Historically, registering with an XMPP service has been difficult. Each server either used customized out-of-band registration mechanisms such as web forms which were difficult to discover, or they used In-Band Registration (XEP-0077)  which could easily be abused by spammers to register large numbers of accounts and which allowed for only limited extensibility.
To solve these issues this specification provides a new in-band registration protocol that allows servers to present the user with a series of "challenges". This allows for both multi-stage proof-of-posession registration flows and spam prevention mechanisms such as proof-of-work functions.
If a server supports registering for or recovering an account using Extensible IBR, it MUST inform the connecting client when returning stream features during the stream negotiation process. This is done by including a <register/> element, qualified by the 'urn:xmpp:register:0' namespace for account registration, or a <recovery/> element qualified by the same namespace for account recovery. The register and recovery features are always voluntary-to-negotiate. The registration and recovery features MUST NOT be advertised before encryption has been negotiated, eg. using direct-TLS or STARTTLS. They SHOULD be advertised at the same time as the SASL authentication feature, meaning that after registration or recovery is completed SASL authentication can proceed.
For recovery or registration, the server MUST include a list of all challenge types which the client may receive during the course of registering or recovering an account. The purpose of this list is to allow clients to detect if registration requires a challenge type which the client does not support, so servers SHOULD only include each type once; the list is merely informative, and should not be relied upon by clients except to ensure that all mechanisms are supported. This list should comprise <challenge/> elements containing a string that uniquely identifies the type of challenge being issued.
A client selects the registration or recovery feature for negotiation by replying with an empty element of the same name and namespace. For example, to attempt account recovery the client would send a <recovery> element qualified by the 'urn:xmpp:register:0' namespace.
The server then replies with a challenge. Challenges take the form of a <challenge/> element qualified by the 'urn:xmpp:register:0' namespace with a 'type' attribute containing the challenge type and containing a challenge data payload.
Type type of a challenge is a value which identifes what sort of payload a client might expect. This document defines a type of 'jabber:x:data' which MUST always contain a data form (an 'x' element with type 'form') as defined by Data Forms (XEP-0004) . Other types may be defined in the future. For example, a challenge containing a data form might look like the following:
After a challenge is received, the client replies to the challenge by sending a <response/> element qualified by the 'urn:xmpp:register:0' namespace or a cancelation as defined later in this document. If the client sends a response, it MUST also include a payload defined by the specific challenge type. In the case of a jabber:x:data challenge, the payload should be a form submission as defined by Data Forms (XEP-0004)  (an 'x' element of type 'submit'). For instance, to reply to the data form challenge from the previous example a client might send:
If after receiving a challenge a client does not wish to continue registration or recovery, it may send an empty <cancel> element qualified by the 'urn:xmpp:register:0' namespace. This informs the server that registration is complete. This is the same as submitting a data form of type 'cancel' in response to a data form challenge.
If the client submits invalid data, or the server wishes to cancel for some other reason, it may reply with an empty <cancel/> element qualified by the 'urn:xmpp:register:0' namespace. If the client successfully completes the challenge, the server MAY return an empty <success/> element qualified by the 'urn:xmpp:register:0' namespace, at which point it may continue with the stream negotiation process. If the server needs more information, for example, in the previous challenge the user entered an email and now the server wishes to ask for a code that was sent to that email, the server MAY send another challenge.
When providing instructions in a data form the server SHOULD use the language specified in the XML stream's current xml:lang, or the closest language for which the server has a translation (eg. based on mutual intelligibility between scripts and languages).
For more information about language tags and matching, see BCP 47 
Servers that allow in-band registration need to take measures to prevent abuse. Common techniques to prevent spam registrations include displaying CAPTCHAs or requiring proof-of-posession of a valid email address or telephone number by sending a unique code (e.g. an HMAC that can later be verified as having originated at the server) to the users email and requiring that they enter the code before continuing. Servers that do not take such measures risk being black listed by other servers in the network.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespace to the registry located at <https://xmpp.org/registrar/stream-features.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
The XMPP Registrar shall maintain a registry of IBR challenge types. Challenge types defined within the XEP series MUST be registered with the XMPP Registrar.
In order to submit new values to this registry, the registrant shall define an XML fragment of the following form and either include it in the relevant XMPP Extension Protocol or send it to the email address <firstname.lastname@example.org>:
For an example registration, see the next section.
This specification defines the following IBR challenge types:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the following definition to the IBR challenge types registry, as described in this document:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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This XMPP Extension Protocol is copyright © 1999 – 2018 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.
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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).
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 primary venue for discussion of XMPP Extension Protocols is the <email@example.com> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <firstname.lastname@example.org>.
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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/