XEP-0388: Extensible SASL Profile

1.1 Terminology¶

Although initiating entities, in general, use SASL, and receiving entities offer it, the SASL specification and common parlance both use "Client " and "Server"; this specification uses Client and Server and assumes C2S links. This is not intended to preclude use of this SASL profile on S2S links. The term "SASL2" is used to mean the new SASL profile specified in this document; however the same RFC 4422 definition of SASL (and SASL profiles) applies.

Examples often use hypothetical SASL mechanisms and sub-extensions; this specification does not intend to make a position on any particular SASL mechanism, and the Mandatory To Implement mechanisms are unaffected.

2.1 SASL mechanism negotiation¶

Servers capable of SASL2 offer a stream feature of <authentication/>, qualified by the "urn:xmpp:sasl:2" namespace. This in turn contains one or more <mechanism/> elements in the same namespace, and potentially other elements (for example, the <hostname/> element defined within Use of Domain-Based Service Names in XMPP SASL Negotiation (XEP-0233) [3]). The mechanism elements MUST only offer mechanisms the server can offer for the JID provided by the client in the "from" attribute of the stream-header. If that attribute is omitted or does not contain a recognised bare JID, the server SHOULD only offer mechanisms it can provide independently of the JID. Note that varying responses based on whether an account exists or not may leak the existence of accounts to unauthorized parties. See the security considerations section for more information. If the "from" attribute is not empty and the domain used in this attribute does not match the domain used in the "to" attribute the server SHOULD respond with an invalid-from error as defined in §4.9.3.9 of RFC 6120 [4].

The feature so advertised, and its child content, SHOULD be stable for the given stream to and from attributes and encryption state, and therefore MAY be cached by clients for later connections.

Note that SASL2 is impossible for clients to initiate without at least one mechanism being available, and therefore MUST NOT be offered if not at least one mechanism can be provided.

The Service Name used by XMPP is unchanged from RFC 6120 [4].

All servers and clients supporting channel-binding MUST implement SASL Channel-Binding Type Capability (XEP-0440) [1].

Additional stream features that can be negotiated as part of a successful SASL authentication can be included in the <inline/> element which is an immediate child of <authentication/>.

<?xml version='1.0'?>
<stream:stream
  from='user@example.org'
  to='example.org'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>

<?xml version='1.0'?>
<stream:stream
  from='example.org'
  id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
  to='user@example.org'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>
<stream:features>
  <authentication xmlns='urn:xmpp:sasl:2'>
    <mechanism>SCRAM-SHA-1</mechanism>
    <mechanism>SCRAM-SHA-1-PLUS</mechanism>
    <inline>
      <!-- Server indicates that XEP-0198 stream resumption can be done "inline" -->
      <sm xmlns='urn:xmpp:sm:3'/>
      <!-- Server indicates support for XEP-0386 Bind 2 -->
      <bind xmlns='urn:xmpp:bind2:1'/>
    </inline>
  </authentication>
  <!-- Channel-binding information provided by XEP-0440 -->
  <sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
    <channel-binding type='tls-server-end-point'/>
    <channel-binding type='tls-exporter'/>
  </sasl-channel-binding>
</stream:features>

2.2 SASL Data Encoding¶

In all cases, both Clients and Servers encode SASL exchanges using Base 64 encoding. This SHOULD NOT include any line wrapping or other whitespace. As the form <element/> is equivalent to <element></element>, these both indicate an empty string. Challenges and responses with no data do not occur in SASL, and so require no special handling. To indicate the absence of an initial response, or the absence of success data, the element is simply not included.

2.3 Initiation¶

Clients, upon observing this stream feature, initiate the authentication by the use of the <authenticate/> top-level element within the same namespace. The nature of this element is to inform the server about properties of the final stream state, as well as initiate authentication itself. To achieve this, it has a single mandatory attribute of "mechanism", with a string value of a mechanism name offered by the Server in the stream feature and an optional child element of <initial-response/>, containing a base64-encoded SASL Initial Response. If the "mechanism" attribute contains a string not previously announced by the server in in the stream feature, the server MUST fail the authentication.

If the stream's from attribute (if present) does not match the non-empty authorization string, the server MUST fail the authentication as defined in Failure.

