Static Client

cryptohack

Task: interactive DH (socket.cryptohack.org 13373) where Bob keeps a STATIC private exponent b; you first eavesdrop Alice<->Bob (p, g, A_alice, B, encrypted flag) then get to send Bob your own parameters. Solution: send {p, g=A_alice, A=dummy}; Bob computes and returns B' = g^b = A_alice^b mod p, which is exactly the Alice<->Bob shared secret, so derive key=sha1(str(B'))[:16] and AES-CBC decrypt the eavesdropped flag with NO discrete log.