E2E Authentication in XMPP (XEP-0416)  specifies certificate requirements for end-to-end authentication. This document describes how such certificates can be obtained directly by an XMPP client from a trusted certificate authority (CA) using the XMPP protocol. This assumes that the CA runs an XMPP server. The CA functionality can be built into the user's server, but this is not a requirement: a client can obtain a certificate from any trusted CA server. In the latter case the user's server should support s2s connectivity with CA servers and, in addition, it may want to trust them if it wishes to accept c2s SASL EXTERNAL authentication (Best Practices for Use of SASL EXTERNAL (XEP-0178) ) for users of those certificates as long as the certificates are issued for the users of this server. In order to improve user experience (UX), an account registration and certificate issuance can be combined into a single step if the account's server supports this specification.
The following rules apply to CA servers:
The following rules apply to CA certificates:
The following rules apply to a certificate signing request:
See also Glossary section of E2E Authentication in XMPP (XEP-0416) : terminology from there is heavily used in this document.
An X.509 certificate and a chain of X.509 certificates are represented by <x509-cert/> and <x509-cert-chain/> elements respectively, qualified by 'urn:xmpp:x509:0' namespace. These elements can be included into other XMPP elements such as messages, subscription requests and so on.
The <x509-cert-chain/> element MUST contain one or many <x509-cert/> elements. Those elements MUST be ordered: each certificate in the chain is signed by the entity identified by the next certificate in the chain. A root certificate MAY be included in the chain (as the last element) and an entity performing certification path validation (RFC 5280 ) MUST be prepared for this: treating a trusted root certificate in the chain as invalid (because it is self-signed) is a common implementation mistake. However, for the sake of optimization and to avoid trivial bugs, including of a root certificate in the chain is NOT RECOMMENDED.
An <x509-cert-chain/> element MAY possess 'name' attribute. The attribute contains a human readable text that uniquely represents the chain for a user, e.g. a device this certificate chain is assigned to.
A certificate chain may be obtained and/or stored as a so called "PEM file" (formalized by RFC 7468 ). In this case the content of this file is trivially mapped to the <x509-cert-chain/> element and vice versa. See also Storage Format.
A certificate signing request (RFC 2986 ) is represented as an <x509-csr/> element qualified by 'urn:xmpp:x509:0' namespace. Character data of the element MUST be a Base64 DER encoded ASN.1 CertificateRequest structure (RFC 2986 ). An <x509-csr/> element MUST NOT contain any child elements. An <x509-csr/> element MAY possess a 'name' attribute: it contains a human readable text that is linked to the 'name' attribute of the <x509-cert-chain/> element being issued, e.g. a device the requested certificate chain will be assigned to. This name also MAY be stored by the CA server as a part of a user profile, e.g. to futher include it in the user's certificates listing.
Given arbitrary data and an X.509 certificate with its private key, a signature of the data is created by computing a signature function from signatureAlgorithm structure of the certificate upon the data and the private key. The result is represented as <x509-signature/> element qualified by 'urn:xmpp:x509:0' namespace. Character data of the element MUST be the Base64 (RFC 4648 ) encoded signature. The element MUST NOT contain any child elements.
Both an XMPP server and a client are supposed to maintain a list of trusted CA certificates. This list MAY be preconfigured or dynamically obtained from a trusted source. In principle, a client MAY choose any CA server extracted from its own list of CA certificates to send a certificate signing request to. However, if a client also wishes to use the certificate for SASL EXTERNAL authentication with its server, it needs to pick a CA server from a mutually trusted CA certificate. For doing this, it MAY retrieve a list of CA certificates from the server and choose a CA server from a mix of the server's list and its own list. The following subsections address the latter use case. If a client has an already registered account and wishes to obtain a certificate for the use in e2e authentication only it MUST directly follow the protocol described in Certificate Issuance section.
An XMPP server willing to disclose its own list of trusted CA certificates to already registered accounts MUST advertise 'urn:xmpp:x509:0' feature. In addition, if it accepts certificates issued by CAs from its list in c2s SASL EXTERNAL authentication, it MUST append an <identity/> element of category 'auth' and type 'cert'. Note that advertising either the feature or the identity alone provides very little knowledge (if any) to a client, so servers are RECOMMENDED to advertise either both of them or none.
