In RSA public-key cryptography, the private and public keys in a pair are mathematically related in that they share the same modulus. The length of the modulus, expressed in bits, is the key length.
To confirm that a particular private key matches the public key contained in a certificate signing request (CSR) and certificate, one must confirm that the moduli of both keys are identical. This can be done straightforwardly with OpenSSL on Linux/Unix, macOS, or Windows (with Windows 10’s Linux subsystem or Cygwin), as follows:
To view the md5 hash of the modulus of the private key:
$ openssl rsa -noout -modulus -in mykey.key | openssl md5
To view the md5 hash of the modulus of the CSR:
$ openssl req -noout -modulus -in mycsr.csr | openssl md5
To view the md5 hash of the modulus of the certificate:
$ openssl x509 -noout -modulus -in mycert.crt | openssl md5
If all three hashes match, the CSR, certificate, and private key are compatible. You can use diff3 to compare the moduli from all three files at once:
$ openssl req -noout -modulus -in mycsr.csr > csr-mod.txt $ openssl x509 -noout -modulus -in mycert.crt > cert-mod.txt $ openssl rsa -noout -modulus -in mykey.key > privkey-mod.txt $ diff3 csr-mod.txt cert-mod.txt privkey-mod.txt
If all three files are identical, diff3 will produce no output. If you only wish to compare two files (e.g. the certificate and the private key), you can just use diff:
$ diff cert-mod.txt privkey-mod.txt
Again, diff will produce no output if the files are identical.