[openssl-users] RSA Digital Signing

[Matt Caswell]

On 27/01/2019 17:43, prithiraj das wrote:
> Hi All,
> 
> Using OpenSSL, I need to implement digital signing. My approach as of now is:
> 1)  At the sender side, generate the hash of the data using sha256.
> 2)  Encrypt the hash of the data using RSA Private key for the purpose of
> signing. Send this encrypted hash and the data from Step 1 to the receiverr side.
> 3)  At the receiver's end, Decrypt the signed data(encrypted hash) using the
> corresponding RSA Public key.
> 4)  Generate hash of the data and verify the decrypted content against this hash
> to verify the signature
> 
> I was thinking of using RSA_private_encrypt() method to do the signing and
> RSA_public_decrypt() method to decrypt the signed hash using the corresponding
> RSA public key. Would the above be a bad approach especially when it comes to
> using the methods mentioned ? Please recommend the methods to be used that would
> be better for the purpose of digital signing and verification using sha256 and
> RSA keys

Consider using the EVP_DigestSign*() functions, rather than the RSA_* ones. This
is the preferred way of doing this:

https://www.openssl.org/docs/man1.1.1/man3/EVP_DigestSign.html

This has the advantages of handling all of the digesting and padding issues for
you. It also gives you greater flexibility to change digest and/or signing
algorithms at a later point if you wish.

Matt