication
protocol exchange, the security layer is applied to all subsequent
data sent over the connection (until another security layer is
negotiated; see also section 6.3). 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. The exact position MUST be defined by
the protocol profile (see section 4 part 5).
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Note that all SASL mechanisms that are unable to negotiate a security
layer automatically select no security layer.
Once the security layer is in effect the protocol stream is processed
by the security layer into buffers of protected data. Each buffer of
protected data 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
protected data buffer MUST be no larger than the maximum size that
was either defined in the mechanism specification or negotiated by
the other side during the authentication protocol exchange. Upon the
receipt of a data buffer which is larger than the defined/negotiated
maximal buffer size the receiver SHOULD close the connection. This
might be a sign of an attack or a buggy implementation.
4. Protocol profile 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 [GSSAPI]. This
service name is made available to the authentication mechanism.
The registry is available at the URL
<http://www.iana.org/assignments/gssapi-service-names>.
2) A definition of the command to initiate the authentication
protocol exchange. This command must have as a parameter the name of
the mechanism being selected by the client.
The command SHOULD have an optional parameter giving an initial
response. If the protocol allows for the initial response, the
protocol profile SHOULD also describe how an empty initial response
is encoded. 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 6.1 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.
The exchange method SHOULD allow the server to include an optional
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data ("optional challenge") with a success notification. This allows
the server to avoid a round trip when using a mechanism which is
defined to have the server send additional data along with the
indication of successful completion. Note that an additional data
with success can't be empty. See section 6.2 of this document for
further information.
4) A protocol profile SHOULD specify a mechanism through which a
client may obtain the names of the SASL mechanisms available to it.
This is typically done through the protocol's extensions or
capabilities mechanism.
5) Identification of the octet where any negotiated security layer
starts to take effect, in both directions.
6) Specify if the protocol profile supports "multiple
authentications" (see section 6.3).
7) If both a Transport Layer Security [TLS] and a SASL security layer
are allowed to be negotiated by the protocol, the protocol profile
MUST define in which order they are applied to a cleartext data sent
over the connection.
8) A protocol profile MAY further refine the definition of an
authorization identity by adding additional syntactic restrictions
and protocol-specific semantics. A protocol profile MUST specify the
form of the authorization identity (since it is protocol specific, as
opposed to the authentication identity, which is mechanism specific)
and how authorization identities are to be compared. Profiles whose
authorization identities are simple user names (e.g. IMAP [RFC 3501])
SHOULD use "SASLprep" profile [SASLprep] of the "stringprep"
algorithm [StringPrep] to prepare these names for matching. The
profiles MAY use a stringprep profile that is more strict than
SASLprep.
9) Where the application-layer protocol does not precisely state how
identities established through SASL relate to identities used
elsewhere (e.g., access controls) in the application-layer protocol,
it may be useful for the application-layer protocol to provide a
facility which the client may use to discover the identity used.
A protocol profile SHOULD NOT attempt to amend the definition of
mechanisms or make mechanism-specific encodings. This breaks the
separation between protocol and mechanism that is fundamental to the
design of SASL. Likewise, SASL mechanisms SHOULD be profile neutral.
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5. Mechanism profile guidelines
Designers of new SASL mechanism should be aware of the following
issues:
1) Authorization identity
While some legacy mechanisms are incapable of transmitting an
authorization identity (which means that for these mechanisms the
authorization identity is always the empty string), newly defined
mechanisms SHOULD be capable of transmitting a non-empty
authorization identity. See also section 3.2.
2) Character string issues
Authentication mechanisms SHOULD encode character strings in UTF-8
[UTF-8] (see [CHARSET-POLICY] for IETF policy regarding character
sets in IETF protocols). In order to avoid interoperability problems
due to differing normalizations, when a mechanisms specifies that
character data is to be used as input to a cryptographic and/or
comparison function, the mechanism specification MUST detail how the
data is to be represented, including any normalizations or other
preparations, to ensure proper function. Designers of mechanisms
SHOULD use the "SASLprep" profile [SASLprep] of the "stringprep"
algorithm [StringPrep] where applicable. However it should be noted
that this rule doesn't apply to authorization identities, as they are
protocol specific.
