XEP-0417: E2E Authentication in XMPP: Certificate Issuance and Revocation

This specification defines a way for a certificate authority to serve certificate signing requests via XMPP in order to issue X.509 certificates for the use in end-to-end and c2s SASL EXTERNAL authentication.
Evgeny Khramtsov
© 1999 – 2020 XMPP Standards Foundation. SEE LEGAL NOTICES.


WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
Standards Track
0.1.01 (2019-03-29)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Draft
  4. Final

1. Introduction

E2E Authentication in XMPP (XEP-EAX0416) [1] 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) [12]) 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.

2. Requirements

2.1 CA Server Requirements

The following rules apply to CA servers:

  1. CA servers MUST fulfill the requirements outlined in E2E Authentication in XMPP (XEP-0416) [1].
  2. CA servers MUST NOT accept HTTP requests over unencrypted connections.
  3. CA servers MUST NOT allow unencrypted XMPP orconnections. HTTPHowever, internal CA server infrastructure MAY be splitted into frontend and backend servers where TLS is "offloaded" to the frontend component. In this case backends will accept unencrypted connections from frontends. Since certificate requests and responses are signed, such deployment possesses little security risks.

2.2 CA Certificate Requirements

The following rules apply to CA certificates:

  1. A CA certificate MUST fulfill the requirements outlined in E2E Authentication in XMPP (XEP-EAX0416) [1].
  2. Additionally, a CA that manages an XMPP server for certificates issuance using the protocol described herein MUST encode an XMPP address of the XMPPrequest serverprocessing entity in its own certificate as an XmppAddr identifier (see Section of RFC 6120 [23]). The nodeaddress andMUST be in the form of 'domain.tld' or 'user@domain.tld', i.e. a resource parts of the addresspart MUST be empty (omitted). AlthoughFor there are several ways to encode domain namessimplicity, in X.509this certificates,document anit's XmppAddr identifier type is chosen to provide an indicationassumed that the CAprocessing acceptsentity certificateis signingrepresented requestsby overthe CA's XMPP server itself and thus the entity's address is called "CA server address". The address MUST be prepared for comparison using PRECIS rules from RFC 7622 [4]: this allows XMPP agnostic software to hash and compare addresses robustly.

2.3 CSR Requirements

The following rules apply to a certificate signing request:

  1. It MUST be an ASN.1 CertificateRequest structure which MUST conform to RFC2986RFC 2986 [5].
  2. Its subject field SHOULD be empty: a subject of the certificate will be generated by the CA server.
  3. It MUST contain extensionRequest attribute requesting a bare XMPP address encoded within subjectAltName as an XmppAddr identifier type (see Section of RFC 6120 [23]). The XMPP address MUST be the one a client wishes to associate the requested certificate with.
  4. It MAY contain other extensionRequest attributes requesting other subjectAltNamesSubject Alternative Names such as rfc822Name (email address), a "tel" URI (RFC 3966 [36]) and so on. In this case a client MUST be prepared to be additionally challenged by the CA server to prove possession of the corresponding identities. A client also MUST be prepared that either those attributes may be either ignored and omitted in the issued certificate or the whole request may be rejected.
  5. It SHOULD NOT contain other extensionRequest attributes.

3. Glossary

CA Server
An XMPP server managed by CA to serve certificate signing requests using the protocol described in this document.
X.509 IBR
A procedure of in-band registration of an XMPP account combined with certificate issuance. See X.509 IBR section for details.
The "Jabber" network
AThe publicly available federated network of XMPP servers and clients running on the Internet.
"Jabber ID" - same as XMPP address (RFC 7622 [4]).
Inactive Certificate
An otherwise valid certificate that is either expired or revoked.

See also Glossary section of E2E Authentication in XMPP (XEP-EAX:0416) [1]: terminology from there is heavily used in this document.

4. X.509 Elements

4.1 Certificate Elements

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.