An XMPP server that supports certificate issuance during account registration MUST report that by offering the SASL EXTERNAL mechanism and by including <x509-register/> element qualified by 'urn:xmpp:x509:0' namespace in <stream:features/> element. A server MUST NOT include the feature alone and a client MUST ignore the feature if the SASL EXTERNAL mechanism is not offered. Note that the SASL EXTERNAL mechanism is only offered for TLS encrypted streams.
Once the server support is determined, a list of CA certificates MAY be retrieved from the server by sending an IQ request containing an empty <x509-ca-list/> element qualified by 'urn:xmpp:x509:0' namespace:
The server responds with an unordered list of <x509-cert/> elements included in an <x509-ca-list/> element:
Note that the important difference, except semantics, between <x509-cert-chain/> and <x509-ca-list/> elements is ordering of their <x509-cert/> elements.
The <x509-ca-list/> element MUST NOT be empty. Upon receiption of an empty <x509-ca-list/> element, a client SHOULD treat it as a server bug or misconfiguration and SHOULD proceed as if the server didn't support c2s SASL EXTERNAL authentication at all.
The server MUST allow unauthenticated clients to retrieve the list if it has reported X.509 IBR support (see Stream Features).
Once a remote list of CA certificates is retrieved from the server, a client MAY merge it with its own local list and then choose an appropriate CA certificate from this mix. A client is free to use any merging algorithm. The simpliest way to do this is to take an intersection of the remote and local lists. If the result is an empty list, a client MAY apply more sofisticated algorithms, such as checking if there are intermediate CA certificates in the remote list whose are signed by some CA from the local list. In any case, prior to merging, a client MUST filter out certificates from both lists which don't contain an XmppAddr identifier (see CA Certificate Requirements for the explanation). When merging is completed, a client proceeds as follows:
The certificate issuance protocol described in this section is designed to work in the presence of network, server and client failures. This in particular means that the use of Stream Management (XEP-0198)  is not assumed, because it's unavailable at legacy servers and during in-band registration. The certificate request is performed as a transaction consisting of an IQ request followed by an optional challenge message and then an IQ response. A transaction diagram is shown below:
To request a certificate, a client generates an ASN.1 CertificateRequest structure following the rules from CSR Requirements section. Note that a client encodes its XMPP address (or the address it wishes to register) as an XmppAddr inside extensionRequest attribute of the structure. The generated structure MUST be retained until successful completion of a transaction. If errors, disconnections or crashes are detected, the same structure MUST be reused for every new transaction (even if another CA server is picked for a retry). The above requirement protects a client from issuing unnecessary certificates (whose number may be limited by certificate authorities).
Upon receiption of a certificate request, a CA server typically generates a challenge. The challenge has two purposes:
When a certificate is issued, a CA server responds with a full chain containing the certificate.
Once a CA certificate is selected, a target XMPP server address is extracted from an XmppAddr identifier of this certificate. The generated ASN.1 CertificateRequest structure is then used to form an <x509-csr/> element as specified under section CSR Element. The element is then included into <x509-request/> element qualified by 'urn:xmpp:x509:0' namespace. The <x509-request/> element MUST possess a 'transaction' attribute containing a random value identifying this CSR transaction: the value MUST be cryptographically strong with at least 128 bits of entropy. Finally, <x509-request/> element is wrapped into IQ request for transmission. The 'to' attribute of the IQ stanza MUST be set to the target CA server address.
If a client already has a certificate issued by this CA server for the client's XMPP address, it MAY include it along with a signature into <x509-request/> element. This certificate is supposed to authenticate a client at the CA server and thus to bypass a challenge procedure. However, the CA server MAY still decide to challenge a client, and a client MUST be prepared for this. The certificate is represented by a single <x509-cert/> element with a single <x509-signature/> element carrying a signature computed upon the ASN.1 DER encoded tbsCertificate structure of the certificate as described in Signature Element:
Upon receiption of a certificate request the CA server MUST check that the bare XMPP address in 'from' attribute matches the value of XmppAddr of the CertificateRequest structure. If the request contains a certificate, the CA server MUST verify its signature and MUST check that XmppAddr from the CertificateRequest structure matches the one from the tbsCertificate structure.
The CA server then decides to either issue a certificate, challenge a client or generate an error. If it has an already issued certificate for this CSR, it MUST respond with the certificate without challenging a client. If it has received another request with the same CSR during a challenge procedure, it MUST abort the running procedure, destroy an internal transaction state and process the request within a new transaction.