The preparation can be potentially performed on the client end (upon
getting user input or retrieving a value from configuration) or on
the server end (upon receiving the value from the client, retrieving
a value from its authentication database or generating a new value in
order to store in in the authentication database). SASL mechanisms
must define which entity (or entities) must perform the preparation.
If preparation fails or results in an empty string, the entity doing
the preparation MUST fail the authentication exchange.
Implementation note: A server end can be represented by multiple
processes. For example, it may consist of the server process itself
that communicated with a client, and a command line utility (a server
agent) that is able to store passwords/hashes in a database that can
be later used by the server. For the server agent the requirement to
"fail the authentication exchange" should be interpreted as a
requirement to refuse to store the data in the database.
3) If the underlying cryptographic technology used by a mechanism
supports data integrity than the mechanism specification MUST
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integrity protect the transmission of an authorization identity and
the negotiation of the security layer.
4) The mechanism should not use the authorization identity in
generation of any long-term cryptographic keys/hashes. The reason is
that different protocols (and sometimes even different
implementations of the same protocol) may use multiple forms of an
authorization identity that are semantically equivalent and some
clients may use one form while other clients use a different form.
5) SASL mechanisms should be designed to minimize the number of round
trips required because SASL can be used with protocols where
connections are short-lived.
6. Specific issues
6.1. 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.)
6.1.1. Client sends data first examples
The following are two examples of an SECURID authentication [SASL-
SECURID] in the SMTP protocol [SMTP]. In the first example below,
the client is trying fast reauthentication by sending the initial
response:
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Welcome client.example.com
S: 250-AUTH GSSAPI SECURID
S: 250 DSN
C: AUTH SECURID AG1hZ251cwAxMjM0NTY3OAA=
S: 235 Authentication successful
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The example below is almost identical to the previous, but here the
client chooses not to use the initial response parameter.
S: 220-smtp.example.com ESMTP Server
C: EHLO client.example.com
S: 250-smtp.example.com Welcome client.example.com
S: 250-AUTH GSSAPI SECURID
S: 250 DSN
C: AUTH SECURID
S: 334
C: AG1hZ251cwAxMjM0NTY3OAA=
S: 235 Authentication successful
Additonal examples that show usage of initial response can be found
in section 7.2.
6.2. Server returns success with additional data
Some mechanisms may specify that additional 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 additional data to be
returned with a success indication, then the server issues the data
as a 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
(with no additional data).
Client implementors should be aware of an additional failure case
that might occur when the profile supports sending the additional
data with success. Imagine that an active attacker is trying to
impersonate the server and sends faked data, which should be used to
authenticate the server to the client, with success. (A similar
situation can happen when either the server and/or the client has a
bug and they calculate different responses.) After checking the data,
the client will think that the authentication exchange has failed,
however the server will think that the authentication exchange has
completed successfully. At this point the client can not abort the
authentication exchange; it SHOULD close the connection instead.
However, if the profile did not support sending of additional data
with success, the client could have aborted the exchange at the very
last step of the authentication exchange.
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6.2.1. Server returns success with additional data examples
The following are two examples of a DIGEST-MD5 authentication [SASL-
DIGEST] in the Extensible Messaging and Presence Protocol [XMPP]. In
the first example below, the server is sending mutual authentication
data with success.
C: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
S: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
version='1.0'>
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>CRAM-MD5</mechanism>
</mechanisms>
</stream:features>
C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</success>
The example below is almost identical to the previous, but here
the server chooses not to use the additional data with success.
