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""" Stream Cryptosystems """
#***************************************************************************** # Copyright (C) 2007 David Kohel <kohel@maths.usyd.edu.au> # # This program is free software: you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation, either version 2 of the License, or # (at your option) any later version. # http://www.gnu.org/licenses/ #*****************************************************************************
""" Linear feedback shift register cryptosystem class """ """ Create a linear feedback shift cryptosystem.
INPUT: A string monoid over a binary alphabet.
OUTPUT:
EXAMPLES::
sage: E = LFSRCryptosystem(FiniteField(2)) sage: E LFSR cryptosystem over Finite Field of size 2
TESTS::
sage: E = LFSRCryptosystem(FiniteField(2)) sage: E == loads(dumps(E)) True
TODO: Implement LFSR cryptosystem for arbitrary rings. The current implementation is limited to the finite field of 2 elements only because of the dependence on binary strings. """ field = FiniteField(2) raise NotImplementedError("Not yet implemented.")
""" Create a LFSR cipher.
INPUT: A polynomial and initial state of the LFSR. """ raise TypeError("Argument key (= %s) must be a list of tuple of length 2" % key) raise TypeError("poly (= %s) must be a polynomial." % poly) raise TypeError("IS (= %s) must be an initial in the key space."%K) raise TypeError("The length of IS (= %s) must equal the degree of poly (= %s)" % (IS, poly))
r""" Return the string representation of this LFSR cryptosystem.
EXAMPLES::
sage: LFSRCryptosystem(FiniteField(2)) LFSR cryptosystem over Finite Field of size 2 """
S = self.cipher_domain() try: return S.encoding(M) except Exception: raise TypeError("Argument M = %s does not encode in the cipher domain" % M)
""" Shrinking generator cryptosystem class """ """ Create a shrinking generator cryptosystem.
INPUT: A string monoid over a binary alphabet.
OUTPUT:
EXAMPLES::
sage: E = ShrinkingGeneratorCryptosystem() sage: E Shrinking generator cryptosystem over Finite Field of size 2 """ raise NotImplementedError("Not yet implemented.")
""" Create a Shrinking generator cipher.
INPUT: A list or tuple consisting of two LFSR ciphers (e1,e2).
OUTPUT: The shrinking generator cipher with key stream generator e1 and decimating cipher e2. """ raise TypeError("Argument key (= %s) must be a list of tuple of length 2" % key) raise TypeError("The key (= (%s,%s)) must be a tuple of two LFSR ciphers." % key)
r""" Return the string representation of this shrinking generator cryptosystem.
EXAMPLES::
sage: ShrinkingGeneratorCryptosystem() Shrinking generator cryptosystem over Finite Field of size 2 """
S = self.cipher_domain() try: return S.encoding(M) except Exception: raise TypeError("Argument M = %s does not encode in the cipher domain" % M)
lbound=None, ubound=None, ntries=100): r""" The Blum-Blum-Shub (BBS) pseudorandom bit generator.
See the original paper by Blum, Blum and Shub [BBS1986]_. The BBS algorithm is also discussed in section 5.5.2 of [MvOV1996]_.
INPUT:
- ``length`` -- positive integer; the number of bits in the output pseudorandom bit sequence.
- ``seed`` -- (default: ``None``) if `p` and `q` are Blum primes, then ``seed`` is a quadratic residue in the multiplicative group `(\ZZ/n\ZZ)^{\ast}` where `n = pq`. If ``seed=None``, then the function would generate its own random quadratic residue in `(\ZZ/n\ZZ)^{\ast}`. If you provide a value for ``seed``, then it is your responsibility to ensure that the seed is a quadratic residue in the multiplicative group `(\ZZ/n\ZZ)^{\ast}`.
- ``p`` -- (default: ``None``) a large positive prime congruent to 3 modulo 4. Both ``p`` and ``q`` must be distinct. If ``p=None``, then a value for ``p`` will be generated, where ``0 < lower_bound <= p <= upper_bound``.
- ``q`` -- (default: ``None``) a large positive prime congruence to 3 modulo 4. Both ``p`` and ``q`` must be distinct. If ``q=None``, then a value for ``q`` will be generated, where ``0 < lower_bound <= q <= upper_bound``.
- ``lbound`` -- (positive integer, default: ``None``) the lower bound on how small each random primes `p` and `q` can be. So we have ``0 < lbound <= p, q <= ubound``. The lower bound must be distinct from the upper bound.
- ``ubound`` -- (positive integer, default: ``None``) the upper bound on how large each random primes `p` and `q` can be. So we have ``0 < lbound <= p, q <= ubound``. The lower bound must be distinct from the upper bound.
- ``ntries`` -- (default: ``100``) the number of attempts to generate a random Blum prime. If ``ntries`` is a positive integer, then perform that many attempts at generating a random Blum prime. This might or might not result in a Blum prime.
OUTPUT:
- A pseudorandom bit sequence whose length is specified by ``length``.
Here is a common use case for this function. If you want this function to use pre-computed values for `p` and `q`, you should pass those pre-computed values to this function. In that case, you only need to specify values for ``length``, ``p`` and ``q``, and you do not need to worry about doing anything with the parameters ``lbound`` and ``ubound``. The pre-computed values `p` and `q` must be Blum primes. It is your responsibility to check that both `p` and `q` are Blum primes.
