380 lines
13 KiB
Python
380 lines
13 KiB
Python
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# -*- coding: utf-8 -*-
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#
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# PublicKey/DSA.py : DSA signature primitive
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#
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# Written in 2008 by Dwayne C. Litzenberger <dlitz@dlitz.net>
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#
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# ===================================================================
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# The contents of this file are dedicated to the public domain. To
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# the extent that dedication to the public domain is not available,
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# everyone is granted a worldwide, perpetual, royalty-free,
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# non-exclusive license to exercise all rights associated with the
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# contents of this file for any purpose whatsoever.
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# No rights are reserved.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# SOFTWARE.
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# ===================================================================
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"""DSA public-key signature algorithm.
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DSA_ is a widespread public-key signature algorithm. Its security is
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based on the discrete logarithm problem (DLP_). Given a cyclic
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group, a generator *g*, and an element *h*, it is hard
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to find an integer *x* such that *g^x = h*. The problem is believed
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to be difficult, and it has been proved such (and therefore secure) for
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more than 30 years.
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The group is actually a sub-group over the integers modulo *p*, with *p* prime.
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The sub-group order is *q*, which is prime too; it always holds that *(p-1)* is a multiple of *q*.
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The cryptographic strength is linked to the magnitude of *p* and *q*.
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The signer holds a value *x* (*0<x<q-1*) as private key, and its public
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key (*y* where *y=g^x mod p*) is distributed.
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In 2012, a sufficient size is deemed to be 2048 bits for *p* and 256 bits for *q*.
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For more information, see the most recent ECRYPT_ report.
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DSA is reasonably secure for new designs.
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The algorithm can only be used for authentication (digital signature).
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DSA cannot be used for confidentiality (encryption).
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The values *(p,q,g)* are called *domain parameters*;
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they are not sensitive but must be shared by both parties (the signer and the verifier).
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Different signers can share the same domain parameters with no security
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concerns.
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The DSA signature is twice as big as the size of *q* (64 bytes if *q* is 256 bit
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long).
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This module provides facilities for generating new DSA keys and for constructing
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them from known components. DSA keys allows you to perform basic signing and
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verification.
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>>> from Crypto.Random import random
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>>> from Crypto.PublicKey import DSA
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>>> from Crypto.Hash import SHA
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>>>
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>>> message = "Hello"
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>>> key = DSA.generate(1024)
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>>> h = SHA.new(message).digest()
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>>> k = random.StrongRandom().randint(1,key.q-1)
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>>> sig = key.sign(h,k)
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>>> ...
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>>> if key.verify(h,sig):
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>>> print "OK"
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>>> else:
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>>> print "Incorrect signature"
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.. _DSA: http://en.wikipedia.org/wiki/Digital_Signature_Algorithm
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.. _DLP: http://www.cosic.esat.kuleuven.be/publications/talk-78.pdf
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.. _ECRYPT: http://www.ecrypt.eu.org/documents/D.SPA.17.pdf
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"""
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__revision__ = "$Id$"
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__all__ = ['generate', 'construct', 'error', 'DSAImplementation', '_DSAobj']
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import sys
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if sys.version_info[0] == 2 and sys.version_info[1] == 1:
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from Crypto.Util.py21compat import *
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from Crypto.PublicKey import _DSA, _slowmath, pubkey
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from Crypto import Random
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try:
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from Crypto.PublicKey import _fastmath
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except ImportError:
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_fastmath = None
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class _DSAobj(pubkey.pubkey):
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"""Class defining an actual DSA key.
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:undocumented: __getstate__, __setstate__, __repr__, __getattr__
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"""
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#: Dictionary of DSA parameters.
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#:
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#: A public key will only have the following entries:
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#:
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#: - **y**, the public key.
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#: - **g**, the generator.
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#: - **p**, the modulus.
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#: - **q**, the order of the sub-group.
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#:
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#: A private key will also have:
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#:
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#: - **x**, the private key.
