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/Lib/hmac.py

http://unladen-swallow.googlecode.com/
Python | 133 lines | 127 code | 2 blank | 4 comment | 2 complexity | 5c2d98e3e4cd83d43377fed3590373a6 MD5 | raw file
  1"""HMAC (Keyed-Hashing for Message Authentication) Python module.
  2
  3Implements the HMAC algorithm as described by RFC 2104.
  4"""
  5
  6import warnings as _warnings
  7
  8trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])
  9trans_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.
 13digest_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
 20class 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
122def 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)