C: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
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to='example.com'
version='1.0'>
S: <stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
version='1.0'>
S: <stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>CRAM-MD5</mechanism>
</mechanisms>
</stream:features>
C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9
ImF1dGgiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
S: <challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>
C: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
6.3. 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.
If 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 a second security layer, then the
second security layer replaces the first. If a security layer is in
effect and a subsequent SASL negotiation selects no security layer,
the original security layer remains in effect.
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Where a protocol profile permits multiple successful SASL
negotiations, the profile MUST detail the effect of a failed SASL
negotiation upon the previously established authentication state.
In particular, it MUST state whether the previously established
authenticated state remain in force or whether the connection is to
revert to an non-authenticated state. Regardless of the specified
effect upon authentication state, the previously negotiated security
layer remains in effect.
7. The EXTERNAL mechanism
The mechanism name associated with external authentication is
"EXTERNAL".
The client sends a single message containing the UTF-8 encoding of
the authorization identity. The message may be empty. The form of the
authorization identity is further restricted by the application-level
protocol's SASL profile.
The server uses information, external to SASL, to determine whether
the client is permitted 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 [IPSec] or TLS [TLS]. However, the client can make no
assumptions as to what information the server can use in determining
client authorization. For example, just because TLS was established,
doesn't mean that the server will use the information provided by
TLS.
If the client sends the empty string as the authorization identity,
the authorization identity is to be derived from authentication
credentials which exist in the system that is providing the external
authentication.
7.1. Formal syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in [ABNF]. Non-terminals referenced
but not defined below are as defined by [UTF-8].
The "extern-resp" rule below defines the message sent from client to
server.
extern-resp = *( UTF8-char-no-nul )
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UTF8-char-no-nul = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4
UTF8-1-no-nul = %x01-7F
7.2. Examples of SASL EXTERNAL
The following is an example of an EXTERNAL authentication in the SMTP
protocol [SMTP]. In this example, the client is proxy authenticating,
sending the authorization identity "fred" using in the (optional)
initial response. The server has obtained the client's
(authentication) identity from an external service, such as IPsec,
and has a security policy that permits that identity to assume the
identity of the asserted authorization identity.
To the protocol profile, the four octet sequence "fred" is an opaque
binary data. The SASL protocol profile for SMTP [SMTP-AUTH] specifies
that server challenges and client responses are encoded in BASE64
[BASE64, section 3]; the BASE64 encoding of "fred" is "ZnJlZA==".
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
S: 250-smtp.example.com
S: 250 AUTH DIGEST-MD5 EXTERNAL
C: AUTH EXTERNAL ZnJlZA==
S: 235 Authentication successful.
The following example is almost identical to the one above, but the
client doesn't request proxy authentication.
S: 220 smtp.example.com ESMTP server ready
C: EHLO jgm.example.com
S: 250-smtp.example.com
S: 250 AUTH DIGEST-MD5 EXTERNAL
C: AUTH EXTERNAL
S: 235 Authentication successful.
The following is an example of an EXTERNAL authentication in the
IMAP4 protocol [IMAP]. IMAP4 doesn't support the initial response
feature of SASL. As in the previous example, the client doesn't
request proxy authentication.
S: * OK IMAP4rev1 Server
C: C01 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 AUTH=DIGEST-MD5 AUTH=EXTERNAL
[...]
C: A01 AUTHENTICATE EXTERNAL
(note that there is a space following the "+" in the following line)
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S: +
C:
S: A01 OK Success
8. IANA Considerations
8.1. Guidelines for IANA
It is requested that IANA updates the SASL mechanisms registry as
follows:
Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
registrations to OBSOLETE. Change the "Published specification"
of the EXTERNAL mechanism to this document. Updated registration
information is provided in Section 8.6.
8.2. Registration procedure
Registration of a SASL mechanism is done by filling in the template
in section 8.5 and sending it via electronic mail to <iana@iana.org>.