Character data of the <x509-cert/> element MUST be a PEMBase64 DER encoded ASN.1 Certificate structure (SectionRFC 5.1 of RFC74685280 [7]) with encapsulation boundaries (BEGIN/END) removed. The <x509-cert/> element MUST NOT contain any child elements.

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 [47]) 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.

Example 1. Certificate Chain
<x509-cert-chain xmlns='urn:xmpp:x509:0'
                 name='Home Desktop'>

A certificate chain may be obtained and/or stored as a so called "PEM file" (formalized by RFC7468RFC 7468 [8]). In this case the content of this file is trivially mapped to the <x509-cert-chain/> element and vice versa. See also Storage Format.

4.2 CSR Element

A certificate signing request (RFC2986RFC 2986 [5]) is represented as an <x509-csr/> element qualified by 'urn:xmpp:x509:0' namespace. Character data of the element MUST be a PEMBase64 DER encoded ASN.1 CertificateRequest structure (SectionRFC 7 of RFC74682986 [5]) with encapsulation boundaries (BEGIN/END) removed. An <x509-csr/> element MUST NOT contain any child elements. The <x509-csr/> element MUST possess a 'transaction' attribute containing a random value identifying a CSR transaction. 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.

Example 2. Certificate Request
<x509-csr xmlns='urn:xmpp:x509:0'
          name='My Phone'>

4.3 Signature Element

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 [59]) encoded signature. The element MUST NOT contain any child elements.

Example 3. Signature
<x509-signature xmlns='urn:xmpp:x509:0'>

5. CA Server Selection

5.1 Overview

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 such as the one described in Section 5 of XEP-EAX, or MAY be a mix of both. 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.

5.12 Determining Server Support

5.12.1 Service Discovery Features

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.

Example 4. Server Advertising X.509 Authentication Support
<iq type='get'
  <query xmlns='http://jabber.org/protocol/disco#info'/>

<iq type='result'
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <identity category='server' type='im'/>
    <identity category='auth' type='cert'/>
    <feature var='urn:xmpp:x509:0'/>

5.12.2 Stream Features

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.

Example 5. Server Advertising X.509 IBR Support
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <x509-register xmlns='urn:xmpp:x509:0'/>

5.23 CA List Retrieval

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:

Example 6. CA List Request
<iq type='get'
  <x509-ca-list xmlns='urn:xmpp:x509:0'/>

The server responds with an unordered list of <x509-cert/> elements included in an <x509-ca-list/> element:

Example 7. CA List Response
<iq type='result'
  <x509-ca-list xmlns='urn:xmpp:x509:0'>
      ... PEMBase64 DER encoded ASN.1 Certificate structure ...
      ... PEMBase64 DER encoded ASN.1 Certificate structure ...

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).

5.34 Merging CA Lists

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 Additional CA Certificate Requirements for the explanation). When merging is completed, a client proceeds as follows:

6. Certificate Issuance

6.1 Overview

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) [610] 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:

CSR Transaction
Client                                CA Server
  |                                        |
  |--------- Certificate Request --------->|
  |                                        |
  |<----- Optional Challenge Message ------|
  |                                        |
  |<------------ Certificate --------------|
  |                                        |
Client                                CA Server

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). Failing to do so during X.509 IBR MAY result into an account deadlock for a prolonged period of time (see Preallocation). Even when a client has an already registered account, theThe above requirement protects ita client from issuing unnecessary certificates (whose number ismay be limited by certificate authorities as outlined in XEP-EAX-CAR).

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.

6.12 Certificate Request

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-csrrequest/> element qualified by 'urn:xmpp:x509:0' namespace. The <x509-request/> element MUST possess a 'transaction' attribute containing a random value identifying this CSR transactiontransaction: the value MUST be cryptographically strong with at least 128 bits of entropy. It MAY contain a 'name' attribute. TheFinally, <x509-csrrequest/> element is then includedwrapped into IQ request for transmission. The 'to' attribute of the IQ stanza MUST be set to the target CA server'sserver address.