Clients SHOULD also include a <user-agent/> element, informing the server about the connecting client. The 'id' attribute is RECOMMENDED, and if present contains a unique stable identifier for the client installation. The contents of the 'id' attribute MUST be a UUID v4. This allows the server to provide functionality such as deriving stable resource identifiers (see Bind 2.0 (XEP-0386) [5]). The child elements <software/> and <device/> MAY be supplied by clients and contain text descriptions of the client software and the device it is installed on. These allow the server to keep the user informed about what devices are connected to their account. Servers MUST NOT expose this information to other entities (such functionality is available in Software Version (XEP-0092) [6] if required).

<authenticate xmlns='urn:xmpp:sasl:2' mechanism='SCRAM-SHA-1-PLUS'>
  <!-- Base64 of: 'p=tls-exporter,,n=user,r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6' -->
  <initial-response>cD10bHMtZXhwb3J0ZXIsLG49dXNlcixyPTEyQzRDRDVDLUUzOEUtNEE5OC04RjZELTE1QzM4RjUxQ0NDNg==</initial-response>
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <software>AwesomeXMPP</software>
    <device>Kiva's Phone</device>
  </user-agent>
</authenticate>

In order to provide support for other desired stream states beyond authentication, additional child elements are used. For example, a hypothetical XEP-0198 session resumption element might be included, and/or Resource Binding requests.

<authenticate xmlns='urn:xmpp:sasl:2' mechanism='BLURDYBLOOP'>
  <initial-response>
    SSBzaG91bGQgbWFrZSB0aGlzIGEgY29tcGV0aXRpb24=
  </initial-response>
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <software>AwesomeXMPP</software>
    <device>Kiva's Phone</device>
  </user-agent>
  <bind xmlns='urn:xmpp:bind:example'/>
</authenticate>

2.4 During Authentication¶

At any time while authentication is in progress, neither Client nor Server sends any element (including stanzas) or other data except the top-level elements defined herein. Clients MUST NOT send whitespace, and MUST send only <response/> elements as appropriate or an <abort/> element to immediately cause an error. Servers MUST disconnect Clients immediately if any other traffic is received. Servers are similarly REQUIRED to send no whitespace, and only the <response/> and completion elements from the section below.

2.5 Challenges and Responses¶

Server Challenges MAY then be sent. Each Challenge MUST be responded to by a Client in a Client Response. These are not extensible, and contain the corresponding base64 encoded SASL data:

<!--
  SCRAM-SHA-1-PLUS challenge issued by the server as defined in RFC 5802.
  Base64 of: 'r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6a09117a6-ac50-4f2f-93f1-93799c2bddf6,s=QSXCR+Q6sek8bf92,i=4096'
-->
<challenge xmlns='urn:xmpp:sasl:2'>
  cj0xMkM0Q0Q1Qy1FMzhFLTRBOTgtOEY2RC0xNUMzOEY1MUNDQzZhMDkxMTdhNi1hYzUwLTRmMmYtOTNmMS05Mzc5OWMyYmRkZjYscz1RU1hDUitRNnNlazhiZjkyLGk9NDA5Ng==
</challenge>

<!--
  The client responds with the base64 encoded SCRAM-SHA-1-PLUS client-final-message (password: 'pencil').
  Base64 of: 'c=cD10bHMtZXhwb3J0ZXIsLMcoQvOdBDePd4OswlmAWV3dg1a1Wh1tYPTBwVid10VU,r=12C4CD5C-E38E-4A98-8F6D-15C38F51CCC6a09117a6-ac50-4f2f-93f1-93799c2bddf6,p=UApo7xo6Pa9J+Vaejfz/dG7BomU='
  The c-attribute contains the GS2-header and channel-binding data blob (32 bytes) as defined in RFC 5802.
-->
<response xmlns='urn:xmpp:sasl:2'>
  Yz1jRDEwYkhNdFpYaHdiM0owWlhJc0xNY29Rdk9kQkRlUGQ0T3N3bG1BV1YzZGcxYTFXaDF0WVBUQndWaWQxMFZVLHI9MTJDNENENUMtRTM4RS00QTk4LThGNkQtMTVDMzhGNTFDQ0M2YTA5MTE3YTYtYWM1MC00ZjJmLTkzZjEtOTM3OTljMmJkZGY2LHA9VUFwbzd4bzZQYTlKK1ZhZWpmei9kRzdCb21VPQ==
</response>

<!-- A server might send: -->
<challenge xmlns='urn:xmpp:sasl:2'>
  U28sIG5leHQgRk9TREVNIC0gMjAxOCwgdGhhdCBpcy4uLg==
</challenge>

<!-- A client might respond: -->
<response xmlns='urn:xmpp:sasl:2'>
  Li4uSSdsbCBidXkgYSBiZWVyIGZvciB0aGUgZmlyc3QgcGVyc29uIHdoby4uLg==
</response>

2.6 Completing Authentication¶

Authentication may complete in one of three ways. It may complete successfully, in which case the client is authenticated. It may also fail, in which case the client is not authenticated and the stream and session state remain entirely unchanged.

