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 = "".join ([chr (x ^ 0x5C) for x in xrange(256)])

     9 trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(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 

    37         if key is _secret_backdoor_key: # cheap

    38             return

    39 

    40         if digestmod is None:

    41             import hashlib

    42             digestmod = hashlib.md5

    43 

    44         if hasattr(digestmod, '__call__'):

    45             self.digest_cons = digestmod

    46         else:

    47             self.digest_cons = lambda d='': digestmod.new(d)

    48 

    49         self.outer = self.digest_cons()

    50         self.inner = self.digest_cons()

    51         self.digest_size = self.inner.digest_size

    52 

    53         if hasattr(self.inner, 'block_size'):

    54             blocksize = self.inner.block_size

    55             if blocksize < 16:

    56                 # Very low blocksize, most likely a legacy value like

    57                 # Lib/sha.py and Lib/md5.py have.

    58                 _warnings.warn('block_size of %d seems too small; using our '

    59                                'default of %d.' % (blocksize, self.blocksize),

    60                                RuntimeWarning, 2)

    61                 blocksize = self.blocksize

    62         else:

    63             _warnings.warn('No block_size attribute on given digest object; '

    64                            'Assuming %d.' % (self.blocksize),

    65                            RuntimeWarning, 2)

    66             blocksize = self.blocksize

    67 

    68         if len(key) > blocksize:

    69             key = self.digest_cons(key).digest()

    70 

    71         key = key + chr(0) * (blocksize - len(key))

    72         self.outer.update(key.translate(trans_5C))

    73         self.inner.update(key.translate(trans_36))

    74         if msg is not None:

    75             self.update(msg)

    76 

    77 ##    def clear(self):

    78 ##        raise NotImplementedError, "clear() method not available in HMAC."

    79 

    80     def update(self, msg):

    81         """Update this hashing object with the string msg.

    82         """

    83         self.inner.update(msg)

    84 

    85     def copy(self):

    86         """Return a separate copy of this hashing object.

    87 

    88         An update to this copy won't affect the original object.

    89         """

    90         other = self.__class__(_secret_backdoor_key)

    91         other.digest_cons = self.digest_cons

    92         other.digest_size = self.digest_size

    93         other.inner = self.inner.copy()

    94         other.outer = self.outer.copy()

    95         return other

    96 

    97     def _current(self):

    98         """Return a hash object for the current state.

    99 

   100         To be used only internally with digest() and hexdigest().

   101         """

   102         h = self.outer.copy()

   103         h.update(self.inner.digest())

   104         return h

   105 

   106     def digest(self):

   107         """Return the hash value of this hashing object.

   108 

   109         This returns a string containing 8-bit data.  The object is

   110         not altered in any way by this function; you can continue

   111         updating the object after calling this function.

   112         """

   113         h = self._current()

   114         return h.digest()

   115 

   116     def hexdigest(self):

   117         """Like digest(), but returns a string of hexadecimal digits instead.

   118         """

   119         h = self._current()

   120         return h.hexdigest()

   121 

   122 def new(key, msg = None, digestmod = None):

   123     """Create a new hashing object and return it.

   124 

   125     key: The starting key for the hash.

   126     msg: if available, will immediately be hashed into the object's starting

   127     state.

   128 

   129     You can now feed arbitrary strings into the object using its update()

   130     method, and can ask for the hash value at any time by calling its digest()

   131     method.

   132     """

   133     return HMAC(key, msg, digestmod)