WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
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)  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 . Providing the bare JID in the "from" attribute, rather than introducing additional nonzas, saves one round-trip, see Extensible SASL Profile (XEP-0388) .
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.
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) , 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.
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.
For upgrades of SCRAM mechanisms as defined in RFC 5802 , 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) .
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) .
The name of the upgrade task MUST NOT conain the "-PLUS" suffix, because channel-binding is not relevant for upgrade tasks.
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.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
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".
4. 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/>.
5. 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/>.