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

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 connections. However, 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-0416) [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 request processing entity in its own certificate as an XmppAddr identifier (see Section 13.7.1.4 of RFC 6120 [3]). The address MUST be in the form of 'domain.tld' or 'user@domain.tld', i.e. a resource part MUST be empty (omitted). For simplicity, in this document it's assumed that the processing entity is represented by the 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 RFC 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 13.7.1.4 of RFC 6120 [3]). The XMPP address MUST be the one a client wishes to associate the requested certificate with.
4. It MAY contain extensionRequest attributes requesting other Subject Alternative Names such as rfc822Name (email address), a "tel" URI (RFC 3966 [6]) 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 ignored and omitted in the issued certificate or the whole request may be rejected.
5. It SHOULD NOT contain other extensionRequest attributes.

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

The publicly available federated network of XMPP servers and clients running on the Internet.

JID

"Jabber ID" - same as XMPP address (RFC 7622 [4]).

Inactive Certificate

An otherwise valid certificate that is either expired or revoked.

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 Base64 DER encoded ASN.1 Certificate structure (RFC 5280 [7]). 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 [7]) 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.

<x509-cert-chain xmlns='urn:xmpp:x509:0'
                 name='Home Desktop'>
  <x509-cert>
    MIICQTCCAeagAwIBAgIBATAKBggqhkjOPQQDAjBFMQswCQYDVQQGEwJBVTETMBEG
    A1UECAwKU29tZS1TdGF0ZTEhMB8GA1UECgwYSW50ZXJuZXQgV2lkZ2l0cyBQdHkg
    THRkMB4XDTE5MDMwMjA2MjMyMloXDTQ2MDcxODA2MjMyMlowHzEdMBsGCSqGSIb3
    DQEJARYOdXNlckBsb2NhbGhvc3QwVjAQBgcqhkjOPQIBBgUrgQQACgNCAAQ/5HQB
    OExPNKkiYVNtPTKSItAewVK5ZyvR7bZFkGCDBOPiqOGoabRufF5xVwmHU7aFd3kL
    jjnLz6qR6SEz/rcEo4HvMIHsMAkGA1UdEwQCMAAwHQYDVR0OBBYEFLO9AFgmoQJV
    sbzcfF3gbR+PRh+fMB8GA1UdIwQYMBaAFNetyKStrn2FIcWjsD2F3HAHuhIjMAsG
    A1UdDwQEAwIF4DAdBgNVHSUEFjAUBggrBgEFBQcDAQYIKwYBBQUHAwIwcwYDVR0R
    BGwwaqAcBggrBgEFBQcIBaAQDA51c2VyQGxvY2FsaG9zdIEOdXNlckBsb2NhbGhv
    c3SGOnJlbG9hZDovLzIyMDI3MjIwMjAxODMxOTkzNDg2ODg1NzA0NzEyMTkzNDMy
    MzIyNUB4bXBwLm9yZy8wCgYIKoZIzj0EAwIDSQAwRgIhAOHsOvXmtDJroR0ggL1l
    v+Gh0t6XdNrGc3Lbd+d+LeZAAiEA1Km0ysJgYO6HBEJouPjxE0G9+Ws5SLDuc/0M
    TvzDgXQ=
  </x509-cert>
  <x509-cert>
    MIIB0DCCAXegAwIBAgIJAPYd1dvKJm6qMAoGCCqGSM49BAMCMEUxCzAJBgNVBAYT
    AkFVMRMwEQYDVQQIDApTb21lLVN0YXRlMSEwHwYDVQQKDBhJbnRlcm5ldCBXaWRn
    aXRzIFB0eSBMdGQwHhcNMTkwMzAyMDYyMzIyWhcNNDYwNzE4MDYyMzIyWjBFMQsw
    CQYDVQQGEwJBVTETMBEGA1UECAwKU29tZS1TdGF0ZTEhMB8GA1UECgwYSW50ZXJu
    ZXQgV2lkZ2l0cyBQdHkgTHRkMFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEYl6sMwVL
    bwGS0c2BLtgaYI5/sQsT+zQ63HfrVdYZ/NqvogSWieQFpkrRCEcewUMVN+5YfA/l
    XnzZRqqdz8kSAKNTMFEwHQYDVR0OBBYEFNetyKStrn2FIcWjsD2F3HAHuhIjMB8G
    A1UdIwQYMBaAFNetyKStrn2FIcWjsD2F3HAHuhIjMA8GA1UdEwEB/wQFMAMBAf8w
    CgYIKoZIzj0EAwIDRwAwRAIgUYLzzS43j55JDmUum41Ixa+p2EslGJqkhC67s1uL
    bWACIEB35gO74quIjqoogP7Hh6L+IdJ8q0yiCS1xxAqmvTjL
  </x509-cert>
</x509-cert-chain>

A certificate chain may be obtained and/or stored as a so called "PEM file" (formalized by RFC 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 (RFC 2986 [5]) 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 [5]). 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.

