rsax509m2cryptopkcs#1

How to convert from X509 to PKCS1 encoded RSA key

For context, I'm trying to use in M2Crypto in place of Python-RSA, but this question isn't really language or library specific.

If I have an RSA public key in X509 format (starts with ----BEGIN PUBLIC KEY---), what do I need to do to convert it to PKCS1 (starts with ----BEGIN RSA PUBLIC KEY----) format?

This thread seems to describe going in the opposite direction.

Solution

  • Essenitally you need OpenSSL (Linux application). With OpenSSL you can run following command to convert from X509 to PKCS1:

    openssl rsa -pubin -in x509_public.key  -RSAPublicKey_out > rsa_public.key

    I had to extract a SSL-Certificate and the PKCS1-Keys from a PFX-file. To do so I used following commands:

    openssl pkcs12 -in pfx_file.pfx -clcerts -nokeys -out certificate.cer

    (Extracting the certificate)

    openssl pkcs12 -in pfx_file.pfx -nocerts -nodes -out rsa_pair.key

    (Extracting the RSA keypair)

    openssl rsa -in rsa_pair.key -out rsa_private.key

    (Extracting the RSA private key)

    openssl rsa -in rsa_pair.key -pubout -out x509_public.key

    (Extracting the "wrong" X509 Public Key)

    openssl rsa -pubin -in x509_public.key  -RSAPublicKey_out > rsa_public.key

    (Finally extracting the RSA-PKCS1 public key)

    There are probably some better ways, but I searched a long time to get here. I hope I could save others from this long searching through the web.