How to turn a X509 Certificate in to a Certificate Signing Request

With an existing X509 Certificate and it's corresponding private key, OpenSSL makes it simple to recreate the CSR that was used to generate the Certificate:

$ openssl x509 -x509toreq -in my.crt -out my.csr -signkey my.key

my.crt is your existing certificate and my.key is your existing key. The CSR (in PKCS#10 format) will be placed in my.csr

See Also:

• Verifying that a Private Key Matches a Certificate
• Verifying that a Certificate is issued by a CA