OpenSSL: Check If Private Key Matches SSL Certificate & CSR

When you are dealing with lots of different SSL Certificates, it is quite easy to forget which certificate goes with which Private Key.

Or, for example, which CSR has been generated using which Private Key.

From the Linux command line, you can easily check whether an SSL Certificate or a CSR match a Private Key using the OpenSSL utility.

To make sure that the files are compatible, you can print and compare the values of the SSL Certificate modulus, the Private Key modulus and the CSR modulus.

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Check Compatibility

When you create a Private Key and CSR to obtain an SSL Certificate, OpenSSL generates some internal data called a modulus.

OpenSSL stores the modulus in the Private Key, as well as in the CSR and therefore in the SSL Certificate itself.

If you are using either the incorrect Private Key or the SSL Certificate – you will receive an error as follows: [error] Unable to configure RSA server Private Key [error] SSL Library Error: x509 certificate routines:X509_check_private_key:key values mismatch.

So if you got the similar error – it is time to check whatever your Private Key matches the SSL Certificate by comparing their modulus.

[Error] … key values mismatch: Your Private Key and SSL Certificate must contain the same modulus, otherwise the web-server won’t start.

Let’s print the values of the modulus of the Private Key, the SSL Certificate and the CSR with the conversion of them to md5 hashes to make the comparison more convenient.

Print the md5 hash of the SSL Certificate modulus:

$ openssl x509 -noout -modulus -in CERTIFICATE.crt | openssl md5

Print the md5 hash of the CSR modulus:

$ openssl req -noout -modulus -in CSR.csr | openssl md5

Print the md5 hash of the Private Key modulus:

$ openssl rsa -noout -modulus -in PRIVATEKEY.key | openssl md5

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If the md5 hashes are the same, then the files (SSL Certificate, Private Key and CSR) are compatible.