A certificate request MAY be challenged by the CA server. The CA server MUST challenge the request if it is not authenticated by an attached certificate and the CA server has no additional knowledge on whether the request has arrived from an authenticated client session or not. It MAY challenge the request otherwise, for example, if it has detected some errors (e.g. too many issued certificates) and wants the human user to perform some actions in order to resolve the problem.
To challenge the request, the CA server responds with an <x509-challenge/> element. The element MUST possess 'uri' attribute containing an URI. It also MUST possess a 'transaction' attribute with the value copied from a 'transaction' attribute of the <x509-request/> element. The <x509-challenge/> element MUST contain exactly one <x509-signature/> element carrying a signature computed upon HMAC-SHA256 hash of the URI with the value of 'transaction' attribute being a key, using the CA certificate (see Signature Element). The challenge element is then included into message stanza for transmission. The value of 'to' attribute of the message MUST be copied from the value of 'from' attribute of the IQ request.
In this version of the protocol the URI MUST be an HTTPS URL. A client is supposed to open this URL in a web browser for a user to process the challenge. The content of the URL is opaque to a human user and thus SHOULD NOT be rendered in a client's user interface.
In the above example the signature is computed upon HMAC-SHA256('0b421ff9e2b15fa582691afba57e8b72', 'https://ca.shakespeare.lit/csr/cOemft/8EQTH8') where the first argument is a key and the second argument is a value.
Upon receiption of a challenge a client MUST follow these rules:
If all the checks have passed, a client spawns an URI handler and waits for the certificate response. If either of the checks has failed, a client MUST ignore the message if it's performing X.509 IBR and MAY reply with an appropriate error otherwise.
When the CA server successfully issued a certificate it MUST respond with an IQ result containing the full certificate chain represented as an <x509-cert-chain/> containing the issued certificate represented as an <x509-cert/> element. Note that according to the defined ordering, this certificate MUST always be the first element in the chain. The server MUST NOT respond with an empty <x509-cert-chain/> element. If the original <x509-csr/> element has possessed a 'name' attribute, its value MUST be copied to 'name' attribute of <x509-cert-chain/> element.
Upon receiption of a response matching the request a client proceeds as follows:
If all the checks succeed, the transaction is considered to be completed. At this point a client MAY release the ASN.1 CertificateRequest structure.
If either of the checks fails, a client MUST behave as if it received an error response with a permanent condition (see Certificate Request Error section).
If the CA server refuses to issue a certificate it MUST generate a corresponding stanza error. If the error is generated due to challenge failure, <error/> element MUST contain <x509-challenge-failed/> element qualified by 'urn:xmpp:x509:0' namespace.
When a client receives an error response, it considers the transaction as failed and MUST destroy internal IQ and transaction states.
In the case of a temporary failure, a client MAY repeat the request to the same CA server. In the case of a permanent failure, a client MUST choose another CA server if it has decided to retry. In both cases, the attributes and character data of <x509-csr/> element of the new request MUST be the same. However, a client MUST generate new values for 'transaction' attribute of <x509-request/> element and for 'id' attribute of the IQ stanza.
A client MUST NOT process an URI from <gone/> error condition and MUST treat this condition as a permanent failure. A <redirect/> error condition has a special meaning and is described in the section below.
A CA server may detect that another CA has previously issued a certificate for this XMPP address (refer to E2E Authentication in XMPP (XEP-0416)  for details). In this case the CA server MUST redirect a client to the appropriate CA server. It does so by responding with an error with <redirect/> condition (see Section 184.108.40.206 of RFC 6120 ). An URI from <redirect/> element MUST be an XMPP URI (RFC 5122 ) containing a bare XMPP address of the CA server that a client is being redirected to. A <redirect/> element MUST also contain a single <x509-signature/> element carrying a signature computed upon HMAC-SHA256 hash of the URI with the value of 'transaction' attribute from <x509-request/> element being a key, using a certificate of the responding CA.
In the above example the signature is computed upon HMAC-SHA256('0b421ff9e2b15fa582691afba57e8b72', 'xmpp:ca.denmark.lit') where the first argument is a key and the second argument is a value.
