An XMPP client typically needs a user name and a password to log into an account. Many clients provide a mechanism to store these credentials to automatically log in into an account. While this practice is very user friendly, it is a security risk for some devices and services. Mobile devices like a phone or a laptop may get stolen, providing the thief with the required password. Phones are particularly insecure: providing the password on the keypad for each log in is too complicated and the risk of losing the phone is high. A bot which needs access to a user's account needs to store the password or request the user to enter it every time it is started.
A solution to this problem is to allow a client to log in without knowing the password. XMPP as specified in RFC 3920  and updated in RFC 6120  allows the use of any SASL mechanism (see RFC 4422 ) in the authentication of XMPP entities, including the SASL EXTERNAL mechanism. Best Practices for Use of SASL EXTERNAL (XEP-0178)  defines the usage of X.509 certificates used in the TLS handshake.
XEP-0178 assumes that the certificates used for SASL EXTERNAL are signed by a trusted CA. This could be a problem for the average user: signing a certificate can be both complicated and expensive (in terms of time and money). If the device gets stolen, the user also needs to provide the required information to the CA to revoke the certificate, and the server needs to keep its list of revoked certificates up-to-date. The end-to-end security mechanism described in C2C Authentication Using TLS (XEP-0250)  relies on self-signed certificates (although CA-issued certificates are allowed). A client capable of secure end-to-end communicate already has a self-signed X.509 certificate for that purpose. The same client certificate should be used for a client to log in. Since the certificates are not signed by a trusted CA, the server must be aware of the list of certificates that are used by the user's clients. This document describes how to manage the list of client certificates.
To manage their certificates, this protocol describes a way for clients to store, query and remove certificates on their server. These certificates can be generated by the client itself, for example to replace a password-based login, or the certificates can be imported by the user.
When a user wants to add a certficate, the client uploads it to the server. It does this by sending an <iq/> with an <append/> payload qualified by the 'urn:xmpp:saslcert:1' namespace. The <append/> element MUST include an <x509cert/> element containing the Base64 encoded DER data of the certificate. The client also MUST provide a unique name for the certificate as a <name/> element to make it possible for the user to manage their different client certificates.
The server either returns an empty result or an error. The server MUST return an error when a client tries to add a certificate using a name that is already used by a certificate. From then on the user can use that certificate to authenticate using SASL EXTERNAL.
The client adding the certificate does not need to be the client using it. A user may use a client to upload a certificate for a bot. The bot creates its certificate and private key, and the user uploads the certificate to the server. After that procedure the bot can log in to the XMPP network without ever knowing the user's password.
An optional element <no-cert-management/> inside the append element indicates that a client logged in with that certificate is not allowed to add or remove certificates from the list. A server MAY allow such a client to query the list of certificates.
A user may want to get a list of all certificates that can be used for SASL EXTERNAL. The client can query the list of the items by sending an <iq/> stanza with an <items/> payload qualified by the 'urn:xmpp:saslcert:1' namespace.
The server then returns the list of all known certificates including the user provided name. For every certificate, the server MAY include a <users/> element with a <resource/> for every resource that is currently logged in and used that certificate with SASL EXTERNAL.
To make it easier to transition to a new certificate when the current one expires, without requiring the user to enter their password again, a client can upload a new certificate to replace its current certificate. After the new certificate is added to the server, it MAY want to disable the old certificate to keep the list of certificates short. Without that, the list will grow indefinitely, making the certificate handling for the user more difficult. Once a certificate is removed it can no longer be used for SASL EXTERNAL. A server MAY automatically remove expired certificates for this list.
A client can disable a certificate by sending an <iq/> stanza containing a <disable/> element qualified by the 'urn:xmpp:saslcert:1' namespace and containing a <name/> element with the user-provided name for the certificate.
The user may want to revoke a certificate from a stolen or compromised device. The mechanism is similar to disabling a certificate. The difference is that if a client is logged in with that compromised certificate using SASL EXTERNAL, the server SHOULD close the stream to that client forcing a log out of the client.
A client can revoke a certificate by sending an <iq/> stanza containing a <revoke/> element qualified by the 'urn:xmpp:saslcert:1' namespace and containing a <name/> element with the user provided name for the certificate.
When using SASL EXTERNAL, the certificate does not need to be signed by a trusted entity if the certificate was uploaded by a user. The server still MUST reject the certificate if it is expired. In a company environment the server MAY only accept signed certificates; the behavior depends on the company's security policy. A free public XMPP server MUST allow self-signed certificates and certificates signed by a CA unknown to the server.
The client certificate SHOULD include a JID as defined in sections 18.104.22.168. and 22.214.171.124. in RFC 6120: a JID MUST be represented as an XmppAddr, i.e., as a UTF8String within an otherName entity inside the subjectAltName.
The protocol flow is almost the same as described in XEP-0178, except for the resource binding in step 12. If the client used a certificate with a single XmppAddr field and that field contained a full JID, or if the certificate had multiple XmppAddr fields and the address that was used as authorization identity was a full JID, then the server should only allow the client to use the resource of that JID. If a client with the same resource is currently logged in it SHOULD be logged out by the server.
If a server supports storage of client side certificates, it MUST advertise that fact by including the feature "urn:xmpp:saslcert:1" in response to a Service Discovery (XEP-0030)  request:
This specification allows the user to manipulate an alternative way to log into the server. The certificates are not required to be signed and any certificate can be used. Therefore the server MUST reject any communication described in this document if the link between client and server is not secured with both STARTTLS and SASL to prevent a man-in-the-middle from modifying the certificate.
In-Band Registration (XEP-0077)  defines a mechanism to change the password without knowing the current one. If the server supports password change it MUST return not-authorized for clients logged in using SASL EXTERNAL and MAY include a password change form requiring the old password.
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/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
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.
The XML schema will be provided in a later version of this document.
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Initial published version.
Add Security Considerations, improve reference to XEP-0178.