Example 8. Certificate Request
<iq type='get'
  <x509-csrrequest xmlns='urn:xmpp:x509:0'
    <x509-csr name='My Second Phone'>
      ... PEMBase64 DER encoded ASN.1 CertificateRequest structure ...

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:

Example 9. Authenticated Certificate Request

<iq type='get'
  <x509-request xmlns='urn:xmpp:x509:0'
    <x509-csr name='My Second Phone'>
      ... Base64 DER encoded ASN.1 CertificateRequest structure ...
      ... Base64 DER encoded ASN.1 Certificate structure ...
      ... Base64 encoded signature ...

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.

6.23 Certificate Challenge

A certificate request MAY be challenged by the CA server. The CA server MAYMUST challenge athe 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 dochallenge sothe forrequest many reasonsotherwise, for example, if it MAY want to identify a human user in order to prevent massive creation of certificates by a single person. Another possible case is when the CA serverhas 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 dochallenge sothe 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 original <x509-csrrequest/> element. The <x509-challenge/> element MUST contain exactly one <x509-signature/> element carringcarrying a signature computed upon concatenationHMAC-SHA256 hash of the valuesURI fromwith the value of 'transaction' andattribute 'uri'being attributesa 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.

Example 910. CA Server Sends Challenge
<message type='normal'
  <x509-challenge xmlns='urn:xmpp:x509:0'
      ... Base64 encoded signature ...

In the above example the signature is computed upon '4UGObuJYf7yY8ucndbmHhttps: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:

  1. A client checks that the value of 'from' attribute matches the CA server address.
  2. A client checks that the value of 'transaction' attribute matches the identifier of the running transaction.
  3. A client verifies the signature using the public key of the CA certificate.

If all the checks have passed, a client spawns an URI handler and waits for the certificate response. OtherwiseIf either of the checks has failed, a client MAYMUST ignore the message orif it's performing X.509 IBR and MAY reply with aan corresponding stanzaappropriate error otherwise.

6.34 Certificate Response

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.

Example 1011. Certificate Response
<iq type='result'
  <x509-cert-chain xmlns='urn:xmpp:x509:0'
                   name='My Second Phone'>
      ... PEMBase64 DER encoded ASN.1 Certificate structure ...
      ... PEMBase64 DER encoded ASN.1 Certificate structure ...

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).

6.45 Certificate Request Error

If the CA server refuses to issue a certificate it MUST generate a corresponding stanza error. In this case <error/> element MUST possess 'by' attribute with the value of the CA server's address. If the error is generated due to challenge failure, <error/> element MUST contain <x509-challenge-failed/> element qualified by 'urn:xmpp:x509:0' namespace.

Example 1112. CA Server Error
<iq type='error'
  <error type='auth'
    <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <x509-challenge-failed xmlns='urn:xmpp:x509:0'/>

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 'name' attributeattributes 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-csrrequest/> 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 conditionscondition has a special meaning and is described in the section below.

6.6 Certificate Request Redirection

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) [1] 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 of RFC 6120 [3]). An URI from <redirect/> element MUST be an XMPP URI (RFC 5122 [11]) 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.

Example 13. Redirection

<iq type='error'
  <error type='modify'
    <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
      ... Base64 encoded signature ...

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:

  1. It MUST destroy internal IQ and transaction states.
  2. It MUST check that the error has arrived from the requested CA server.
  3. If the <error/> element possesses 'by' attribute, a client MUST check that its value equal to requested CA server address.
  4. It MUST verify the signature from <x509-signature/> element using a public key of the requested CA.
  5. It MUST extract a new CA server address from an URI of <redirect/> element and MUST treatcheck that the corresponding CA certificate is trusted. Depending on the use case, the trust is evaluated by consulting either local or merged list of CA certificates.