Upon receiption of a redirection response matching the request a client proceeds as follows:
If either of these checks fails, a client MUST behave as if it received an error response with a permanent condition (see Certificate Request Error section). Otherwise, a client SHOULD request a certificate from the CA server it was redirected to.
A client MUST be prepared to receive redirection during a challenge procedure. The procedure itself will be aborted by the CA server in this case if needed.
A client MUST be prepared for multiple redirections (this might happen during resolution of replication conflicts at CA servers), but MUST detect loops. Since the number of trusted CAs is limited, a number of redirections will always be finite as long as a client checks for loops.
If a client detects a request timeout, i.e. neither challenge nor response have arrived in the assumed time, it MUST behave as if it received an error response with a temporary condition (see Certificate Request Error section).
A registered client at any time MAY revoke its certificate. To accomplish this it MUST create an IQ stanza containing <x509-revoke/> element qualified by 'urn:xmpp:x509:0' namespace. The element MUST contain:
The IQ stanza MUST be sent to the CA server that has issued the certificate, i.e. extracted from XmppAddr of the corresponding CA certificate.
The CA server MUST verify the signature using the public key of the certificate, MUST perform its revocation procedure (e.g. appending the certificate's serial to the corresponding CRL) and, in the case of success, MUST respond with an empty IQ result. If the revocation is not needed (e.g. the certificate is expired or already revoked), the CA server MUST still respond with an empty IQ result.
In the case of failure, the CA server MUST respond with a corresponding stanza error. Depending on the error type, a client MAY either repeat the request or give up.
Upon successful revocation, a client MAY retract the corresponding published item (see Certificates Discovery section).
A client SHOULD revoke all its certificates prior to cancelling the account registration (Section 3.2 of In-Band Registration (XEP-0077) ).
The protocol supports certificate issuance during account registration. Thus the requested certificate can be also used in SASL EXTERNAL authentication with the server where the account is being registered. The rationale for this is at least twofold:
The registration protocol described in this section is called X.509 In-Band Registration (X.509 IBR).
It is important to note that X.509 IBR replaces account creation defined in In-Band Registration (XEP-0077)  and doesn't extend it. However, ordinary IBR can still be used to cancel account registration, because X.509 IBR doesn't provide such functionality.
X.509 IBR may also be used to restore access to the account by requesting a new certificate from the CA server that has previously issued certificates for the account's XMPP address. A client will be redirected to this CA server as described in Certificate Request Redirection section.
A server reports X.509 IBR support as specified under section Stream Features.
Once a client has learnt server support from the stream features, it MUST retrieve a list of CA certificates from the server as specified under section CA List Retrieval. Then a client merges the server's list with its local list as described in section Merging CA Lists and chooses a CA certificate from the mix. A client then follows the procedure described in Certificate Issuance section.
Upon receiption of a certificate request, the server checks that:
If either of these checks fails, the server MUST generate a corresponding stanza error. If the error is generated because the account is already registered, the error condition MUST be <conflict/>.
If all the checks succeed, the server routes the request as described below.
If the server has accepted the request it MUST set 'from' attribute of the IQ stanza with the value of the XMPP address being registered and MUST forward the request towards the CA server. Since the client doesn't have a binded session at the server, the standard routing rules (Section 8.5 of RFC 6121 ) cannot be used to route back CA responses. In order to find the corresponding client's stream statelessly, the server MAY append a resource part to the XMPP address in 'from' attribute. The resource MAY contain arbitrary data needed by the server to detect the client's stream location. Note that the data MUST NOT be more than 1023 octets in length (Section 3.4 of RFC 7622 ). Prior to forwarding of a CA response to a client, the server MAY remove 'to' attribute from the response, however, this is not strictly speaking needed since a client is supposed not to check its value (see "Implementation Note" of Section 220.127.116.11 of RFC 6120 ).
In general, there is no way for the server to know whether certificate issuance was successful or not: even though the server is able to inspect CA responses, their delivery to a client is not guaranteed. So the only reliable way to mark an account as registered is at the first successful SASL EXTERNAL authentication. It marks it by storing an association of the account's XMPP address with its certificate as described in Client-to-Server Authentication.
During c2s SASL EXTERNAL authentication a server MUST reject the certificate if either of the following is true:
If the certificate is accepted, a server consults its storage to find previously stored associations of the XMPP address with a certificate:
A server MAY destroy inactive accounts, i.e. accounts with all associated certificates being either expired or revoked. However, a server SHOULD NOT destroy an inactive account if it has at least one associated certificate that was expired or revoked less than a month ago.