Finally, it may have completed successfully, but further interaction is required - for example, a password change or second-factor authentication.

<success xmlns='urn:xmpp:sasl:2'>
  <!-- Base64 of: 'v=msVHs/BzIOHDqXeVH7EmmDu9id8=' -->
  <additional-data>
    dj1tc1ZIcy9CeklPSERxWGVWSDdFbW1EdTlpZDg9
  </additional-data>
  <authorization-identifier>user@example.org</authorization-identifier>
</success>

<success xmlns='urn:xmpp:sasl:2'>
  <additional-data>
    ip/AeIOfZXKBV+fW2smE0GUB3I//nnrrLCYkt0Vj
  </additional-data>
  <authorization-identifier>juliet@montague.example/Balcony/a987dsh9a87sdh</authorization-identifier>
</success>

<success xmlns='urn:xmpp:sasl:2'>
  <additional-data>
    SGFkIHlvdSBnb2luZywgdGhlcmUsIGRpZG4ndCBJPw==
  </additional-data>
  <authorization-identifier>juliet@montague.example</authorization-identifier>
  <resumed xmlns='urn:xmpp:sm:3' h='345' previd='124'/>
</success>

<failure xmlns='urn:xmpp:sasl:2'>
  <aborted xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
  <optional-application-specific xmlns='urn:something:else'/>
  <text>This is a terrible example.</text>
</failure>

<continue xmlns='urn:xmpp:sasl:2'>
  <additional-data>
    SSdtIGJvcmVkIG5vdy4=
  </additional-data>
  <tasks>
    <task>HOTP-EXAMPLE</task>
    <task>TOTP-EXAMPLE</task>
  </tasks>
  <text>This account requires 2FA</text>
</continue>

<!-- Client starts TOTP-EXAMPLE task -->
<next xmlns='urn:xmpp:sasl:2' task='TOTP-EXAMPLE'>
  <totp xmlns="urn:totp:example">
    SSd2ZSBydW4gb3V0IG9mIGlkZWFzIGhlcmUu
  </totp>
</next>

<!-- Server provides needed data to Client -->
<task-data xmlns='urn:xmpp:sasl:2'>
  <totp xmlns="urn:totp:example">
    94d27acffa2e99a42ba7786162a9e73e7ab17b9d
  </totp>
</task-data>

<!-- Client responds with requested TOP data -->
<task-data xmlns='urn:xmpp:sasl:2'>
  <totp xmlns="urn:totp:example">
    OTRkMjdhY2ZmYTJlOTlhNDJiYTc3ODYxNjJhOWU3M2U3YWIxN2I5ZAo=
  </totp>
</task-data>

<!-- Server indicates successful completion of TOTP-EXAMPLE task -->
<success xmlns='urn:xmpp:sasl:2'>
  <totp xmlns="urn:totp:example">
    SGFkIHlvdSBnb2luZywgdGhlcmUsIGRpZG4ndCBJPw==
  </totp>
  <authorization-identifier>juliet@montague.example</authorization-identifier>
</success>

4.1 Service Name¶

The service name SHALL be "xmpp", as defined by RFC 6120.

4.2 Mechanism negotiation¶

Servers list mechanisms in the <mechanisms/> list in response to a <request/> by the client. (See Initiation).

4.3 Message Definitions¶

4.4 Non-Empty Authorization Strings¶

If a Client specifies an authorization string which is non-empty, the identifier is normalized by treating it as a JID, and performing normalization as described in RFC 7622.

Any non-empty authorization string MUST be considered an error if the stream's from attribute (if present) does not match.

4.5 Aborting¶

Clients MAY abort unilaterally at any time before the authentication completed by sending an <abort/> element. This element MAY contain a <text/> element containing a textual representation of the reason as well as any other element defined in some future protocol.