If either of these errorschecks fails, a client MUST behave as if it received an error response with a permanent failurescondition (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.

6.57 Certificate Request Timeout

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).

7. Certificate Revocation

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:

There MUST be exactly one <x509-cert/> element and exactly one <x509-signature/> element.

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.

Example 1214. Certificate Revocation Request
<iq type='set'
  <x509-revoke xmlns='urn:xmpp:x509:0'>
      ... PEMBase64 DER encoded ASN.1 Certificate structure ...
      ... Base64 encoded signature ...

The CA server MUST verify the signature using the public key of the certificate, MUST appendperform 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: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.

Example 1315. Certificate Revocation RequestSuccess
<iq type='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) [712]).

8. X.509 IBR

8.1 Overview

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).

AIt serveris important to note that supports X.509 IBR MUSTreplaces reportaccount creation defined in In-Band Registration (XEP-0077) [12] 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.

8.12 IBR Client Rules

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. The server MUST allow unregistered clients to retrieve this list if it has reported X.509 IBR support. Then a client merges the server's list with its local list as described in section Merging CA Lists and choosechooses a CA certificate from the mix. A client then follows the procedure described in Certificate Issuance section.

Once a certificate is obtained, it is RECOMMENDED to perform SASL EXTERNAL authentication with the server as soon as possible in order for the server to mark the account as registered (see Registration Mark).

8.23 IBR Server Rules

Upon receiption of a certificate request, the server checks that:

  1. The IQ stanza possesses 'to' attribute.
  2. The ASN.1 CertificateRequest structure inside <x509-csr/> element contains an XMPP address (that the client requests to register, see CSR Requirements). To avoid backward incompatibilities, the server SHOULD NOT check or validate other fields of the structure.
  3. The server is responsible for the domain of the XMPP address.
  4. An XMPP address in 'to' attribute is a trusted CA server and the server allows this CA to issue certificates for the users of thethis domain.
  5. There is no already registered or preallocated (see Preallocation)existing account matching the XMPP address being registered. If the account isalready preallocatedexists, an ASN.1 CertificateRequest structure from the preallocationserver MUST matchcheck that there is at least one active certificate associated with the oneaccount from(see Client-to-Server Authentication), or, otherwise, the requestonly possible mechanism to authenticate this account is SASL EXTERNAL.

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 or preallocated, the error condition MUST be <conflict/>.

If all the checks succeed, the server preallocates an account and routes the request as described below.

8.2.1 Preallocation In order to prevent registration of the same account by different human users, the server MUST temporary preallocate an account upon receiption of a certificate request and later MUST mark it as permanently registered and/or release the preallocation. The server preallocates an account by storing an association of the XMPP address with the ASN.1 CertificateRequest structure from the request. The server MUST keep an account preallocated for a period long enough for a client to complete the issuance and authentication. The server MUST NOT allow registration of preallocated accounts using different methods (e.g. In-Band Registration (XEP-0077) [7]).

8.23.21 Routing

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 yet have ana accountbinded session at the server, the standard routing rules (Section 8.5 of RFC 6121 [813]) 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 [94]). 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 of RFC 6120 [23]). Registration Mark

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. WhenIt marks it by storing an association of the accountaccount's isXMPP finallyaddress marked,with theits servercertificate MUSTas releasedescribed thein preallocationClient-to-Server Authentication.

9. 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.

910. Certificates Discovery

A client MAY use local PEP storage (Personal Eventing Protocol (XEP-0163) [1014]) 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.

Example 1416. Publishing Certificate Chain
<iq type='set' id='announce1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:x509:0'>
      <item id='304402206242a7b554e2f1a1bd790758'>
        <x509-cert-chain xmlns='urn:xmpp:x509:0'
            ... PEMBase64 DER encoded ASN.1 Certificate structure ...
            ... PEMBase64 DER encoded ASN.1 Certificate structure ...

