'Get one of the keys for authentication/encryption.
@type c: L{bytes}
@param c: The letter identifying which key this is.
@type sharedSecret: L{bytes}
@param sharedSecret: The shared secret K.
@type exchangeHash: L{bytes}
@param exchangeHash: The hash H from key exchange.
@rtype: L{bytes}
@return: The derived key.'

def _getKey(self, c, sharedSecret, exchangeHash):

    hashProcessor = _kex.getHashProcessor(self.kexAlg)
    k1 = hashProcessor((((sharedSecret + exchangeHash) + c) + self.sessionID))
    k1 = k1.digest()
    k2 = hashProcessor(((sharedSecret + exchangeHash) + k1)).digest()
    return (k1 + k2)
