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ProtoXEP: pre-auth key generation
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Sam Whited
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inbox/preauth-ibr.xml
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inbox/preauth-ibr.xml
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<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Pre-auth Registration Key Generation and Validation</title>
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<abstract>
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This specification updates XEP-0401 and XEP-0445 by specifying a shared
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format for the pre-authenticated registration token.
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</abstract>
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&LEGALNOTICE;
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<number>xxxx</number>
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<status>ProtoXEP</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0401</spec>
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<spec>XEP-0445</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>preauth-token</shortname>
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&sam;
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<revision>
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<version>0.0.1</version>
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<date>2021-06-06</date>
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<initials>ssw</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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Both &xep0401; and &xep0445; specify a mechanism for requesting a token from
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a server that can be exchanged for registration at a later date.
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However, neither XEP defines the format of this token, or a recommended
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algorithm for generating it.
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This means that each server may choose a unique format and that any token
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issuing entity must either only support a specific servers token format, or
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must connect to the server with an admin account so that it can ask the
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server for tokens.
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</p>
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<p>
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This specification rectifies this by specifying a server-agnostic format for
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pre-auth tokens and an algorithm for generating them.
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This enables third-party trusted services that share a private key to
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sign a token that can later be verified by the server to register a user.
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</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<ul>
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<li>
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Tokens issued by a third-party authorization server MUST be verifiable by
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any authentication server supporting this specification.
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</li>
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<li>
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Generated tokens MUST be URL safe.
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</li>
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<li>
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<p>
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The algorithm for generating tokens SHOULD allow verification of the
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following inputs:
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</p>
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<ul>
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<li>an optional username,</li>
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<li>a list of hostnames for which the user can register,</li>
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<li>and an optional expiration time.</li>
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</ul>
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</li>
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</ul>
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</section1>
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<section1 topic='Glossary' anchor='glossary'>
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<dl>
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<di>
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<dt>Authentication server</dt>
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<dd>
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The server authenticating the user using IBR or SASL, normally the XMPP
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server.
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</dd>
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</di>
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<di>
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<dt>Authorization server</dt>
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<dd>
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A server issuing a token authorizing the user to register.
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This may be the XMPP server, or another entity that shares its private
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key.
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</dd>
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</di>
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<di>
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<dt>IBR</dt>
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<dd>
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In-band Registration, as defined by either &xep0077; or &xep0389;
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</dd>
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</di>
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<di>
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<dt>Key</dt>
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<dd>
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A shared secret that is used to sign and validate tokens.
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</dd>
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</di>
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</dl>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<ul>
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<li>
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As the admin of a social media network I want to allow my users to
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self-provision accounts on our instant messaging platform without
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involving the XMPP admin or requiring that the provisioning service speak
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with the XMPP server.
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</li>
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<li>
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As the admin of an XMPP server I want to generate tokens that can be
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redeemed for an account at a later date even though the XMPP server is
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temporarily down for maintenance.
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</li>
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<li>
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As the representative of a chat network attending a conference, I want to
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be able to print invitations to sign up for an account at our server even
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though the conference WiFi is spotty and I may not be able to remain
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connected to the server.
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Furthermore, I want to use a different private key than the one used by
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user invites so that I can see how many users from the conference
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registered.
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</li>
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</ul>
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</section1>
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<section1 topic='Business Rules' anchor='rules'>
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<p>
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The following algorithm is used to generate tokens where "," is a separator
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and not part of any actual input and <tt>$key</tt> is the shared secret key.
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All uses of base64 are the Raw URL encoding (with no padding characters)
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defined in &rfc4648;
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HMAC-SHA256 is a Keyed-Hash Message Authentication Code (see &nistfips198a;)
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using the SHA256 hashing algorithm (see &nistfips180-2;).
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</p>
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<code>
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// Current time rounded up and converted to milliseconds.
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expiration = ($currentTime + 1e6 -1) / 1e6
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jids = {
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JID1, ':',
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JID2, ':',
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JID3, ':',
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…
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JIDFinal
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}
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signature = HMAC-SHA256
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(
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$key,
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$jids, ':',
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$expiration
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)
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token = {
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base64-raw-url($signature), ':',
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base64-raw-url($jids), ':',
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$expiration,
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}
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</code>
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</section1>
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<section1 topic='Implementation Notes' anchor='impl'>
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<p>
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If the shared key is longer than the block size it will be hashed by some
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HMAC implementations, otherwise it is left unhashed.
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This is not represented in the algorithm above, but if it is not done the
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user must determine whether the input key needs hashing themselves.
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Input keys shorter than the block size are not hashed.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>
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The secret key SHOULD be at least half the length of the SHA256 output (ie.
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16 bytes).
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No key stretching is performed by this algorithm, so the user should take
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care to pick a long key.
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</p>
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<p>
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Creation of the secret key MUST NOT require communication between the
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authentication server, the authorization server, or a third party such as a
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database.
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</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This document requires no action from the ®ISTRAR;</p>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<p>This document does not define an XML namespace requiring a schema.</p>
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</section1>
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</xep>
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