convert RSA public key to RSA DER

Question

Hello., I have id_rsa.pub key generated by ssh-keygen. I need to prigrammically convert id_rsa.pub to RSA DER formatted key.

How it is possible? Preferably ruby language but I would be glad for algorythm in any language.

Answer 1

If you use ssh-keygen to generate a key:

$ ssh-keygen

Then you can just use openssl to pull out the public key and write it in the DER format like this:

$ openssl rsa -in id_rsa -out pub.der -outform DER -pubout
writing RSA key

You can view the DER output as PEM like this:

$ openssl rsa -in pub.der -inform DER -pubin -text

I don't use Ruby, so I don't know how easy it is to use OpenSSL from Ruby.

Edit: I answered too quickly -- you wrote id_rsa.pub and you may not have the id_rsa itself. Another Stack Overflow question is for the reverse conversion, but the source code found there might help: Convert pem key to ssh-rsa format Once you have PEM you can use openssl to conver the PEM to DER.