IANA has the right to reject obviously bogus registrations, but will
perform no review of claims made in the registration form. SASL
mechanism registrations are currently available at the URL
<http://www.iana.org/assignments/sasl-mechanisms>.
There is no naming convention for SASL mechanisms; any name that
conforms to the syntax of a SASL mechanism name can be registered.
However an IETF Standards Track document may reserve a portion of the
SASL mechanism namespace ("family of SASL mechanisms") for its own
use, amending the registration rules for that portion of the
namespace. Each family of SASL mechanisms MUST be identified by a
prefix.
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.
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8.3. Comments on SASL mechanism registrations
Comments on registered SASL mechanisms should first be sent to the
"owner" of the mechanism and/or to the SASL WG mailing list.
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.
8.4. 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 usage" 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.
8.5. Registration template
Subject: Registration of SASL mechanism X
Family of SASL mechanisms: (YES or NO)
SASL mechanism name (or prefix for the family):
Security considerations:
Published specification (optional, recommended):
Person & email address to contact for further information:
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Intended usage:
(One of COMMON, LIMITED USE or OBSOLETE)
Owner/Change controller:
(Any other information that the author deems interesting may be
added below this line.)
8.6. The EXTERNAL mechanism registration
It is requested that the SASL Mechanism registry [IANA-SASL] entry
for the EXTERNAL mechanism be updated to reflect that this document
now provides its technical specification.
Subject: Updated Registration of SASL mechanism EXTERNAL
Family of SASL mechanisms: NO
SASL mechanism name: EXTERNAL
Security considerations: See RFC XXXX, section 9.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for EXTERNAL
9. Security considerations
Security issues are discussed throughout this memo.
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
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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.
In order to detect Man-in-the-middle (MITM) attacks the client MAY
list available SASL mechanisms both before and after the SASL
security layer is negotiated. This allows the client to detect
active attacks that remove mechanisms from the server's list of
supported mechanisms, and allows the client to ensure that it is
using the best mechanism supported by both client and server. New
protocol profiles SHOULD require servers to make the list of SASL
mechanisms available for the initial authentication available to the
client after security layers are established. Some older protocols
do not require this (or don't support listing of SASL mechanisms once
authentication is complete); for these protocols clients MUST NOT
treat an empty list of SASL mechanisms after authentication as a MITM
attack.
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.
When use of a security layer is negotiated by the authentication
protocol exchange, the receiver should handle gracefully any
protected data buffer larger than the defined/negotiated maximal
size. In particular, it must not blindly allocate the amount of
memory specified in the buffer size field, as this might cause the
"out of memory" condition. If the receiver detects a large block, it
SHOULD close the connection.
Distributed server implementations need to be careful in how they
trust other parties. In particular, authentication secrets should
only be disclosed to other parties that are trusted to manage and use
those secrets in manner acceptable to disclosing party. Applications
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using SASL assume that SASL security layers providing data
confidentiality are secure even when an attacker chooses the text to
be protected by the security layer. Similarly applications assume
that the SASL security layer is secure even if the attacker can
manipulate the ciphertext output of the security layer. New SASL
mechanisms MUST meet these assumptions.
"stringprep" and Unicode security considerations apply to
authentication identities, authorization identities and passwords.
The EXTERNAL mechanism provides no security protection; it is
vulnerable to spoofing by either client or server, active attack, and
eavesdropping. It should only be used when external security
mechanisms are present and have sufficient strength.
10. References
10.1. Normative References
[ABNF] Crocker, D. (Ed.), Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997
[ASCII] American National Standards Institute, "Code Extension
Techniques for Use with the 7-bit Coded Character Set of American
National Standard Code (ASCII) for Information Interchange", FIPS PUB
35, 1974
[CHARSET-POLICY] Alvestrand, H., "IETF Policy on Character Sets and
Languages", RFC 2277, BCP 18, January 1998
[GSSAPI] Linn, J., "Generic Security Service Application Program
Interface, Version 2, Update 1", RFC 2743, January 2000
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 19, March 1997
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version 3.0" (Reading,
MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the
"Unicode Standard Annex #27: Unicode 3.1"
(http://www.unicode.org/reports/tr27/) and by the "Unicode Standard
Annex #28: Unicode 3.2" (http://www.unicode.org/reports/tr28/).
