Прямо по ссылке, которую я только что дал:
RSA
We encode the public key using the DER-encoded PKIX format.
We encode the private key as a PKCS1 key using ASN.1 DER.
To sign a message, we first hash it with SHA-256 and then sign it using the RSASSA-PKCS1-V1.5-SIGN method, as originally defined in RSA PKCS#1 v1.5.