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 def _strxor(s1, s2):
   7     """Utility method. XOR the two strings s1 and s2 (must have same length).
   8     """
   9     return "".join(map(lambda x, y: chr(ord(x) ^ ord(y)), s1, s2))
  10
  11 # The size of the digests returned by HMAC depends on the underlying
  12 # hashing module used.
  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     """RFC2104 HMAC class.
  22
  23     This supports the API for Cryptographic Hash Functions (PEP 247).
  24     """
  25
  26     def __init__(self, key, msg = None, digestmod = None):
  27         """Create a new HMAC object.
  28
  29         key:       key for the keyed hash object.
  30         msg:       Initial input for the hash, if provided.
  31         digestmod: A module supporting PEP 247.  *OR*
  32                    A hashlib constructor returning a new hash object.
  33                    Defaults to hashlib.md5.
  34         """
  35
  36         if key is _secret_backdoor_key: # cheap
  37             return
  38
  39         if digestmod is None:
  40             import hashlib
  41             digestmod = hashlib.md5
  42
  43         if callable(digestmod):
  44             self.digest_cons = digestmod
  45         else:
  46             self.digest_cons = lambda d='': digestmod.new(d)
  47
  48         self.outer = self.digest_cons()
  49         self.inner = self.digest_cons()
  50         self.digest_size = self.inner.digest_size
  51
  52         blocksize = 64
  53         ipad = "\x36" * blocksize
  54         opad = "\x5C" * blocksize
  55
  56         if len(key) > blocksize:
  57             key = self.digest_cons(key).digest()
  58
  59         key = key + chr(0) * (blocksize - len(key))
  60         self.outer.update(_strxor(key, opad))
  61         self.inner.update(_strxor(key, ipad))
  62         if msg is not None:
  63             self.update(msg)
  64
  65 ##    def clear(self):
  66 ##        raise NotImplementedError, "clear() method not available in HMAC."
  67
  68     def update(self, msg):
  69         """Update this hashing object with the string msg.
  70         """
  71         self.inner.update(msg)
  72
  73     def copy(self):
  74         """Return a separate copy of this hashing object.
  75
  76         An update to this copy won't affect the original object.
  77         """
  78         other = HMAC(_secret_backdoor_key)
  79         other.digest_cons = self.digest_cons
  80         other.digest_size = self.digest_size
  81         other.inner = self.inner.copy()
  82         other.outer = self.outer.copy()
  83         return other
  84
  85     def digest(self):
  86         """Return the hash value of this hashing object.
  87
  88         This returns a string containing 8-bit data.  The object is
  89         not altered in any way by this function; you can continue
  90         updating the object after calling this function.
  91         """
  92         h = self.outer.copy()
  93         h.update(self.inner.digest())
  94         return h.digest()
  95
  96     def hexdigest(self):
  97         """Like digest(), but returns a string of hexadecimal digits instead.
  98         """
  99         return "".join([hex(ord(x))[2:].zfill(2)
 100                         for x in tuple(self.digest())])
 101
 102 def new(key, msg = None, digestmod = None):
 103     """Create a new hashing object and return it.
 104
 105     key: The starting key for the hash.
 106     msg: if available, will immediately be hashed into the object's starting
 107     state.
 108
 109     You can now feed arbitrary strings into the object using its update()
 110     method, and can ask for the hash value at any time by calling its digest()
 111     method.
 112     """
 113     return HMAC(key, msg, digestmod)