[Stringprep] Hoffman, P., Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, December 2002.
[SASLprep] Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords", Work in progress, draft-ietf-sasl-saslprep-XX.txt.
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[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, STD 63, November 2003.
10.2. Informative References
[SASL-GSSAPI] Melnikov, A., "SASL GSSAPI mechanisms", work in
progress, draft-ietf-sasl-gssapi-XX.txt, November 2003
[SASL-DIGEST] Leach, P., Newman, C., Melnikov, A., "Using Digest
Authentication as a SASL Mechanism", work in progress, draft-ietf-
sasl-rfc2831bis-XX.txt, replaces RFC 2831
[SASL-OTP] Newman, C., "The One-Time-Password SASL Mechanism", RFC
2444, October 1998.
[SASL-SECURID] Nystrom, M., "The SecurID(r) SASL Mechanism", RFC
2808, April 2000.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
[SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
Being revised by Siemborski, R., "SMTP Service Extension for
Authentication", work in progress, draft-siemborski-rfc2554bis-
XX.txt.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence Protocol
(XMPP): Core", work in progress, draft-ietf-xmpp-core-XX.txt.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC-INSTRUCTIONS] Postel, J., Reynolds, J., "Instructions to RFC
Authors", RFC 2223, October 1997.
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", http://www.iana.org/assignments/sasl-mechanisms.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[IPSec] Kent, S., and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[Sec-Glossary] Shirey, R., "Internet Security Glossary", RFC 2828,
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May 2000.
11. Editor's Address
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex,
TW12 2BX, United Kingdom
Email: Alexey.Melnikov@isode.com
URI: http://www.melnikov.ca/
12. Acknowledgments
This document is a revision of RFC 2222 written by John G. Myers. He
also contributed significantly to this revision.
Contributions of many members of the SASL mailing list are gratefully
acknowledged, in particular Kurt Zeilenga, Peter Saint-Andre, Rob
Siemborski, Magnus Nystrom, Jeffrey Hutzelman, Hallvard B Furuseth,
Tony Hansen, Simon Josefsson, Abhijit Menon-Sen, RL 'Bob' Morgan, Sam
Hartman, Tim Alsop and Luke Howard for proofreading the document and
various editorial suggestions.
13. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
14. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
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:
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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.
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.
Appendix B. Relationship to other documents
This document obsoletes RFC 2222. It replaces all portions of RFC
2222 excepting sections 7.1 (Kerberos version 4 mechanism), 7.2
(GSSAPI mechanism), 7.3 (S/Key mechanism). The Kerberos version 4
(KERBEROS_IV) and S/Key (SKEY) mechanisms are now viewed as obsolete.
The GSSAPI mechanism is now separately specified [SASL-GSSAPI].
Appendix C. Changes since RFC 2222
The GSSAPI mechanism was removed. It is now specified in a separate
document [SASL-GSSAPI].
The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has
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been removed.
The "SKEY" mechanism described in RFC 2222 is obsolete and has been
removed. It has been replaced by the OTP mechanism [SASL-OTP].
The introduction has been substantially reorganized and clarified.
Clarified the definition and semantics of the authorization identity.
Prohibited the NUL character in authorization identities.
Added a section on character string issues.
The word "must" in the first paragraph of the "Protocol profile
requirements" section was changed to "MUST".
Specified that protocol profiles SHOULD provide a way for clients to
discover available SASL mechanisms.
Made the requirement that protocol profiles specify the semantics of
the authorization identity optional to the protocol profile.
Clarified that such a specification is a refinement of the definition
in the base SASL spec.
