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Table of Contents

• Get a certificate with a CRL
• Getting the certificate chain
• Combining the CRL and the Chain
• OpenSSL Verify
• Revoked certificate

This article shows you how to manually verfify a certificate against a CRL. CRL stands for Certificate Revocation List and is one way to validate a certificate status. It is an alternative to the OCSP, Online Certificate Status Protocol.

You can read more about CRL's on Wikipedia.

If you want to validate a certificate against an OCSP, see my article on that here.

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We will be using OpenSSL in this article. I'm using the following version:

$ openssl version
OpenSSL 1.0.2 22 Jan 2015

Get a certificate with a CRL

First we will need a certificate from a website. I'll be using Wikipedia as an example here. We can retreive this with the following openssl command:

openssl s_client -connect wikipedia.org:443 2>&1 < /dev/null | sed -n '/-----BEGIN/,/-----END/p'

Save this output to a file, for example, wikipedia.pem:

openssl s_client -connect wikipedia.org:443 2>&1 < /dev/null | sed -n '/-----BEGIN/,/-----END/p' > wikipedia.pem

Now, check if this certificate has an CRL URI:

openssl x509 -noout -text -in wikipedia.pem | grep -A 4 'X509v3 CRL Distribution Points'
X509v3 CRL Distribution Points: 
    Full Name:
      URI:http://crl.globalsign.com/gs/gsorganizationvalsha2g2.crl

If it does not give any output, the certificate has no CRL URI. You cannot valdiate it against a CRL.

Download the CRL:

wget -O crl.der http://crl.globalsign.com/gs/gsorganizationvalsha2g2.crl

The CRL will be in DER (binary) format. The OpenSSL command needs it in PEM (base64 encoded DER) format, so convert it:

openssl crl -inform DER -in crl.der -outform PEM -out crl.pem

Getting the certificate chain

It is required to have the certificate chain together with the certificate you want to validate. So, we need to get the certificate chain for our domain, wikipedia.org. Using the -showcerts option with openssl s_client, we can see all the certificates, including the chain:

openssl s_client -connect wikipedia.org:443 -showcerts 2>&1 < /dev/null

Results in a lot of output, but what we are interested in is the following:

 1 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance CA-3
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

As you can see, this is number 1. Number 0 is the certificate for Wikipedia, we already have that. If your site has more certificates in its chain, you will see more here. Save them all, in the order OpenSSL sends them (as in, first the one which directly issued your server certificate, then the one that issues that certificate and so on, with the root or most-root at the end of the file) to a file, named chain.pem.

You can use the following command to save all the certificates OpenSSL command returns to a file named chain.pem. See [this article for more information)[https://raymii.org/s/articles/OpenSSL - Get all certificates from a website in plain text.html).

OLDIFS=$IFS; IFS=':' certificates=$(openssl s_client -connect wikipedia.org:443 -showcerts -tlsextdebug -tls1 2>&1 </dev/null | sed -n '/-----BEGIN/,/-----END/ {/-----BEGIN/ s/^/:/; p}'); for certificate in ${certificates#:}; do echo $certificate | tee -a chain.pem ; done; IFS=$OLDIFS 

Combining the CRL and the Chain

The Openssl command needs both the certificate chain and the CRL, in PEM format concatenated together for the validation to work. You can omit the CRL, but then the CRL check will not work, it will just validate the certificate against the chain.

cat chain.pem crl.pem > crl_chain.pem

OpenSSL Verify

We now have all the data we need can validate the certificate.

$ openssl verify -crl_check -CAfile crl_chain.pem wikipedia.pem 
wikipedia.pem: OK

Above shows a good certificate status.

Revoked certificate

If you have a revoked certificate, you can also test it the same way as stated above. The response looks like this:

$ openssl verify -crl_check -CAfile crl_chain.pem revoked-test.pem 
revoked-test.pem: OU = Domain Control Validated, OU = PositiveSSL, CN = xs4all.nl
error 23 at 0 depth lookup:certificate revoked

You can test this using the certificate and chain on the Verisign revoked certificate test page: .

Tags: articles , certificate , crl , ocsp , openssl , shell , ssl , tls