Servers MAY abort unliterally by sending <failure/> with the <aborted/> error code as defined in Failure.

4.6 Security Layer Effect¶

Security Layers take effect after the SASL mechanism itself (ie, the first negotiation) has completed successfully, after the final octet of the server's <success> or <continue>. See Success and Continue.

4.7 Security Layer Order¶

Option (a) is used - any SASL Security Layer is applied first to data being sent, and TLS applied last.

4.8 Multiple Authentication¶

Although the <continue/> concept does use tasks analogous to multiple SASL sequences, only the first SASL mechanism used is considered an authentication, and only the first can negotiate a security layer.

In particular, once <success/> or <continue/> has been sent by the server, any further <authenticate/> element MUST result in a stream error.

5.1 Syntactic Equivalence¶

Where no additional features that SASL2 makes available are used, the flow of information is identical to the original SASL profile. This example shows the new syntax and draws the reader's attention to the differences.

<!--
  Client sends stream header
-->
<stream:stream
  from='alice@example.org'
  to='example.org'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>

<!--
  Server sends stream features.
  These are identical to SASL1, but within a different namespace.
-->
<stream:features>
  <authentication xmlns='urn:xmpp:sasl:2'>
    <mechanism>PLAIN</mechanism>
    <mechanism>SCRAM-SHA-1</mechanism>
  </authentication>
</stream:features>

<!--
  Client intiates authentication.
  Beyond the element local name and namespace,
  the main distinction is that initial-response data is held within an element,
  so the "=" special case no longer applies.
-->
<authenticate xmlns='urn:xmpp:sasl:2' mechanism='PLAIN'>
  <initial-response>AGFsaWNlQGV4YW1wbGUub3JnCjM0NQ==</initial-response>
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <software>AwesomeXMPP</software>
    <device>Kiva's Phone</device>
  </user-agent>
</authenticate>

<!--
  This completes in one step, so the Server sends a success.
  A SASL2 success always includes the authorization identifier:
-->
<success xmlns='urn:xmpp:sasl:2'>
  <authorization-identifier>alice@example.org</authorization-identifier>
</success>

<!--
  The server immediately sends a new set of stream features at this point.
  There is no stream restart.
-->
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <required/>
  </bind>
</stream:features>

<!--
  The client now continues with resource binding.
-->

Use of SASL2 in this simple scenario saves one round-trip (due to the lack of stream restart).

5.2 Once More, This Time With CRAM¶

In this example, multiple potential round-trips are saved by negotiating resource binding and stream management inline.

<!--
  Client sends stream header
-->
<stream:stream
  from='tim@example.org'
  to='example.org'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>

<!--
  Server sends stream features.
-->
<stream:features>
  <mechanisms xmlns='urn:xmpp:sasl:2'>
    <mechanism>PLAIN</mechanism>
    <mechanism>CRAM-MD5</mechanism>
  </mechanisms>
</stream:features>

<!--
  Client intiates authentication.
  Here, no initial response is needed, so none is sent.
-->
<authenticate xmlns='urn:xmpp:sasl:2' mechanism='CRAM-MD5'>
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <software>AwesomeXMPP</software>
    <device>Kiva's Phone</device>
  </user-agent>
</authenticate>

<!--
  CRAM-MD5 starts with a server challenge:
-->
<challenge xmlns='urn:xmpp:sasl:2'>
  PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
</challenge>

<!--
  And the client responds:
-->
<response xmlns='urn:xmpp:sasl:2>
  dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
</response>

<!--
  This completes, so the Server sends a success.
-->
<success xmlns='urn:xmpp:sasl:2'>
  <authorization-identifier>tim@example.org</authorization-identifier>
</success>

<!--
  The server immediately sends a new set of stream features at this point.
  There is no stream restart.
-->
<stream:features>
  <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
    <required/>
  </bind>
</stream:features>

<!--
  The client now continues with resource binding.
-->

5.3 Advanced Usage¶

This moves into the deeply hypothetical. A binding extension is posited, alongside an unrealistic 2FA mechanism which somehow mutually authenticates because why not.

