加密X509证书公钥

Question

All, I am working on IDP initiated web sso for a project and the Service Provider has this requirement to RSA encrypt the AES-128 symmetric key(ECB with PKCS1 padding) and Base64 encode that before adding this to the AttributeStatement section of SAML 2.0 response .My understanding is RSA encryption is asymmetric and the key we have to encrypt is the Service Providers public key.But we dont have SP's public certificate ,so I decided to RSA encrypt our X509Certicate's public key

This is the code I have and it throws a Bad Length Cryptographic exception on calling the Encrypt method on RSACryptoServiceProvider class.Does anyone know what the issue?

using(var rsa = cert.PublicKey.Key as RSACryptoServiceProvider)
            {
                rsa.KeySize = 1024;                
                byte[] encryptedKey = rsa.Encrypt(cert.GetPublicKey(),false);                
                encodedPublicKey =  Convert.ToBase64String(encryptedKey);
            }

Answer 1

The amount of data that you are able to encrypt using RSA public key must be (I don't know the exact equation) less than the length of the public key.

In case of encrypting AES-128 you will be encrypting 128 bits.