mod_client_certs

Introduction

XEP-0257 specifies a protocol for clients to store and manage client side certificates. When a client presents a stored client side certificate during the TLS handshake, it can log in without supplying a password (using SASL EXTERNAL). This makes it possible to have multiple devices accessing an account, without any of them needing to know the password, and makes it easier to revoke access for a single device.

Details

Each user can add their own certificates. These do not need to be signed by a trusted CA, yet they do need to be valid at the time of logging in and they should include an subjectAltName with otherName "id-on-xmppAddr" with the JID of the user.

Generating your certificate

  1. To generate your own certificate with a "id-on-xmppAddr" attribute using the command line openssl tool, first create a file called client.cnf with contents:

    [req] prompt = no
    x509_extensions = v3_extensions
    req_extensions = v3_extensions
    distinguished_name = distinguished_name
    
    [v3_extensions]
    extendedKeyUsage = clientAuth
    keyUsage = digitalSignature,keyEncipherment
    basicConstraints = CA:FALSE
    subjectAltName = @subject_alternative_name
    
    [subject_alternative_name]
    otherName.0 =
    1.3.6.1.5.5.7.8.5;FORMAT:UTF8,UTF8:hamlet@shakespeare.lit
    
    [distinguished_name]
    commonName = Your Name
    emailAddress = hamlet@shakespeare.lit
  2. Replace the values for otherName.0 and commonName and emailAddress with your own values. The JID in otherName.0 can either be a full JID or a bare JID, in the former case, the client can only use the resource specified in the resource. There are many other fields you can add, however, for SASL EXTERNAL, they will have no meaning. You can add more JIDs as otherName.1, otherName.2, etc.
  3. Create a private key (as an example, a 4096 bits RSA key):

    openssl genrsa -out client.key 4096
  4. Create the certificate request:

    openssl req -key client.key -new -out client.req -config client.cnf -extensions v3_extensions
  5. Sign it yourself:

    openssl x509 -req -days 365 -in client.req -signkey client.key -out client.crt -extfile client.cnf -extensions v3_extensions

The 365 means the certificate will be valid for a year starting now.

The client.key must be kept secret, and is only needed by clients connecting using this certificate. The client.crt file contains the certificate that should be sent to the server using XEP-0257, and is also needed by clients connecting to the server. The client.req file is not needed anymore.

Configuration

(None yet)

Compatibility

0.9 Works
0.8 Untested. Probably doesn't.

Clients

(None?)

TODO

Possible options to add to the configuration: