SASL Upgrade Tasks This specification provides a way to upgrade to newer SASL mechanisms using SASL2 tasks. This XMPP Extension Protocol is copyright © 1999 – 2024 by the XMPP Standards Foundation (XSF). Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation. ## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ## In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages. This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA). 0480 Experimental Standards Track Standards Council XMPP Core RFC 5802 XEP-0388 sut Thilo Molitor thilo+xmpp@eightysoft.de thilo.molitor@juforum.de 0.2.0 2024-12-01 tm Fix SCRAM upgrade description and XML schema. 0.1.0 2023-05-04 XEP Editor: ks Promote to Experimental. 0.0.2 2022-12-30 tm Move SASL upgrade tasks from XEP-0388. 0.0.1 2022-10-19 tm Initial version.

While Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>. provides a modern and extensible way to use SASL in XMPP, it lacks support for SASL mechanism upgrades.

Modern XMPP server deployments typically store only the hash of a user's password, to improve account security. At times, it may be desirable for servers to upgrade to newer or different hash algorithms, e.g. so they can offer different authentication mechanisms for improved security or interoperability. Due to the security properties of one-way cryptographic hash algorithms, it is not possible for the server to extract the original data and simply hash it in a new format. To perform such upgrades, the cooperation of the client is necessary - because it has, or can obtain from the user, the original password to derive a hash from.

This specification fills that gap by providing a pluggable way to perform such SASL mechanism upgrades using Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>. tasks to provide the server with the needed data it does not yet have.

This specification also provides a concrete definition of SCRAM upgrade tasks in Section 3.

Clients capable of SASL mechanism upgrades defined herein MUST send the desired bare JID they want to authenticate for in the "from" attribute of the stream-header unless they don't know it (e.g. when using the GSS-API SASL mechanism etc.) according to section 4.7.1 of RFC 6120 RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.. Providing the bare JID in the "from" attribute, rather than introducing additional nonzas, saves one round-trip, see Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>..

To inform the client which SASL mechanism upgrades it supports, the server adds <upgrade/> elements in the namespace "urn:xmpp:sasl:upgrade:0", each containing the name of one upgrade task, to the SASL2 <authentication/> element inside the stream features.

Upgrade task names SHOULD have a prefix of "UPGR-" (to distinguish them from "normal" SASL mechanisms) followed by the SASL mechanism name to upgrade to, and if multiple mechanisms differ only in their support for channel-binding (e.g. SCRAM's -PLUS variants), implementations MUST use only the names of variants without channel-binding for the task names, because mechanism upgrades are independent of any channel-binding. Finally, upgrade tasks MUST NOT transmit plaintext passwords (or any reversible encoding of them) if the SASL mechanism to upgrade allows this to be avoided.

SCRAM-SHA-1 SCRAM-SHA-1-PLUS UPGR-BLOOP2 UPGR-BLOOP-42 ]]>

The client SHOULD always request one or more upgrade tasks it recognises. To do this, it includes the <upgrade/> element namespaced to "urn:xmpp:sasl:upgrade:0" in its <authenticate/> element listing the upgrade tasks it wants to perform, as specified in the Initiation section of Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>., one <upgrade> element for each task.

Upon successfully authenticating the client (including any secondary authentication steps required for the account), but before the final <success/> would be sent, the server sends a <continue/> element, which MUST contain a single task, matching whatever was selected by the client. If the client selected more than one upgrade task, as sequence of upgrade tasks occur. The client then initiates this upgrade task by providing a corresponding <next/> element providing the task name and optionally including any further child-elements as defined by the specification for this concrete upgrade task.

UPGR-BLOOP2 UPGR-BLOOP-42 cD10bHMtZXhwb3J0ZXIsLG49dXNlcixyPTEyQzRDRDVDLUUzOEUtNEE5OC04RjZELTE1QzM4RjUxQ0NDNg== AwesomeXMPP Kiva's Phone ]]> [...] SSdtIGJvcmVkIG5vdy4= UPGR-BLOOP2 This account requires an upgrade to BLOOP2 as requested by the client ]]>

Upon receiving the <next/> element for the upgrade, the server provides the elements and data needed for the client to calculate the requested data. The concrete elements and exchanges needed for the upgrade are specific to individual tasks. These tasks may be documented in other documents.

 element --> UGxlYXNlIHVwZ3JhZGUgbWUh U28sIG5leHQgRk9TREVNIC0gMjAxOCwgdGhhdCBpcy4uLg== BLOOP2 is cool! Li4uSSdsbCBidXkgYSBiZWVyIGZvciB0aGUgZmlyc3QgcGVyc29uIHdoby4uLg== 2 SSdtIGJvcmVkIG5vdy4= UPGR-BLOOP-42 This account requires an upgrade to BLOOP-42 as requested by the client UGxlYXNlIHVwZ3JhZGUgbWUh U28sIG5leHQgRk9TREVNIC0gMjAxOCwgdGhhdCBpcy4uLg== BLOOP-42 is cool! Li4uSSdsbCBidXkgYSBiZWVyIGZvciB0aGUgZmlyc3QgcGVyc29uIHdoby4uLg== 42 user@example.org ]]>

For upgrades of SCRAM mechanisms as defined in RFC 5802 RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>., 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 base64 encoded salt (omitting the "s=" prefix, see RFC 5802 RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>. for the concrete definition of valid salts) and an attribute named "iterations" containing the iteration count as defined in that RFC (omitting the "i=" prefix). The <salt/> element is contained within a <task-data/> wrapper element as defined in Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>. and MUST NOT be empty.

The client then calculates the RFC 5802 RFC 5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms <http://tools.ietf.org/html/rfc5802>. SaltedPassword and sends back its base64 encoded value inside a <hash/> element namespaced to "urn:xmpp:scram-upgrade:0". The <hash/> element MUST NOT be empty and is contained within a <task-data/> wrapper element as defined in Extensible SASL Profile (XEP-0388) XEP-0388: Extensible SASL Profile <https://xmpp.org/extensions/xep-0388.html>..

The name of the upgrade task MUST NOT conain the "-PLUS" suffix, because channel-binding is not relevant for upgrade tasks.

SCRAM-SHA-1 SCRAM-SHA-1-PLUS UPGR-SCRAM-SHA-256 UPGR-SCRAM-SHA-512 UPGR-SCRAM-SHA-256 cD10bHMtZXhwb3J0ZXIsLG49dXNlcixyPTEyQzRDRDVDLUUzOEUtNEE5OC04RjZELTE1QzM4RjUxQ0NDNg== AwesomeXMPP Kiva's Phone [...] dj1tc1ZIcy9CeklPSERxWGVWSDdFbW1EdTlpZDg9 UPGR-SCRAM-SHA-256 QV9TWENSWFE2c2VrOGJmX1o= BzOnw3Pc5H4bOLlPZ/8JAy6wnTpH05aH21KW2+Xfpaw= user@example.org ]]>

For compatibility purposes, the server SHOULD keep the older authentication data (password hashes etc.) of all configured mechanisms to continue offering the same mechanisms as before.

Clients SHOULD use channel-binding, if available, when requesting an upgrade to make sure no MITM can eavesdrop that hash and subsequently use it for authentication. Note that a client can always choose to not upgrade SASL mechanisms 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) 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/>..

This specification does not need any interaction with the XMPP Registrar 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/>..

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Thanks to Matthew Wild and Dave Cridland for all of their valuable feedback and improvements.