Lib/hmac.py

   1 """HMAC (Keyed-Hashing for Message Authentication) Python module.
   2
   3 Implements the HMAC algorithm as described by RFC 2104.
   4 """
   5
   6 import warnings as _warnings
   7
   8 trans_5C = bytes((x ^ 0x5C) for x in range(256))
   9 trans_36 = bytes((x ^ 0x36) for x in range(256))
  10
  11 # The size of the digests returned by HMAC depends on the underlying
  12 # hashing module used.  Use digest_size from the instance of HMAC instead.
  13 digest_size = None
  14
  15 # A unique object passed by HMAC.copy() to the HMAC constructor, in order
  16 # that the latter return very quickly.  HMAC("") in contrast is quite
  17 # expensive.
  18 _secret_backdoor_key = []
  19
  20 class HMAC:
  21     """RFC 2104 HMAC class.  Also complies with RFC 4231.
  22
  23     This supports the API for Cryptographic Hash Functions (PEP 247).
  24     """
  25     blocksize = 64  # 512-bit HMAC; can be changed in subclasses.
  26
  27     def __init__(self, key, msg = None, digestmod = None):
  28         """Create a new HMAC object.
  29
  30         key:       key for the keyed hash object.
  31         msg:       Initial input for the hash, if provided.
  32         digestmod: A module supporting PEP 247.  *OR*
  33                    A hashlib constructor returning a new hash object.
  34                    Defaults to hashlib.md5.
  35
  36         Note: key and msg must be bytes objects.
  37         """
  38
  39         if key is _secret_backdoor_key: # cheap
  40             return
  41
  42         if not isinstance(key, bytes):
  43             raise TypeError("expected bytes, but got %r" % type(key).__name__)
  44
  45         if digestmod is None:
  46             import hashlib
  47             digestmod = hashlib.md5
  48
  49         if hasattr(digestmod, '__call__'):
  50             self.digest_cons = digestmod
  51         else:
  52             self.digest_cons = lambda d=b'': digestmod.new(d)
  53
  54         self.outer = self.digest_cons()
  55         self.inner = self.digest_cons()
  56         self.digest_size = self.inner.digest_size
  57
  58         if hasattr(self.inner, 'block_size'):
  59             blocksize = self.inner.block_size
  60             if blocksize < 16:
  61                 # Very low blocksize, most likely a legacy value like
  62                 # Lib/sha.py and Lib/md5.py have.
  63                 _warnings.warn('block_size of %d seems too small; using our '
  64                                'default of %d.' % (blocksize, self.blocksize),
  65                                RuntimeWarning, 2)
  66                 blocksize = self.blocksize
  67         else:
  68             _warnings.warn('No block_size attribute on given digest object; '
  69                            'Assuming %d.' % (self.blocksize),
  70                            RuntimeWarning, 2)
  71             blocksize = self.blocksize
  72
  73         if len(key) > blocksize:
  74             key = self.digest_cons(key).digest()
  75
  76         key = key + bytes(blocksize - len(key))
  77         self.outer.update(key.translate(trans_5C))
  78         self.inner.update(key.translate(trans_36))
  79         if msg is not None:
  80             self.update(msg)
  81
  82 ##    def clear(self):
  83 ##        raise NotImplementedError, "clear() method not available in HMAC."
  84
  85     def update(self, msg):
  86         """Update this hashing object with the string msg.
  87         """
  88         if not isinstance(msg, bytes):
  89             raise TypeError("expected bytes, but got %r" % type(msg).__name__)
  90         self.inner.update(msg)
  91
  92     def copy(self):
  93         """Return a separate copy of this hashing object.
  94
  95         An update to this copy won't affect the original object.
  96         """
  97         other = self.__class__(_secret_backdoor_key)
  98         other.digest_cons = self.digest_cons
  99         other.digest_size = self.digest_size
 100         other.inner = self.inner.copy()
 101         other.outer = self.outer.copy()
 102         return other
 103
 104     def _current(self):
 105         """Return a hash object for the current state.
 106
 107         To be used only internally with digest() and hexdigest().
 108         """
 109         h = self.outer.copy()
 110         h.update(self.inner.digest())
 111         return h
 112
 113     def digest(self):
 114         """Return the hash value of this hashing object.
 115
 116         This returns a string containing 8-bit data.  The object is
 117         not altered in any way by this function; you can continue
 118         updating the object after calling this function.
 119         """
 120         h = self._current()
 121         return h.digest()
 122
 123     def hexdigest(self):
 124         """Like digest(), but returns a string of hexadecimal digits instead.
 125         """
 126         h = self._current()
 127         return h.hexdigest()
 128
 129 def new(key, msg = None, digestmod = None):
 130     """Create a new hashing object and return it.
 131
 132     key: The starting key for the hash.
 133     msg: if available, will immediately be hashed into the object's starting
 134     state.
 135
 136     You can now feed arbitrary strings into the object using its update()
 137     method, and can ask for the hash value at any time by calling its digest()
 138     method.
 139     """
 140     return HMAC(key, msg, digestmod)