To uniquely identify a certificate chain within the node, the value of 'id' attribute of the <item/> element MUST containbe equal to first 16 octets from a signatureValue (Section of RFC 5280 [47]) of the first certificate in the chain, represented in lowercased hexadecimal encoding. For instance, athe value of 'id' attribute from the example above corresponds to the signature from the example below.

Example 1517. Certificate Signature

1011. Implementation Notes

1011.1 Storage Format

For compatibility with other programs, a client SHOULD store an obtained certificate chain in PEM format (RFC7468RFC 7468 [8]) 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.

1011.2 SASL Mechanism Transitioning

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.

1011.3 Mobile OS Considerations

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.

11.4 CA Migration

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.

11.5 Account Recovery

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.

11.6 Debugging

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.

1112. Security Considerations

12.1 Identity Spoofing

TODOA 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) [1]). 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.

12.2 MITM

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.

12.3 Registration Race

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) [1].

12.4 CA List Poisoning

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.

1213. IANA Considerations

None required.

1314. XMPP Registrar Considerations

The urn:xmpp:x509:0 namespace needs to be registered.

1415. XML Schema


16. Acknowledgements

Special thanks to Wiktor Kwapisiewicz for spotting a few security flaws.


Appendix A: Document Information

XMPP Standards Foundation
Standards Track
Last Updated
Approving Body
XMPP Council
XMPP Core, XMPP IM, RFC 2986, RFC 4648, RFC 5280, RFC 7468, RFC 7622, XEP-0001, XEP-0163, XEP-0178, XEP-0416
Superseded By
Short Name
Source Control

This document in other formats: XML  PDF

Appendix B: Author Information

Evgeny Khramtsov


This XMPP Extension Protocol is copyright © 1999 – 2020 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.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

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).

Visual Presentation

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Appendix D: Relation to XMPP

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.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

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".

Appendix G: Notes

1. XEP-0416: E2E Authentication in XMPP <https://xmpp.org/extensions/xep-0416.html>.

2. XEP-0178: Best Practices for Use of SASL EXTERNAL <https://xmpp.org/extensions/xep-0178.html>.

23. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

34. RFC 7622: Extensible Messaging and Presence Protocol (XMPP): Address Format <http://tools.ietf.org/html/rfc7622>.

5. RFC 2986: PKCS #10: Certification Request Syntax Specification - Version 1.7 <http://tools.ietf.org/html/rfc2986>.

6. RFC 3966: The tel URI for Telephone Numbers <http://tools.ietf.org/html/rfc3966>.

47. RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile <http://tools.ietf.org/html/rfc5280>.

58. RFC 7468: Textual Encodings of PKIX, PKCS, and CMS Structures <http://tools.ietf.org/html/rfc7468>.

9. RFC 4648: The Base16, Base32, and Base64 Data Encodings <http://tools.ietf.org/html/rfc4648>.

610. XEP-0198: Stream Management <https://xmpp.org/extensions/xep-0198.html>.

711. RFC 5122: Internationalized Resource Identifiers (IRIs) and Uniform Resource Identifiers (URIs) for the Extensible Messaging and Presence Protocol (XMPP) <http://tools.ietf.org/html/rfc5122>.

12. XEP-0077: In-Band Registration <https://xmpp.org/extensions/xep-0077.html>.

813. RFC 6121: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc6121>.

9. RFC 7622: Extensible Messaging and Presence Protocol (XMPP): Address Format <http://tools.ietf.org/html/rfc7622>.1014. XEP-0163: Personal Eventing Protocol <https://xmpp.org/extensions/xep-0163.html>.

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

  1. Version 0.1.1 (2019-03-29)
  2. Version 0.1.0 (2019-03-29)
    Accepted by vote of Council on 2019-03-13.
    XEP Editor (jsc)
  3. Version 0.0.1 (2019-03-11)

    First draft.