Given two SSH2 keys how do I check that they belong to the same key pair in Java?

Question

I'm trying to find a way to validate that two SSH2 keys, one private and one public, belong to the same key pair. I have used JSch for loading and parsing the private key.

Update: A snippet that could show how to regenerate the public key from the private key (SSH2 RSA) would solve the problem.

Answer 1

You could do this with the BouncyCastle lightweight API.

For example:

InputStream in = new FileInputStream("path/to/private/key");
AsymmetricKeyParameter privateKey = PrivateKeyFactory.createKey(in);
RSAPrivateCrtKeyParameters rsaPrivateKey = (RSAPrivateCrtKeyParameters)privateKey;
BigInteger modulus = rsaPrivateKey.getModulus();
BigInteger publicExponent = rsaPrivateKey.getPublicExponent();
RSAKeyParameters publicKeyParams = new RSAKeyParameters(false, modulus, publicExponent);

The RSAKeyParameters class represents the actual key.

Hope that helps!