From 2f6c124e127b5dd98723e7e75a9825c4ed8bd5c7 Mon Sep 17 00:00:00 2001
 From: Paul Howarth <paul@city-fan.org>
 Date: Fri, 23 Feb 2018 13:03:13 +0000
 Subject: [PATCH] Backport of fix for CVE-2018-6594 from pycryptodome
 
 When creating ElGamal keys, the generator wasn't a square residue: ElGamal
 encryption done with those keys cannot be secure under the DDH assumption.
 
 More details:
 - https://github.com/TElgamal/attack-on-pycrypto-elgamal
 - https://github.com/Legrandin/pycryptodome/issues/90
 - https://github.com/dlitz/pycrypto/issues/253
 
 This commit is a backport to pycrypto of Legrandin/pycryptodome@99c27a3b
 Thanks to Weikeng Chen.
 ---
  lib/Crypto/PublicKey/ElGamal.py | 30 +++++++++++++++---------------
  1 file changed, 15 insertions(+), 15 deletions(-)
 
 diff --git a/lib/Crypto/PublicKey/ElGamal.py b/lib/Crypto/PublicKey/ElGamal.py
 index 0ab07fc8..064e42bf 100644
 --- a/lib/Crypto/PublicKey/ElGamal.py
 +++ b/lib/Crypto/PublicKey/ElGamal.py
 @@ -154,33 +154,33 @@ def generate(bits, randfunc, progress_func=None):
          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')