"""HMAC (Keyed-Hashing for Message Authentication) Python module.

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Implements the HMAC algorithm as described by RFC 2104.

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"""

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import warnings as _warnings

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from operator import _compare_digest as compare_digest

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trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])

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trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(256)])

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# The size of the digests returned by HMAC depends on the underlying

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# hashing module used. Use digest_size from the instance of HMAC instead.

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digest_size = None

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# A unique object passed by HMAC.copy() to the HMAC constructor, in order

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# that the latter return very quickly. HMAC("") in contrast is quite

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# expensive.

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_secret_backdoor_key = []

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class HMAC:

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"""RFC 2104 HMAC class. Also complies with RFC 4231.

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This supports the API for Cryptographic Hash Functions (PEP 247).

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"""

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blocksize = 64 # 512-bit HMAC; can be changed in subclasses.

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def __init__(self, key, msg = None, digestmod = None):

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"""Create a new HMAC object.

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key: key for the keyed hash object.

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msg: Initial input for the hash, if provided.

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digestmod: A module supporting PEP 247. *OR*

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A hashlib constructor returning a new hash object.

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Defaults to hashlib.md5.

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"""

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if key is _secret_backdoor_key: # cheap

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return

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if digestmod is None:

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import hashlib

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digestmod = hashlib.md5

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if hasattr(digestmod, '__call__'):

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self.digest_cons = digestmod

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else:

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

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self.outer = self.digest_cons()

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self.inner = self.digest_cons()

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self.digest_size = self.inner.digest_size

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

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blocksize = self.inner.block_size

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if blocksize < 16:

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# Very low blocksize, most likely a legacy value like

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

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

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

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RuntimeWarning, 2)

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blocksize = self.blocksize

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else:

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

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

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RuntimeWarning, 2)

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blocksize = self.blocksize

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if len(key) > blocksize:

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

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

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

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

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if msg is not None:

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self.update(msg)

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## def clear(self):

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

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def update(self, msg):

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

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"""

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self.inner.update(msg)

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def copy(self):

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

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

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"""

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other = self.__class__(_secret_backdoor_key)

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other.digest_cons = self.digest_cons

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other.digest_size = self.digest_size

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other.inner = self.inner.copy()

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other.outer = self.outer.copy()

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return other

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def _current(self):

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

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

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"""

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h = self.outer.copy()

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

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return h

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def digest(self):

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

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

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

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updating the object after calling this function.

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"""

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h = self._current()

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return h.digest()

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def hexdigest(self):

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

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"""

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h = self._current()

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return h.hexdigest()

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

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

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key: The starting key for the hash.

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

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state.

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

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

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method.

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"""

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return HMAC(key, msg, digestmod)

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