In 2004, I set up a small certification authority using OpenSSL on Linux and the simple management scripts provided with OpenVPN. In accordance with the guides I found at the time, I set the validity period for the root CA certificate to 10 years. Since then, I have signed many certificates for OpenVPN tunnels, web sites and e-mail servers, all of which also have a validity period of 10 years (this may have been wrong, but I didn't know better at the time).
I have found many guides about setting up a CA, but only very little information about its management, and in particular, about what has to be done when the root CA certificate expires, which will happen some time in 2014. So I have the following questions:
- Will the certificates that have a validity period extending after the expiry of the root CA certificate become invalid as soon as the latter expires, or will they continue to be valid (because they were signed during the validity period of the CA certificate)?
- What operations are needed to renew the root CA certificate and ensure a smooth transition over its expiry?
- Can I somehow re-sign the current root CA certificate with a different validity period, and upload the newly-signed cert to clients so that client certificates remain valid?
- Or do I need to replace all client certificates with new ones signed by a new root CA certificate?
- When should the root CA certificate be renewed? Close to expiry, or a reasonable time before expiry?
- If the renewal of the root CA certificate becomes a major piece of work, what can I do better now to ensure a smoother transition at the next renewal (short of setting the validity period to 100 years, of course)?
The situation is made slightly more complicated by the fact that my only access to some of the clients is through an OpenVPN tunnel that uses a certificate signed by the current CA certificate, so if I have to replace all client certs, I will need to copy the new files to the client, restart the tunnel, cross my fingers and hope that it comes up afterwards.
Answer
Keeping the same private key on your root CA allows for all certificates to continue to validate successfully against the new root; all that's required of you is to trust the new root.
The certificate signing relationship is based on a signature from the private key; keeping the same private key (and, implicitly, the same public key) while generating a new public certificate, with a new validity period and any other new attributes changed as needed, keeps the trust relationship in place. CRLs, too, can continue over from the old cert to the new, as they are, like certificates, signed by the private key.
So, let's verify!
Make a root CA:
openssl req -new -x509 -keyout root.key -out origroot.pem -days 3650 -nodes
Generate a child certificate from it:
openssl genrsa -out cert.key 1024
openssl req -new -key cert.key -out cert.csr
Sign the child cert:
openssl x509 -req -in cert.csr -CA origroot.pem -CAkey root.key -create_serial -out cert.pem
rm cert.csr
All set there, normal certificate relationship. Let's verify the trust:
# openssl verify -CAfile origroot.pem -verbose cert.pem
cert.pem: OK
Ok, so, now let's say 10 years passed. Let's generate a new public certificate from the same root private key.
openssl req -new -key root.key -out newcsr.csr
openssl x509 -req -days 3650 -in newcsr.csr -signkey root.key -out newroot.pem
rm newcsr.csr
And.. did it work?
# openssl verify -CAfile newroot.pem -verbose cert.pem
cert.pem: OK
But.. why? They're different files, right?
# sha1sum newroot.pem
62577e00309e5eacf210d0538cd79c3cdc834020 newroot.pem
# sha1sum origroot.pem
c1d65a6cdfa6fc0e0a800be5edd3ab3b603e1899 origroot.pem
Yes, but, that doesn't mean that the new public key doesn't cryptographically match the signature on the certificate. Different serial numbers, same modulus:
# openssl x509 -noout -text -in origroot.pem
Serial Number:
c0:67:16:c0:8a:6b:59:1d
...
RSA Public Key: (1024 bit)
Modulus (1024 bit):
00:bd:56:b5:26:06:c1:f6:4c:f4:7c:14:2c:0d:dd:
3c:eb:8f:0a:c0:9d:d8:b4:8c:b5:d9:c7:87:4e:25:
8f:7c:92:4d:8f:b3:cc:e9:56:8d:db:f7:fd:d3:57:
1f:17:13:25:e7:3f:79:68:9f:b5:20:c9:ef:2f:3d:
4b:8d:23:fe:52:98:15:53:3a:91:e1:14:05:a7:7a:
9b:20:a9:b2:98:6e:67:36:04:dd:a6:cb:6c:3e:23:
6b:73:5b:f1:dd:9e:70:2b:f7:6e:bd:dc:d1:39:98:
1f:84:2a:ca:6c:ad:99:8a:fa:05:41:68:f8:e4:10:
d7:a3:66:0a:45:bd:0e:cd:9d
# openssl x509 -noout -text -in newroot.pem
Serial Number:
9a:a4:7b:e9:2b:0e:2c:32
...
