Backport importlib to at least Python 2.5 by getting rid of use of str.format.
[python.git] / Lib / hmac.py
blob538810630cf3ad5a471082e74f0a1e7d990b10c1
1 """HMAC (Keyed-Hashing for Message Authentication) Python module.
3 Implements the HMAC algorithm as described by RFC 2104.
4 """
6 import warnings as _warnings
8 trans_5C = "".join ([chr (x ^ 0x5C) for x in xrange(256)])
9 trans_36 = "".join ([chr (x ^ 0x36) for x in xrange(256)])
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
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 = []
20 class HMAC:
21 """RFC 2104 HMAC class. Also complies with RFC 4231.
23 This supports the API for Cryptographic Hash Functions (PEP 247).
24 """
25 blocksize = 64 # 512-bit HMAC; can be changed in subclasses.
27 def __init__(self, key, msg = None, digestmod = None):
28 """Create a new HMAC object.
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 """
37 if key is _secret_backdoor_key: # cheap
38 return
40 if digestmod is None:
41 import hashlib
42 digestmod = hashlib.md5
44 if hasattr(digestmod, '__call__'):
45 self.digest_cons = digestmod
46 else:
47 self.digest_cons = lambda d='': digestmod.new(d)
49 self.outer = self.digest_cons()
50 self.inner = self.digest_cons()
51 self.digest_size = self.inner.digest_size
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
68 if len(key) > blocksize:
69 key = self.digest_cons(key).digest()
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)
77 ## def clear(self):
78 ## raise NotImplementedError, "clear() method not available in HMAC."
80 def update(self, msg):
81 """Update this hashing object with the string msg.
82 """
83 self.inner.update(msg)
85 def copy(self):
86 """Return a separate copy of this hashing object.
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
97 def _current(self):
98 """Return a hash object for the current state.
100 To be used only internally with digest() and hexdigest().
102 h = self.outer.copy()
103 h.update(self.inner.digest())
104 return h
106 def digest(self):
107 """Return the hash value of this hashing object.
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.
113 h = self._current()
114 return h.digest()
116 def hexdigest(self):
117 """Like digest(), but returns a string of hexadecimal digits instead.
119 h = self._current()
120 return h.hexdigest()
122 def new(key, msg = None, digestmod = None):
123 """Create a new hashing object and return it.
125 key: The starting key for the hash.
126 msg: if available, will immediately be hashed into the object's starting
127 state.
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.
133 return HMAC(key, msg, digestmod)