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How do I confirm that a private key matches a CSR and certificate?

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.

Note: There are multiple third-party online tools that allow you to check a match between a certificate and a private key. However, for the sake of security, it is generally preferable if any operations involving your private key are done on the computer where it is installed. Remember that if your private key is exposed, your website’s security is compromised. Bearing this in mind, you should exercise all due caution when handing your private key over the Internet to a third party for examination.
If you have any questions or need assistance, please contact us at Support@SSL.com, 1-SSL-Certificate (1-775-237-8434) or just use the Live Chat option at the bottom right of this page. And, as always, thank you for choosing SSL.com!

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