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 trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])
   7 trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(256)])
   8
   9 # The size of the digests returned by HMAC depends on the underlying
  10 # hashing module used.  Use digest_size from the instance of HMAC instead.
  11 digest_size = None
  12
  13 # A unique object passed by HMAC.copy() to the HMAC constructor, in order
  14 # that the latter return very quickly.  HMAC("") in contrast is quite
  15 # expensive.
  16 _secret_backdoor_key = []
  17
  18 class HMAC:
  19     """RFC2104 HMAC class.
  20
  21     This supports the API for Cryptographic Hash Functions (PEP 247).
  22     """
  23     blocksize = 64  # 512-bit HMAC; can be changed in subclasses.
  24
  25     def __init__(self, key, msg = None, digestmod = None):
  26         """Create a new HMAC object.
  27
  28         key:       key for the keyed hash object.
  29         msg:       Initial input for the hash, if provided.
  30         digestmod: A module supporting PEP 247.  *OR*
  31                    A hashlib constructor returning a new hash object.
  32                    Defaults to hashlib.md5.
  33         """
  34
  35         if key is _secret_backdoor_key: # cheap
  36             return
  37
  38         if digestmod is None:
  39             import hashlib
  40             digestmod = hashlib.md5
  41
  42         if callable(digestmod):
  43             self.digest_cons = digestmod
  44         else:
  45             self.digest_cons = lambda d='': digestmod.new(d)
  46
  47         self.outer = self.digest_cons()
  48         self.inner = self.digest_cons()
  49         self.digest_size = self.inner.digest_size
  50
  51         blocksize = self.blocksize
  52         if len(key) > blocksize:
  53             key = self.digest_cons(key).digest()
  54
  55         key = key + chr(0) * (blocksize - len(key))
  56         self.outer.update(key.translate(trans_5C))
  57         self.inner.update(key.translate(trans_36))
  58         if msg is not None:
  59             self.update(msg)
  60
  61 ##    def clear(self):
  62 ##        raise NotImplementedError, "clear() method not available in HMAC."
  63
  64     def update(self, msg):
  65         """Update this hashing object with the string msg.
  66         """
  67         self.inner.update(msg)
  68
  69     def copy(self):
  70         """Return a separate copy of this hashing object.
  71
  72         An update to this copy won't affect the original object.
  73         """
  74         other = self.__class__(_secret_backdoor_key)
  75         other.digest_cons = self.digest_cons
  76         other.digest_size = self.digest_size
  77         other.inner = self.inner.copy()
  78         other.outer = self.outer.copy()
  79         return other
  80
  81     def _current(self):
  82         """Return a hash object for the current state.
  83
  84         To be used only internally with digest() and hexdigest().
  85         """
  86         h = self.outer.copy()
  87         h.update(self.inner.digest())
  88         return h
  89
  90     def digest(self):
  91         """Return the hash value of this hashing object.
  92
  93         This returns a string containing 8-bit data.  The object is
  94         not altered in any way by this function; you can continue
  95         updating the object after calling this function.
  96         """
  97         h = self._current()
  98         return h.digest()
  99
 100     def hexdigest(self):
 101         """Like digest(), but returns a string of hexadecimal digits instead.
 102         """
 103         h = self._current()
 104         return h.hexdigest()
 105
 106 def new(key, msg = None, digestmod = None):
 107     """Create a new hashing object and return it.
 108
 109     key: The starting key for the hash.
 110     msg: if available, will immediately be hashed into the object's starting
 111     state.
 112
 113     You can now feed arbitrary strings into the object using its update()
 114     method, and can ask for the hash value at any time by calling its digest()
 115     method.
 116     """
 117     return HMAC(key, msg, digestmod)