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 = "".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)