<!--
  Client sends stream header
-->
<stream:stream
  from='alice@example.org'
  to='example.org'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>

<!--
  Server sends stream features.
  Note that whilst megabind is advertised, the unrealistic 2FA mechanism is not.
-->
<stream:features>
  <authentication xmlns='urn:xmpp:sasl:2'>
    <mechanism>BLURDYBLOOP</mechanism>
    <inline>
      <megabind xmlns='urn:example:megabind'/>
    </inline>
  </authentication>
</stream:features>

<!--
  Client intiates authentication.
-->
<authenticate xmlns='urn:xmpp:sasl:2' mechanism='BLURDYBLOOP'>
  <initial-response>
    SW5pdGlhbCBSZXNwb25zZQ==
  </initial-response>
  <user-agent id='d4565fa7-4d72-4749-b3d3-740edbf87770'>
    <software>AwesomeXMPP</software>
    <device>Kiva's Phone</device>
  </user-agent>
  <megabind xmlns='urn:example:megabind'>
    <resource>this-one-please</resource>
  </megabind>
</authenticate>

<!--
  Maybe BLURDYBLOOP does this:
-->
<challenge xmlns='urn:xmpp:sasl:2'>
  PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
</challenge>

<!--
  And the client responds:
-->
<response xmlns='urn:xmpp:sasl:2>
  dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
</response>

<!--
  This completes, so the Server sends a continue, with the
  success data (probably mutual auth) from BLURDYBLOOP.
  The next task required is the unrealistic-2FA.
  Note that the session is - probably - bound at this point, but
  the authorization-identifier is not passed to the client until
  the full authentication sequence is completed.
-->
<continue xmlns='urn:xmpp:sasl:2'>
  <additional-data>
    QWRkaXRpb25hbCBEYXRh
  </additional-data>
  <tasks>
    <task>UNREALISTIC-2FA</task>
  </tasks>
</continue>

<!--
  The client supports the unrealistic-2FA, so can move onto the next task:
-->
<next xmlns='urn:xmpp:sasl:2' task='UNREALISTIC-2FA'>
  <parameters xmlns='urn:example:unrealistic2fa'>
    VW5yZWFsaXN0aWMgMkZBIElS
  </parameters>
</next>

<!--
  This 2FA process is both unrealistic and also uses multiple round-trips.
-->
<task-data xmlns='urn:xmpp:sasl:2'>
  <question xmlns='urn:example:unrealistic2fa'>
    PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
  </question>
</task>

<!--
  The client responds here.
-->
<task-data xmlns='urn:xmpp:sasl:2'>
  <response xmlns='urn:example:unrealistic2fa'>
    dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
  </respons>
</task>

<!--
  Finally, the server sends a success.
  Here, we have an authzid which is a full jid, since the session is now bound.
  The result element here contains the mutual authentication data for the
  unrealistic 2FA, and doesn't refer back to the BLURDYBLOOP exchange.
-->
<success xmlns='urn:xmpp:sasl:2'>
  <result xmlns='urn:example:unrealistic2fa'>
    VW5yZWFsaXN0aWMgMkZBIG11dHVhbCBhdXRoIGRhdGE=
  </result>
  <authorization-identifier>
    alice@example.org/this-one-please
  </authorization-identifier>
</success>

Although the unrealistic 2FA here uses 2 round-trips (real ones will probably use one), the embedding of resource binding as shown here means that a second RTT is saved by SASL2, and there's no net change. A more realistic example would see RTTs saved, and additional negotiations could be added to further reduce RTTs.

7.1 Protocol Namespaces¶

This specification defines the following XML namespace:

• urn:xmpp:sasl:2

7.2 Stream Features¶

The XMPP Registrar includes "urn:xmpp:sasl:2" in its registry of stream features (see <https://xmpp.org/registrar/stream-features.html>).

<feature>
  <ns>urn:xmpp:sasl:2</ns>
  <name>sasl2</name>
  <element>authentication</element>
  <desc>Indicate support for Extensible SASL Profile</desc>
  <doc>&xep0388;</doc>
</feature>

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1. XEP-0440: SASL Channel-Binding Type Capability <https://xmpp.org/extensions/xep-0440.html>.

2. RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>.

3. XEP-0233: Use of Domain-Based Service Names in XMPP SASL Negotiation <https://xmpp.org/extensions/xep-0233.html>.

4. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

5. XEP-0386: Bind 2.0 <https://xmpp.org/extensions/xep-0386.html>.

6. XEP-0092: Software Version <https://xmpp.org/extensions/xep-0092.html>.

7. RFC 8446: The Transport Layer Security (TLS) Protocol Version 1.3 <http://tools.ietf.org/html/rfc8446>.

8. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.