Here is another common use case. If you want the function to generate its own values for `p` and `q`, you must specify the lower and upper bounds within which these two primes must lie. In that case, you must specify values for ``length``, ``lbound`` and ``ubound``, and you do not need to worry about values for the parameters ``p`` and ``q``. The parameter ``ntries`` is only relevant when you want this function to generate ``p`` and ``q``.
.. NOTE::
Beware that there might not be any primes between the lower and upper bounds. So make sure that these two bounds are "sufficiently" far apart from each other for there to be primes congruent to 3 modulo 4. In particular, there should be at least two distinct primes within these bounds, each prime being congruent to 3 modulo 4. This function uses the function :func:`random_blum_prime() <sage.crypto.util.random_blum_prime>` to generate random primes that are congruent to 3 modulo 4.
ALGORITHM:
The BBS algorithm as described below is adapted from the presentation in Algorithm 5.40, page 186 of [MvOV1996]_.
#. Let `L` be the desired number of bits in the output bit sequence. That is, `L` is the desired length of the bit string. #. Let `p` and `q` be two large distinct primes, each congruent to 3 modulo 4. #. Let `n = pq` be the product of `p` and `q`. #. Select a random seed value `s \in (\ZZ/n\ZZ)^{\ast}`, where `(\ZZ/n\ZZ)^{\ast}` is the multiplicative group of `\ZZ/n\ZZ`. #. Let `x_0 = s^2 \bmod n`. #. For `i` from 1 to `L`, do
#. Let `x_i = x_{i-1}^2 \bmod n`. #. Let `z_i` be the least significant bit of `x_i`.
#. The output pseudorandom bit sequence is `z_1, z_2, \dots, z_L`.
EXAMPLES:
A BBS pseudorandom bit sequence with a specified seed::
sage: from sage.crypto.stream import blum_blum_shub sage: blum_blum_shub(length=6, seed=3, p=11, q=19) 110000
You could specify the length of the bit string, with given values for ``p`` and ``q``::
sage: blum_blum_shub(length=6, p=11, q=19) # random 001011
Or you could specify the length of the bit string, with given values for the lower and upper bounds::
sage: blum_blum_shub(length=6, lbound=10**4, ubound=10**5) # random 110111
Under some reasonable hypotheses, Blum-Blum-Shub [BBS1982]_ sketch a proof that the period of the BBS stream cipher is equal to `\lambda(\lambda(n))`, where `\lambda(n)` is the Carmichael function of `n`. This is verified below in a few examples by using the function :func:`lfsr_connection_polynomial() <sage.crypto.lfsr.lfsr_connection_polynomial>` (written by Tim Brock) which computes the connection polynomial of a linear feedback shift register sequence. The degree of that polynomial is the period. ::
sage: from sage.crypto.stream import blum_blum_shub sage: from sage.crypto.util import carmichael_lambda sage: carmichael_lambda(carmichael_lambda(7*11)) 4 sage: s = [GF(2)(int(str(x))) for x in blum_blum_shub(60, p=7, q=11, seed=13)] sage: lfsr_connection_polynomial(s) x^3 + x^2 + x + 1 sage: carmichael_lambda(carmichael_lambda(11*23)) 20 sage: s = [GF(2)(int(str(x))) for x in blum_blum_shub(60, p=11, q=23, seed=13)] sage: lfsr_connection_polynomial(s) x^19 + x^18 + x^17 + x^16 + x^15 + x^14 + x^13 + x^12 + x^11 + x^10 + x^9 + x^8 + x^7 + x^6 + x^5 + x^4 + x^3 + x^2 + x + 1
TESTS:
Make sure that there is at least one Blum prime between the lower and upper bounds. In the following example, we have ``lbound=24`` and ``ubound=30`` with 29 being the only prime within those bounds. But 29 is not a Blum prime. ::
sage: from sage.crypto.stream import blum_blum_shub sage: blum_blum_shub(6, lbound=24, ubound=30, ntries=10) Traceback (most recent call last): ... ValueError: No Blum primes within the specified closed interval.
Both the lower and upper bounds must be greater than 2::
sage: blum_blum_shub(6, lbound=2, ubound=3) Traceback (most recent call last): ... ValueError: Both the lower and upper bounds must be > 2. sage: blum_blum_shub(6, lbound=3, ubound=2) Traceback (most recent call last): ... ValueError: Both the lower and upper bounds must be > 2. sage: blum_blum_shub(6, lbound=2, ubound=2) Traceback (most recent call last): ... ValueError: Both the lower and upper bounds must be > 2.
The lower and upper bounds must be distinct from each other::
sage: blum_blum_shub(6, lbound=3, ubound=3) Traceback (most recent call last): ... ValueError: The lower and upper bounds must be distinct.
The lower bound must be less than the upper bound::
sage: blum_blum_shub(6, lbound=4, ubound=3) Traceback (most recent call last): ... ValueError: The lower bound must be less than the upper bound. """ # sanity checks raise ValueError("The length of the bit string must be positive.") raise ValueError("Either specify values for p and q, or specify values for the lower and upper bounds.") # Use pre-computed Blum primes. Both the parameters p and q are # assumed to be Blum primes. No attempts are made to ensure that they # are indeed Blum primes. # generate random Blum primes within specified bounds randq = random_blum_prime(lbound, ubound, ntries=ntries) # no pre-computed primes given, and no appropriate bounds given else: raise ValueError("Either specify values for p and q, or specify values for the lower and upper bounds.") # By now, we should have two distinct Blum primes. # If no seed is provided, select a random seed. s = zmod.random_element().lift() # start generating pseudorandom bits |