Extensible SASL Profile (XEP-0388) [1] section 3 defines a way to upgrade to newer SASL mechanisms by providing the server with the needed data for that mechanism using upgrade tasks.
This specification uses those Extensible SASL Profile (XEP-0388) [1] upgrade tasks to upgrade to newer SCRAM mechanisms using hash methods, which the server does not yet have the required data for.
This protocol was designed to allow the client to upgrade to newer SCRAM mechanisms.
For upgrades of SCRAM mechanisms as defined in RFC 5802 [2], the server has to provide the needed data for the client to calculate the SaltedPassword as defined in this RFC (or some RFC updating it), namely the iteration count and salt. To do so the server sends a <salt/> element namespaced to "urn:xmpp:scram-upgrade:0" containing the salt and an attribute named "iteration" containing the iteration count as defined in that RFC, omitting the "s=" and "i=" prefix. The <salt/> element is contained within a <task-data/> wrapper element as defined in Extensible SASL Profile (XEP-0388) [1].
The client then calculates the SaltedPassword and sends back its base64 encoded value inside a <hash/> element namespaced to "urn:xmpp:scram-upgrade:0". The <hash/> element is contained within a <task-data/> wrapper element as defined in Extensible SASL Profile (XEP-0388) [1].
The name of the upgrade task MUST NOT conain the "-PLUS" suffix, because channel-binding is not relevant for upgrade tasks.
<!-- The Server provides the SCRAM upgrade task requested by the client in its <authenticate/> element -->
<continue xmlns='urn:xmpp:sasl:2'>
<!-- Base64 of: 'v=msVHs/BzIOHDqXeVH7EmmDu9id8=' -->
<additional-data>
dj1tc1ZIcy9CeklPSERxWGVWSDdFbW1EdTlpZDg9
</additional-data>
<tasks>
<task>UPGR-SCRAM-SHA-256</task>
</tasks>
</continue>
<!-- The Client initiates the task -->
<next xmlns='urn:xmpp:sasl:2' task='UPGR-SCRAM-SHA-256'/>
<!--
The server sends the required salt and iteration count.
-->
<task-data xmlns='urn:xmpp:sasl:2'>
<salt xmlns='urn:xmpp:scram-upgrade:0' iterations='4096'>
A_SXCRXQ6sek8bf_Z
</salt>
</task-data>
<!-- The client responds with the base64 encoded SaltedPassword. -->
<task-data xmlns='urn:xmpp:sasl:2'>
<hash xmlns='urn:xmpp:sasl:2'>
BzOnw3Pc5H4bOLlPZ/8JAy6wnTpH05aH21KW2+Xfpaw=
</hash>
</task-data>
<!--
Finally, the server sends a success after adding the salted SHA-256 hash to its database.
A SASL2 success always includes the authorization identifier.
-->
<success xmlns='urn:xmpp:sasl:2'>
<authorization-identifier>user@example.org</authorization-identifier>
</success>
Sending the password hash to the server (after authentication) is only protected by TLS and possibly channel-binding. Clients SHOULD use channel-binding, if available, to make sure no MITM can eavesdrop that hash and subsequently use it for authentication.
Note that a client can always choose to not take server-offered upgrades to newer SCRAM hash algorithms if it can not use channel-binding or the connection is otherwise deemed not secure enough.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [3].
This specification does not need any interaction with the XMPP Registrar [4].
<?xml version='1.0' encoding='utf-8'?>
<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace="urn:xmpp:scram-upgrade:0"
xmlns="urn:xmpp:scram-upgrade:0"
elementFormDefault="qualified">
<xs:element name="parameters">
<xs:complexType>
<xs:sequence>
<xs:element name="salt" type="base64Data" minOccurs="1" maxOccurs="1"/>
<xs:element name="iterations" type="iterationCount" minOccurs="1" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="hash" type="base64Data"/>
<xs:simpleType name="base64Data">
<xs:restriction base="xs:base64Binary"/>
</xs:simpleType>
<xs:simpleType name="iterationCount">
<xs:restriction base="xs:integer">
<xs:minInclusive value="1"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>.
2. RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>.
3. 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/>.
4. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
@report{molitor2022sut,
title = {SCRAM Upgrade Tasks},
author = {Molitor, Thilo},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2022-10-19/2022-10-19},
}END