How to turn a X509 Certificate in to a Certificate Signing Request
Steps for taking an existing X509 Certificate and turning it in to a CSR
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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