diff options
| -rw-r--r-- | cryptography/fernet.py | 34 |
1 files changed, 22 insertions, 12 deletions
diff --git a/cryptography/fernet.py b/cryptography/fernet.py index f923911a..214b9374 100644 --- a/cryptography/fernet.py +++ b/cryptography/fernet.py @@ -4,6 +4,8 @@ import os import struct import time +import cffi + import six from cryptography.hazmat.primitives import padding, hashes @@ -15,6 +17,25 @@ class InvalidToken(Exception): pass +ffi = cffi.FFI() +ffi.cdef(""" +bool constant_time_compare(uint8_t *, size_t, uint8_t *, size_t); +""") +lib = ffi.verify(""" +#include <stdbool.h> + +bool constant_time_compare(uint8_t *a, size_t len_a, uint8_t *b, size_t len_b) { + if (len_a != len_b) { + return false; + } + int result = 0; + for (size_t i = 0; i < len_a; i++) { + result |= a[i] ^ b[i]; + } + return result == 0; +} +""") + class Fernet(object): def __init__(self, key): super(Fernet, self).__init__() @@ -75,7 +96,7 @@ class Fernet(object): h.update(data[:-32]) hmac = h.digest() - if not constant_time_compare(hmac, data[-32:]): + if not lib.constant_time_compare(hmac, len(hmac), data[-32:], 32): raise InvalidToken decryptor = BlockCipher( @@ -90,14 +111,3 @@ class Fernet(object): except ValueError: raise InvalidToken return unpadded - - -def constant_time_compare(a, b): - # TOOD: replace with a cffi function - assert isinstance(a, bytes) and isinstance(b, bytes) - if len(a) != len(b): - return False - result = 0 - for i in range(len(a)): - result |= six.indexbytes(a, i) ^ six.indexbytes(b, i) - return result == 0 |