--- a/lib/Crypto/PublicKey/ElGamal.py +++ b/lib/Crypto/PublicKey/ElGamal.py @@ -153,33 +153,33 @@ def generate(bits, randfunc, progress_fu if number.isPrime(obj.p, randfunc=randfunc): break # Generate generator g - # See Algorithm 4.80 in Handbook of Applied Cryptography - # Note that the order of the group is n=p-1=2q, where q is prime if progress_func: progress_func('g\n') while 1: + # Choose a square residue; it will generate a cyclic group of order q. + obj.g = pow(number.getRandomRange(2, obj.p, randfunc), 2, obj.p) + # We must avoid g=2 because of Bleichenbacher's attack described # in "Generating ElGamal signatures without knowning the secret key", # 1996 - # - obj.g = number.getRandomRange(3, obj.p, randfunc) - safe = 1 - if pow(obj.g, 2, obj.p)==1: - safe=0 - if safe and pow(obj.g, q, obj.p)==1: - safe=0 + if obj.g in (1, 2): + continue + # Discard g if it divides p-1 because of the attack described # in Note 11.67 (iii) in HAC - if safe and divmod(obj.p-1, obj.g)[1]==0: - safe=0 + if (obj.p - 1) % obj.g == 0: + continue + # g^{-1} must not divide p-1 because of Khadir's attack # described in "Conditions of the generator for forging ElGamal # signature", 2011 ginv = number.inverse(obj.g, obj.p) - if safe and divmod(obj.p-1, ginv)[1]==0: - safe=0 - if safe: - break + if (obj.p - 1) % ginv == 0: + continue + + # Found + break + # Generate private key x if progress_func: progress_func('x\n')