A client MAY use local PEP storage (Personal Eventing Protocol (XEP-0163) ) in order to publish its certificates so other peers can discover them. It MUST do this by including each certificate chain represented as <x509-cert-chain/> element in a separate pubsub <item/> element and publish each of the items to 'urn:xmpp:x509:0' node. Note well: a single item corresponds to a single certificate chain.
To uniquely identify a certificate chain within the node, the value of 'id' attribute of the <item/> element MUST be equal to first 16 octets from a signatureValue (Section 18.104.22.168 of RFC 5280 ) of the first certificate in the chain, represented in lowercased hexadecimal encoding. For instance, the value of 'id' attribute from the example above corresponds to the signature from the example below.
For compatibility with other programs, a client SHOULD store an obtained certificate chain in PEM format (RFC 7468 ) written to a file with ".pem" extension. Alternatively, a client MAY store it in other formats, but SHOULD provide a procedure for exporting in PEM format.
When an already registered client detects server support e.g. after software upgrade, it may ask the user to request a certificate and transition to SASL EXTERNAL authentication (although the exact question may not contain these technical details). In order to avoid confusion, a client should check if it has a mutually trusted CA certificate with the server as specified under CA List Retrieval and Merging CA Lists sections before asking for transitioning.
In order to optimize battery consumption some mobile operating systems have very strong limitations for background processes. This may become a problem for a client running a challenge procedure: the procedure is typically interactive and thus the client process may be preempted and killed. A possible workaround is to store the request state in durable storage and, when the challenge is passed and the client process is restarted, consult the storage and repeat the request if needed. Since CA servers are prepared to resend responses for already issued certificates without challenging, a client doesn't need to disturb a human user again in order to receive the certificate.
A user may decide to change a certificate authority and request certificates from a new CA server. Since all user's certificates are required to be issued by the same CA, a user's client has to revoke all its certificates (see Certificate Revocation) prior to switching CA servers. However, when all certificates are revoked, the account is vulnerable because an attacker may request a certificate for the account's XMPP address from another CA and thus gain control over the account and spoof its identity. TODO: find a solution for this.
A client certificate might be lost, e.g. due to the device being lost or damaged. A client is able to restore access to the account by requesting another certificate using X.509 IBR. In this case a client will be redirected to the appropriate CA server as described in Certificate Request Redirection section. When the account is recovered, it is RECOMMENDED to revoke the lost certificate: a CA server SHOULD provide such functionality during a challenge procedure.
To simplify investigation of errors, an XMPP entity that generated an error SHOULD possess 'by' attribute in <error/> element containing its XMPP address and SHOULD include <text/> element containing a human readable description of the error.
A compromised server may try to request a certificate on behalf of an already registered user in order to spoof the user's identity. The root CAs are coordinated to avoid issuing certificates for the same XMPP address by different CA servers (see E2E Authentication in XMPP (XEP-0416) ). That means that all user's certificates must be issued by the same CA. Therefore, a rogue server may only request a certificate from the CA that has previously issued a certificate for the user. However, in this case the server must authenticate itself at the CA server (by passing a challenge) because the CA already has an account for the user (which was created at the first issuance). Thus, the attack is considered to be inefficient as long as the challenge is hard enough.
The protocol packets from this specification are protected from modification in transit. Firstly, all connections are required to be TLS protected. This protects from man-in-the-middle attacks at the network level. Secondly, all requests and successful responses are signed and are required to be verified. This protects from compromised middle-boxes, e.g. CA frontend servers.
Hypothetically, several users may almost simultaneously try to register the same XMPP address by sending certificate requests to different CA servers. Due to delays in replica propagation among CA servers, they might issue certificates for the same XMPP address to different users. It's up to the CA servers to resolve such conflicts as outlined in E2E Authentication in XMPP (XEP-0416) .
In order to prevent dissemination of fake root certificates, a client MUST NOT absorb into its local list of CA certificates any of CA certificates retrieved from the server (as described in CA List Retrieval). In other words, a client MUST NOT treat its server as a trusted source of CA certificates.
The urn:xmpp:x509:0 namespace needs to be registered.
Special thanks to Wiktor Kwapisiewicz for spotting a few security flaws.
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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.
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Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
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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".
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