RSA Public Key: (1024 bit)
Modulus (1024 bit):
00:bd:56:b5:26:06:c1:f6:4c:f4:7c:14:2c:0d:dd:
3c:eb:8f:0a:c0:9d:d8:b4:8c:b5:d9:c7:87:4e:25:
8f:7c:92:4d:8f:b3:cc:e9:56:8d:db:f7:fd:d3:57:
1f:17:13:25:e7:3f:79:68:9f:b5:20:c9:ef:2f:3d:
4b:8d:23:fe:52:98:15:53:3a:91:e1:14:05:a7:7a:
9b:20:a9:b2:98:6e:67:36:04:dd:a6:cb:6c:3e:23:
6b:73:5b:f1:dd:9e:70:2b:f7:6e:bd:dc:d1:39:98:
1f:84:2a:ca:6c:ad:99:8a:fa:05:41:68:f8:e4:10:
d7:a3:66:0a:45:bd:0e:cd:9d
Let's go a little further to verify that it's working in real world certificate validation.
Fire up an Apache instance, and let's give it a go (debian file structure, adjust as needed):
# cp cert.pem /etc/ssl/certs/
# cp origroot.pem /etc/ssl/certs/
# cp newroot.pem /etc/ssl/certs/
# cp cert.key /etc/ssl/private/
We'll set these directives on a VirtualHost listening on 443 - remember, the newroot.pem root certificate didn't even exist when cert.pem was generated and signed.
SSLEngine on
SSLCertificateFile /etc/ssl/certs/cert.pem
SSLCertificateKeyFile /etc/ssl/private/cert.key
SSLCertificateChainFile /etc/ssl/certs/newroot.pem
Let's check out how openssl sees it:
# openssl s_client -showcerts -CAfile newroot.pem -connect localhost:443
Certificate chain
0 s:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=server.lan
i:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
1 s:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
i:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
-----BEGIN CERTIFICATE-----
MIICHzCCAYgCCQCapHvpKw4sMjANBgkqhkiG9w0BAQUFADBUMQswCQYDVQQGEwJB
...
-----END CERTIFICATE-----
(this should match the actual contents of newroot.pem)
...
Verify return code: 0 (ok)
Ok, and how about a browser using MS's crypto API? Gotta trust the root, first, then it's all good, with the new root's serial number:
And, we should still be working with the old root, too. Switch Apache's config around:
SSLEngine on
SSLCertificateFile /etc/ssl/certs/cert.pem
SSLCertificateKeyFile /etc/ssl/private/cert.key
SSLCertificateChainFile /etc/ssl/certs/origroot.pem
Do a full restart on Apache, a reload won't switch the certs properly.
# openssl s_client -showcerts -CAfile origroot.pem -connect localhost:443
Certificate chain
0 s:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=server.lan
i:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
1 s:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
i:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/CN=root
-----BEGIN CERTIFICATE-----
MIIC3jCCAkegAwIBAgIJAMBnFsCKa1kdMA0GCSqGSIb3DQEBBQUAMFQxCzAJBgNV
...
-----END CERTIFICATE-----
(this should match the actual contents of origroot.pem)
...
Verify return code: 0 (ok)
And, with the MS crypto API browser, Apache's presenting the old root, but the new root's still in the computer's trusted root store. It'll automatically find it and validate the cert against the trusted (new) root, despite Apache presenting a different chain (the old root). After stripping the new root from trusted roots and adding the original root cert, all is well:
So, that's it! Keep the same private key when you renew, swap in the new trusted root, and it pretty much all just works. Good luck!
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