<title>Client Certificate Management for SASL EXTERNAL</title>
<abstract>This specification defines a method to manage client certificates that can be used with SASL External to allow clients to log in without a password.</abstract>
<p>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. Mobile devices like a phone or a laptop may get stolen, providing the thief with the required password. Mobile 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.</p>
<p>A solution to this problem is to allow a client to log in without knowing the password. XMPP as specified in &rfc3920; and provisionally clarified in &rfc3920bis; allows the use of any SASL mechanism (see &rfc4422;) in the authentication of XMPP entities, including the SASL EXTERNAL mechanism. &xep0178; defines the usage of X.509 certificates used in the TLS handshake.</p>
<p>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 is both an expensive and a complicated procedure. 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 &xep0250; 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 users' clients. This document describes how to manage the list of client certificates.</p>
<p>From the user's point of view the initial client configuration is the same. The user needs to provide the user name and the password for the client to connect to the XMPP server. The main difference is that the client MUST NOT store the password for later use and MUST create an X.509 certificate for future logins. It does not matter if the certificate is self-signed or issued by a trusted third party.</p>
<p>After the client has logged in and created the certificate, it uploads the certificate to the server. The XML representation of the certificate from &xep0189; is used to describe a certificate. The client also SHOULD provide a name for the certificate to make it possible for the user to manage the different client certificates.</p>
<examplecaption='Client Uploads an X.509 Certificate'><![CDATA[
<p>The server either returns an empty result or an error. From now on the client can use that certificate to authenticate the user using SASL EXTERNAL.</p>
<p>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 (and may also sign the certificate outside the X.509 infrastructure to be used in &xep0189;). After that procedure the bot can log in to the XMPP network and even participate in secure end-to-end communication without ever knowing the user's password.</p>
<p>An optional element <no-cert-management/> inside the keyinfo 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.</p>
<section2topic='Request a list of all Certificates'>
<p>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 items iq stanza.</p>
<examplecaption='Client requests List of X.509 Certificates'><![CDATA[
<p>A client has to create a new certification before its current one expires. 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.</p>
<p>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.</p>
<p>The protocol flow is similar to the one described in XEP-0178. Only step 9 is different: the certificate does not need to be signed by a trusted entity if the certificate was uploaded by the 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.</p>
<p>The client certificate SHOULD include a JID as defined in sections 15.2.1.2. and 15.2.1.3. in rfc3920bis: a JID MUST be represented as an XmppAddr, i.e., as a UTF8String within an otherName entity inside the subjectAltName.</p>
<p>If the subjectAltName contains a full JID the server MUST force the client to use the given resource during resource binding. The client is only allowed to use the provided resource name. If a client with the same resource name is currently logged in and that client is not forced to use that resource name, it SHOULD be logged out by the server.</p>
<p>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 to modify the certificate.</p>
<p>&xep0077; 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.</p>
<examplecaption='Password Change'><![CDATA[
<iqtype='set'to='shakespeare.lit'id='change1'>
<queryxmlns='jabber:iq:register'>
<username>hamlet</username>
<password>newpass</password>
</query>
</iq>
]]></example>
<p>If the sending client has logged in using SASL EXTERNAL and the server supports XEP-0077 password changes it returns a form to change the password. If the client has logged in with the current password, the server MAY change the password without a form as specified in XEP-0077.</p>
<examplecaption='Server Returns Password Change Form With Error'><