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keydata = ['y', 'g', 'p', 'q', 'x']
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def __init__(self, implementation, key):
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self.implementation = implementation
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self.key = key
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def __getattr__(self, attrname):
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if attrname in self.keydata:
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# For backward compatibility, allow the user to get (not set) the
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# DSA key parameters directly from this object.
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return getattr(self.key, attrname)
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else:
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raise AttributeError("%s object has no %r attribute" % (self.__class__.__name__, attrname,))
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def sign(self, M, K):
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"""Sign a piece of data with DSA.
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:Parameter M: The piece of data to sign with DSA. It may
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not be longer in bit size than the sub-group order (*q*).
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:Type M: byte string or long
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:Parameter K: A secret number, chosen randomly in the closed
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range *[1,q-1]*.
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:Type K: long (recommended) or byte string (not recommended)
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:attention: selection of *K* is crucial for security. Generating a
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random number larger than *q* and taking the modulus by *q* is
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**not** secure, since smaller values will occur more frequently.
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Generating a random number systematically smaller than *q-1*
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(e.g. *floor((q-1)/8)* random bytes) is also **not** secure. In general,
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it shall not be possible for an attacker to know the value of `any
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bit of K`__.
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:attention: The number *K* shall not be reused for any other
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operation and shall be discarded immediately.
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:attention: M must be a digest cryptographic hash, otherwise
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an attacker may mount an existential forgery attack.
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:Return: A tuple with 2 longs.
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.. __: http://www.di.ens.fr/~pnguyen/pub_NgSh00.htm
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"""
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return pubkey.pubkey.sign(self, M, K)
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def verify(self, M, signature):
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"""Verify the validity of a DSA signature.
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:Parameter M: The expected message.
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:Type M: byte string or long
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:Parameter signature: The DSA signature to verify.
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:Type signature: A tuple with 2 longs as return by `sign`
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:Return: True if the signature is correct, False otherwise.
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"""
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return pubkey.pubkey.verify(self, M, signature)
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def _encrypt(self, c, K):
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raise TypeError("DSA cannot encrypt")
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def _decrypt(self, c):
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raise TypeError("DSA cannot decrypt")
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def _blind(self, m, r):
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raise TypeError("DSA cannot blind")
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def _unblind(self, m, r):
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raise TypeError("DSA cannot unblind")
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def _sign(self, m, k):
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return self.key._sign(m, k)
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def _verify(self, m, sig):
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(r, s) = sig
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return self.key._verify(m, r, s)
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def has_private(self):
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return self.key.has_private()
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def size(self):
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return self.key.size()
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def can_blind(self):
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return False
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def can_encrypt(self):
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return False
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def can_sign(self):
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return True
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def publickey(self):
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return self.implementation.construct((self.key.y, self.key.g, self.key.p, self.key.q))
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def __getstate__(self):
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d = {}
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for k in self.keydata:
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try:
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d[k] = getattr(self.key, k)
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except AttributeError:
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pass
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return d
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def __setstate__(self, d):
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if not hasattr(self, 'implementation'):
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self.implementation = DSAImplementation()
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t = []
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for k in self.keydata:
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if k not in d:
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break
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t.append(d[k])
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self.key = self.implementation._math.dsa_construct(*tuple(t))
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def __repr__(self):
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attrs = []
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for k in self.keydata:
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if k == 'p':
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attrs.append("p(%d)" % (self.size()+1,))
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elif hasattr(self.key, k):
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attrs.append(k)
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if self.has_private():
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attrs.append("private")
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# PY3K: This is meant to be text, do not change to bytes (data)
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return "<%s @0x%x %s>" % (self.__class__.__name__, id(self), ",".join(attrs))
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class DSAImplementation(object):
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"""
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A DSA key factory.
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This class is only internally used to implement the methods of the
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`Crypto.PublicKey.DSA` module.
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"""
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def __init__(self, **kwargs):
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"""Create a new DSA key factory.
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:Keywords:
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use_fast_math : bool
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Specify which mathematic library to use:
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- *None* (default). Use fastest math available.