<x509-csr xmlns='urn:xmpp:x509:0'
          name='My Phone'>
  MIIBSDCB7wIBADAfMR0wGwYJKoZIhvcNAQkBFg51c2VyQGxvY2FsaG9zdDBWMBAG
  ByqGSM49AgEGBSuBBAAKA0IABE470MZk43MAKM/HJEXbVU0cOemftIucZi+2Ug9T
  JK4s2J5AqrL84Fznv1zF5dT1Mu2QcwxxCMWEIC/a7/3UfFigcTBvBgkqhkiG9w0B
  CQ4xYjBgMAkGA1UdEwQCMAAwCwYDVR0PBAQDAgXgMB0GA1UdJQQWMBQGCCsGAQUF
  BwMBBggrBgEFBQcDAjAnBgNVHREEIDAeoBwGCCsGAQUFBwgFoBAMDnVzZXJAbG9j
  YWxob3N0MAoGCCqGSM49BAMCA0gAMEUCIQDrWmWBB5/W5+r1AXh7eQOXBHlAAZ5E
  VdF1wXTWUbc1TwIgWQNht5Xu2Z4zOkvnyfh7+fy4L8EQTH8TclPUDaUO1z8=
</x509-csr>

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

<x509-signature xmlns='urn:xmpp:x509:0'>
  MIIB8zCCAZqgAwIBAgIBATAKBggqhkjOPQQDAjBFMQswCQYDVQQGEwJBVTETMBEG
  A1UECAwKU29tZS1TdGF0ZTEhMB8GA1UECgwYSW50ZXJuZXQgV2lkZ2l0cyBQdHkg
  THRkMB4XDTE5MDMwMjE1Mzk0N1oXDTQ2MDcxODE1Mzk0N1owHzEdMBsGCSqGSIb3
  DQEJARYOdXNlckBsb2NhbGhvc3QwVjAQBgcqhkjOPQIBBgUrgQQACgNCAAROO9DG
  ZONzACjPxyRF21VNHDnpn7SLnGYvtlIPUySuLNieQKqy/OBc579cxeXU9TLtkHMM
  cQjFhCAv2u/91HxYo4GjMIGgMAkGA1UdEwQCMAAwHQYDVR0OBBYEFOf7dRPkxIxf
  z7HWUJ+NPezUiZr+MB8GA1UdIwQYMBaAFL5BMCaVAMvN6fxvJSnb0qpwHT/+MAsG
  A1UdDwQEAwIF4DAdBgNVHSUEFjAUBggrBgEFBQcDAQYIKwYBBQUHAwIwJwYDVR0R
  BCAwHqAcBggrBgEFBQcIBaAQDA51c2VyQGxvY2FsaG9zdDAKBggqhkjOPQQDAgNH
  ADBEAiBtdFGoGZ/TcxeOiQ2cau9irgEEP6kW6yEU81VRmjG6OQIgTxhH3vMvgONn
  BXXI8cAFM5iOo5JDq/TW/pV729SFVwg=
</x509-signature>

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. 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.

5.2 Determining Server Support¶

<iq type='get'
    to='shakespeare.lit'
    id='features'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>

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

<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL</mechanism>
  </mechanisms>
  <x509-register xmlns='urn:xmpp:x509:0'/>
</stream:features>

5.3 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:

<iq type='get'
    to='shakespeare.lit'
    id='ca-list'>
  <x509-ca-list xmlns='urn:xmpp:x509:0'/>
</iq>

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

<iq type='result'
    from='shakespeare.lit'
    id='ca-list'>
  <x509-ca-list xmlns='urn:xmpp:x509:0'>
    <x509-cert>
      ... Base64 DER encoded ASN.1 Certificate structure ...
    </x509-cert>
    ...
    <x509-cert>
      ... Base64 DER encoded ASN.1 Certificate structure ...
    </x509-cert>
  </x509-ca-list>
</iq>

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).

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

• If the result is not an empty list, a client picks from this list whatever CA certificate it finds appropriate, extracts an XMPP server address from an XmppAddr identifier of this certificate and sends a certificate signing request to this server as described in Certificate Issuance section.
• If the result is an empty list, a client SHOULD proceed as if the server didn't support c2s SASL EXTERNAL authentication at all.

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) [10] 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:

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). 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:

• If this is a first certificate request, i.e. a CA server has not previously issued certificates for this XMPP address, the challenge is meant to create an account for this address at the CA server.
• Otherwise, the challenge is used to authenticate the request against an already existing account.

When a certificate is issued, a CA server responds with a full chain containing the certificate.

6.2 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-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.

