Timing leak example

timing_leak.py

import timeit

TOKEN = b"super_secret_token"


# Which one is better? 
def is_authorized_one(provided_token: bytes, expected_token: bytes) -> bool:
  return provided_token == expected_token

def is_authorized_two(provided_token: bytes, expected_token: bytes) -> bool:

   if len(provided_token) != len(expected_token):
       return False
   result = 0
   for i in range(len(provided_token)):
       result |= provided_token[i] ^ expected_token[i]
   return result == 0
  

# check they're working when correct
assert is_authorized_one(TOKEN, TOKEN) == True
assert is_authorized_two(TOKEN, TOKEN) == True

# check they're working when incorrect
assert is_authorized_one(b'bad_token', TOKEN) == False
assert is_authorized_two(b'bad_token', TOKEN) == False


# time the function calls
print("Same length, bad token:")
normal_compare = timeit.timeit(
    stmt="is_authorized_one(b'SuPeR_SeCrEt_ToKen', TOKEN)",
    setup="from __main__ import is_authorized_one, TOKEN",
    number=1000
)
print(f"  Normal: {normal_compare}")

constant_time = timeit.timeit(
    stmt="is_authorized_two(b'SuPeR_SeCrEt_ToKen', TOKEN)",
    setup="from __main__ import is_authorized_two, TOKEN",
    number=1000)
print(f"  Constant time: {constant_time}")
print(normal_compare / constant_time)

print()

print("Same length, good token:")
normal_compare = timeit.timeit(
    stmt="is_authorized_one(b'super_secret_token', TOKEN)",
    setup="from __main__ import is_authorized_one, TOKEN",
    number=1000
)
print(f"  Normal: {normal_compare}")
constant_time = timeit.timeit(
    stmt="is_authorized_two(b'super_secret_token', TOKEN)",
    setup="from __main__ import is_authorized_two, TOKEN",
    number=1000)
print(f"  Constant time: {constant_time}")
print(normal_compare / constant_time)