What metod KeyAgreement.generateSecret(String algorithm) does?

Question

We implemented Diffie-Hellman Key Exchange algorithm:

KeyAgreement aKeyAgree = KeyAgreement.getInstance("DH");

keyAgreement.init(myPrivateKey);
keyAgreement.doPhase(otherPublicKey)

Now we need to generate a secret to use for AES encryption. There is method generateSecret(String algorithm). I think I should call it with 'AES' argument.

But for DH I use 512-bit length public keys so the secret should be 512-bit length too. But AES allows 256-bit length keys as maximum. The plain method generateSecret() without parameters returns 512-bit DH secret. But what generateSecret(String) does? How it transform 512-bit secret to 256/128-bit AES key?

Answer 1

Diffie-Hellman is a key agreement protocol; AES is a symmetric encryption algorithm.

The no-argument generateSecret() call provides a key of the default size for the key agreement algorithm. Passing the algorithm name to generateSecret will give you a key with the appropriate size for your algorithm (i.e. shortened to 256 bits for AES).

I found a page that shows an example.

http://www.exampledepot.com/egs/javax.crypto/KeyAgree.html

I would expect you can just replace "DES" in the example with "AES" or whatever symmetric key algorithm you'd like to use.

Answer 2

This method just truncate generatedSecret().