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- *True* . Use fast math.
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- *False* . Use slow math.
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default_randfunc : callable
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Specify how to collect random data:
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- *None* (default). Use Random.new().read().
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- not *None* . Use the specified function directly.
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:Raise RuntimeError:
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When **use_fast_math** =True but fast math is not available.
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"""
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use_fast_math = kwargs.get('use_fast_math', None)
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if use_fast_math is None: # Automatic
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if _fastmath is not None:
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self._math = _fastmath
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else:
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self._math = _slowmath
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elif use_fast_math: # Explicitly select fast math
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if _fastmath is not None:
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self._math = _fastmath
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else:
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raise RuntimeError("fast math module not available")
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else: # Explicitly select slow math
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self._math = _slowmath
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self.error = self._math.error
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# 'default_randfunc' parameter:
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# None (default) - use Random.new().read
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# not None - use the specified function
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self._default_randfunc = kwargs.get('default_randfunc', None)
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self._current_randfunc = None
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def _get_randfunc(self, randfunc):
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if randfunc is not None:
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return randfunc
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elif self._current_randfunc is None:
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self._current_randfunc = Random.new().read
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return self._current_randfunc
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def generate(self, bits, randfunc=None, progress_func=None):
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"""Randomly generate a fresh, new DSA key.
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:Parameters:
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bits : int
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Key length, or size (in bits) of the DSA modulus
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*p*.
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It must be a multiple of 64, in the closed
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interval [512,1024].
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randfunc : callable
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Random number generation function; it should accept
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a single integer N and return a string of random data
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N bytes long.
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If not specified, a new one will be instantiated
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from ``Crypto.Random``.
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progress_func : callable
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Optional function that will be called with a short string
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containing the key parameter currently being generated;
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it's useful for interactive applications where a user is
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waiting for a key to be generated.
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:attention: You should always use a cryptographically secure random number generator,
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such as the one defined in the ``Crypto.Random`` module; **don't** just use the
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current time and the ``random`` module.
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:Return: A DSA key object (`_DSAobj`).
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:Raise ValueError:
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When **bits** is too little, too big, or not a multiple of 64.
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"""
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# Check against FIPS 186-2, which says that the size of the prime p
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# must be a multiple of 64 bits between 512 and 1024
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for i in (0, 1, 2, 3, 4, 5, 6, 7, 8):
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if bits == 512 + 64*i:
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return self._generate(bits, randfunc, progress_func)
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# The March 2006 draft of FIPS 186-3 also allows 2048 and 3072-bit
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# primes, but only with longer q values. Since the current DSA
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# implementation only supports a 160-bit q, we don't support larger
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# values.
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raise ValueError("Number of bits in p must be a multiple of 64 between 512 and 1024, not %d bits" % (bits,))
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def _generate(self, bits, randfunc=None, progress_func=None):
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rf = self._get_randfunc(randfunc)
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obj = _DSA.generate_py(bits, rf, progress_func) # TODO: Don't use legacy _DSA module
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key = self._math.dsa_construct(obj.y, obj.g, obj.p, obj.q, obj.x)
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return _DSAobj(self, key)
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def construct(self, tup):
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"""Construct a DSA key from a tuple of valid DSA components.
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The modulus *p* must be a prime.
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The following equations must apply:
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- p-1 = 0 mod q
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- g^x = y mod p
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- 0 < x < q
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- 1 < g < p
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:Parameters:
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tup : tuple
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A tuple of long integers, with 4 or 5 items
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in the following order:
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1. Public key (*y*).
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2. Sub-group generator (*g*).
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3. Modulus, finite field order (*p*).
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4. Sub-group order (*q*).
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5. Private key (*x*). Optional.
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:Return: A DSA key object (`_DSAobj`).
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"""
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key = self._math.dsa_construct(*tup)
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return _DSAobj(self, key)
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_impl = DSAImplementation()
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generate = _impl.generate
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construct = _impl.construct
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error = _impl.error
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# vim:set ts=4 sw=4 sts=4 expandtab:
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