Added a requirement discouraging protocol profiles from breaking the
separation between protocol and mechanism.
Mentioned that standards track documents may carve out their own
portions of the SASL mechanism namespace and may amend registration
rules for the portion. However registration of individual SASL
mechanisms is still required.
Specified that the authorization identity in the EXTERNAL mechanism
is encoded in UTF-8.
Added a statement that a protocol profile SHOULD allow challenge data
to be sent with a success indication.
Added a security consideration for the EXTERNAL mechanism.
Clarified sections concerning success with additional data.
Cleaned up IANA considerations/registrations and assembled them in
one place.
Updated references and split them into Informative and Normative.
Added text to the Security Considerations section regarding handling
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of extremely large SASL blocks.
Replaced UTF-8 ABNF with the reference to the UTF-8 document.
Added text about SASLprep for authentication identities and
passwords. Described where SASLprep preparation should take place.
Added paragraph about verifying authorization identities.
Added a protocol profile requirement to specify interaction between
SASL and TLS security layers.
Added a protocol profile requirement to specify if it supports
reauthentication.
Removed the text that seemed to suggest that SASL security layer must
not be used when TLS is available.
Created two subsections in 3.2 to talk separately about proxy
authorization and format of the authorization identities.
Made requirement to verify that an authorization identity is correct
by performing SASLprep a SHOULD, instead of a MUST.
Clarified that each SASL mechanism must decide where SASLprep is
taking place.
Added 4 new examples for initial response and additional data with
success.
Added text on checking the list of available SASL mechanisms after
negotiating a security layer.
Added definition of "integrity protection" and "confidentiality
protection".
Added warning about negotiating no layer once a security layer is
negotiated.
Added new section with guidelines to a SASL mechanism designer.
Added a requirement to specify how an empty initial challenge is
encoded if initial response is supported by a protocol.
Clarified that empty "additional data with success" is not allowed.
Replaced "buffers of security encoded data" with "buffers of
protected data" for clarity.
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Clarified that SASL EXTERNAL can be used even with SASL profiles that
don't support initial data with success.
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Status of this Memo .......................................... i
Abstract ..................................................... 2
1. Conventions used in this document ........................ 2
2. Introduction ........................................... 2
3. Authentication mechanisms .............................. 4
3.1. Authentication protocol exchange ....................... 5
3.2. Authorization and authentication identities ............ 6
3.2.1. Authorization identities and proxy authentication .... 6
3.2.2. Authorization Identity Format ........................ 7
3.3. Security layers ........................................ 7
4. Protocol profile requirements .......................... 8
5. Mechanism profile guidelines .......................... 10
6. Specific issues ....................................... 11
6.1. Client sends data first ............................... 11
6.1.1. Client sends data first examples .................... 11
6.2. Server returns success with additional data ........... 12
6.2.1. Server returns success with additional data examples 13
6.3. Multiple authentications .............................. 14
7. The EXTERNAL mechanism ................................ 15
7.1. Formal syntax ......................................... 15
7.2. Examples of SASL EXTERNAL ............................. 16
8. IANA Considerations ................................... 17
8.1. Guidelines for IANA ................................... 17
8.2. Registration procedure ................................ 17
8.3. Comments on SASL mechanism registrations .............. 18
8.4. Change control ........................................ 18
8.5. Registration template ................................. 18
8.6. The EXTERNAL mechanism registration ................... 19
9. Security considerations ................................ 19
10. References ........................................... 21
10.1. Normative References ................................. 21
10.2. Informative References ............................... 22
11. Editor's Address ...................................... 23
12. Acknowledgments ....................................... 23
13. Full Copyright Statement .............................. 23
14. Intellectual Property ................................. 24
Appendix A. Relation of SASL to transport security .......... 24
Appendix B. Relationship to other documents ................. 25
Appendix C. Changes since RFC 2222 .......................... 25
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