<iq type='get'
    to='ca.shakespeare.lit'
    id='csr'>
  <x509-request xmlns='urn:xmpp:x509:0'
                transaction='0b421ff9e2b15fa582691afba57e8b72'>
    <x509-csr name='My Second Phone'>
      ... Base64 DER encoded ASN.1 CertificateRequest structure ...
    </x509-csr>
  </x509-request>
</iq>

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:

<iq type='get'
    to='ca.shakespeare.lit'
    id='csr'>
  <x509-request xmlns='urn:xmpp:x509:0'
                transaction='0b421ff9e2b15fa582691afba57e8b72'>
    <x509-csr name='My Second Phone'>
      ... Base64 DER encoded ASN.1 CertificateRequest structure ...
    </x509-csr>
    <x509-cert>
      ... Base64 DER encoded ASN.1 Certificate structure ...
    </x509-cert>
    <x509-signature>
      ... Base64 encoded signature ...
    </x509-signature>
  </x509-request>
</iq>

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.

6.3 Certificate Challenge¶

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.

<message type='normal'
         from='ca.shakespeare.lit'
         to='romeo@montague.lit/orchard'>
  <x509-challenge xmlns='urn:xmpp:x509:0'
                  transaction='0b421ff9e2b15fa582691afba57e8b72'
                  uri='https://ca.shakespeare.lit/csr/cOemft/8EQTH8'>
    <x509-signature>
      ... Base64 encoded signature ...
    </x509-signature>
  </x509-challenge>
</message>

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:

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. 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.

6.4 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.

<iq type='result'
    from='ca.shakespeare.lit'
    to='romeo@montague.lit/orchard'
    id='csr'>
  <x509-cert-chain xmlns='urn:xmpp:x509:0'
                   name='My Second Phone'>
    <x509-cert>
      ... Base64 DER encoded ASN.1 Certificate structure ...
    </x509-cert>
    ...
    <x509-cert>
      ... Base64 DER encoded ASN.1 Certificate structure ...
    </x509-cert>
  </x509-cert-chain>
</iq>

Upon receiption of a response matching the request a client proceeds as follows:

• It MUST destroy internal IQ and transaction states.
• It MUST check that the response has arrived from the requested CA server.
• It MUST check that the response carries <x509-cert-chain/> element which contains at least one <x509-cert/> element.
• It MUST check that all certificates in the chain are valid according to the rules outlined in E2E Authentication in XMPP (XEP-0416) [1].
• It MUST perform certification path validation (RFC 5280 [7]) to check that the certificate chain is indeed signed by the requested CA.

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).

6.5 Certificate Request Error¶

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.

<iq type='error'
    from='ca.shakespeare.lit'
    to='romeo@montague.lit/orchard'
    id='csr'>
  <error type='auth'
	 by='ca.shakespeare.lit'>
    <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
    <x509-challenge-failed xmlns='urn:xmpp:x509:0'/>
  </error>
</iq>

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.

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 8.3.3.14 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.

<iq type='error'
    from='ca.shakespeare.lit'
    to='romeo@montague.lit/orchard'
    id='csr'>
  <error type='modify'
         by='ca.shakespeare.lit'>
    <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
      xmpp:ca.denmark.lit
    </redirect>
    <x509-signature>
      ... Base64 encoded signature ...
    </x509-signature>
  </error>
</iq>

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 check 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 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.

6.7 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).

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:

• Consequent creation of an account and then a certificate creates burden for users because they typically need to pass a challenge twice: firstly at the local server, and secondly at some CA server. Combining these two steps into one improves user experience in this regard.
• Managing freely available account registration at public servers is not a simple task for operators of these servers. Failing to manage it correctly leads to uncontrolled massive creation of accounts used for SPAM propagation, DoS attacks, etc. and degrades the "Jabber" network as a whole. A server operator may want to delegate a registration and verification procedure to a trusted CA, which is believed to be very good at this task. The operator doesn't lose the userbase in this case: user data and profiles are still located at the operator's server.

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) [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.

8.2 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. 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.

8.3 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 this domain.
5. There is no already existing account matching the XMPP address being registered. If the account already exists, the server MUST check that there is at least one active certificate associated with the account (see Client-to-Server Authentication), or, otherwise, the only 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, the error condition MUST be <conflict/>.

If all the checks succeed, the server routes the request as described below.

11.1 Storage Format¶

For compatibility with other programs, a client SHOULD store an obtained certificate chain in PEM format (RFC 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.

11.2 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.

11.3 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.

12.1 Identity Spoofing¶

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) [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.

Copyright

This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).

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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>.

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

4. 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>.

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

8. 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>.

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

11. 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>.

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

14. XEP-0163: Personal Eventing Protocol <https://xmpp.org/extensions/xep-0163.html>.