Certificates for XMPP/Jabber

I am revamping my XMPP server and I’ve written down notes on how to set up certificates to enable TLS.

I will run Debian Jessie with JabberD 2.x, using the recent jabberd2 jessie-backport. The choice of server software is not significant for the rest of this post.

Running XMPP over TLS is a good idea. So I need a X.509 PKI for this purpose. I don’t want to use a third-party Certificate Authority, since that gives them the ability to man-in-the-middle my XMPP connection. Therefor I want to create my own CA. I prefer tightly scoped (per-purpose or per-application) CAs, so I will set up a CA purely to issue certificates for my XMPP server.

The current XMPP specification, RFC 6120, includes a long section 13.7 that discuss requirements on Certificates.

One complication is the requirement to include an AIA for OCSP/CRLs — fortunately, it is not a strict “MUST” requirement but a weaker “SHOULD”. I note that checking revocation using OCSP and CRL is a “MUST” requirement for certificate validation — some specification language impedence mismatch at work there.

The specification demand that the CA certificate MUST have a keyUsage extension with the digitalSignature bit set. This feels odd to me, and I’m wondering if keyCertSign was intended instead. Nothing in the XMPP document, nor in any PKIX document as far as I am aware of, will verify that the digitalSignature bit is asserted in a CA certificate. Below I will assert both bits, since a CA needs the keyCertSign bit and the digitalSignature bit seems unnecessary but mostly harmless.

My XMPP/Jabber server will be “chat.sjd.se” and my JID will be “simon@josefsson.org”. This means the server certificate need to include references to both these domains. The relevant DNS records for the “josefsson.org” zone is as follows, see section 3.2.1 of RFC 6120 for more background.

_xmpp-client._tcp.josefsson.org.	IN	SRV 5 0 5222 chat.sjd.se.
_xmpp-server._tcp.josefsson.org.	IN	SRV 5 0 5269 chat.sjd.se.

The DNS records or the “sjd.se” zone is as follows:

chat.sjd.se.	IN	A	...
chat.sjd.se.	IN	AAAA	...

The following commands will generate the private key and certificate for the CA. In a production environment, you would keep the CA private key in a protected offline environment. I’m asserting a expiration date ~30 years in the future. While I dislike arbitrary limits, I believe this will be many times longer than the anticipated lifelength of this setup.

openssl genrsa -out josefsson-org-xmpp-ca-key.pem 3744
cat > josefsson-org-xmpp-ca-crt.conf << EOF
[ req ]
x509_extensions = v3_ca
distinguished_name = req_distinguished_name
prompt = no
[ req_distinguished_name ]
CN=XMPP CA for josefsson.org
[ v3_ca ]
basicConstraints = CA:true
keyUsage=critical, digitalSignature, keyCertSign
openssl req -x509 -set_serial 1 -new -days 11147 -sha256 -config josefsson-org-xmpp-ca-crt.conf -key josefsson-org-xmpp-ca-key.pem -out josefsson-org-xmpp-ca-crt.pem

Let’s generate the private key and server certificate for the XMPP server. The wiki page on XMPP certificates is outdated wrt PKIX extensions. I will embed a SRV-ID field, as discussed in RFC 6120 section and RFC 4985. I chose to skip the XmppAddr identifier type, even though the specification is somewhat unclear about it: section says that it “is no longer encouraged in certificates issued by certification authorities” while section says “Use of the ‘id-on-xmppAddr’ format is RECOMMENDED in the generation of certificates”. The latter quote should probably have been qualified to say “client certificates” rather than “certificates”, since the latter can refer to both client and server certificates.

Note the use of a default expiration time of one month: I believe in frequent renewal of entity certificates, rather than use of revocation mechanisms.

openssl genrsa -out josefsson-org-xmpp-server-key.pem 3744
cat > josefsson-org-xmpp-server-csr.conf << EOF
[ req ]
distinguished_name = req_distinguished_name
prompt = no
[ req_distinguished_name ]
CN=XMPP server for josefsson.org
openssl req -sha256 -new -config josefsson-org-xmpp-server-csr.conf -key josefsson-org-xmpp-server-key.pem -nodes -out josefsson-org-xmpp-server-csr.pem
cat > josefsson-org-xmpp-server-crt.conf << EOF
openssl x509 -sha256 -CA josefsson-org-xmpp-ca-crt.pem -CAkey josefsson-org-xmpp-ca-key.pem -set_serial 2 -req -in josefsson-org-xmpp-server-csr.pem -out josefsson-org-xmpp-server-crt.pem -extfile josefsson-org-xmpp-server-crt.conf

With this setup, my XMPP server can be tested by the XMPP IM Observatory. You can see the c2s test results and the s2s test results. Of course, there are warnings regarding the trust anchor issue. It complains about a self-signed certificate in the chain. This is permitted but not recommended — however when the trust anchor is not widely known, I find it useful to include it. This allows people to have a mechanism of fetching the trust anchor certificate should they want to. Some weaker cipher suites trigger warnings, which is more of a jabberd2 configuration issue and/or a concern with jabberd2 defaults.

My jabberd2 configuration is simple — in c2s.xml I add a <id> entity with the “require-starttls”, “cachain”, and “pemfile” fields. In s2s.xml, I have the <pemfile>, <resolve-ipv6>, and <require-tls> entities.

Some final words are in order. While this setup will result in use of TLS for XMPP connections (c2s and s2s), other servers are unlikely to find my CA trust anchor, let alone be able to trust it for verifying my server certificate. I’m happy to read about Peter Saint-Andre’s recent SSL/TLS work, and in particular I will follow the POSH effort.