r"""
Linear feedback shift register (LFSR) sequence commands
Stream ciphers have been used for a long time as a source of
pseudo-random number generators.
S. Golomb [G]_ gives a list of three statistical properties a
sequence of numbers `{\bf a}=\{a_n\}_{n=1}^\infty`,
`a_n\in \{0,1\}`, should display to be considered
"random". Define the autocorrelation of `{\bf a}` to be
.. math::
C(k)=C(k,{\bf a})=\lim_{N\rightarrow \infty} {1\over N}\sum_{n=1}^N (-1)^{a_n+a_{n+k}}.
In the case where `{\bf a}` is periodic with period
`P` then this reduces to
.. math::
C(k)={1\over P}\sum_{n=1}^P (-1)^{a_n+a_{n+k}}.
Assume `{\bf a}` is periodic with period `P`.
- balance: `|\sum_{n=1}^P(-1)^{a_n}|\leq 1`.
- low autocorrelation:
.. math::
C(k)= \left\{ \begin{array}{cc} 1,& k=0,\\ \epsilon, & k\not= 0. \end{array} \right.
(For sequences satisfying these first two properties, it is known
that `\epsilon=-1/P` must hold.)
- proportional runs property: In each period, half the runs have
length `1`, one-fourth have length `2`, etc.
Moreover, there are as many runs of `1`'s as there are of
`0`'s.
A general feedback shift register is a map
`f:{\bf F}_q^d\rightarrow {\bf F}_q^d` of the form
.. math::
\begin{array}{c} f(x_0,...,x_{n-1})=(x_1,x_2,...,x_n),\\ x_n=C(x_0,...,x_{n-1}), \end{array}
where `C:{\bf F}_q^d\rightarrow {\bf F}_q` is a given
function. When `C` is of the form
.. math::
C(x_0,...,x_{n-1})=a_0x_0+...+a_{n-1}x_{n-1},
for some given constants `a_i\in {\bf F}_q`, the map is
called a linear feedback shift register (LFSR).
Example of a LFSR Let
.. math::
f(x)=a_{{0}}+a_{{1}}x+...+a_{{n}}{x}^n+...,
.. math::
g(x)=b_{{0}}+b_{{1}}x+...+b_{{n}}{x}^n+...,
be given polynomials in `{\bf F}_2[x]` and let
.. math::
h(x)={f(x)\over g(x)}=c_0+c_1x+...+c_nx^n+... \ .
We can compute a recursion formula which allows us to rapidly
compute the coefficients of `h(x)` (take `f(x)=1`):
.. math::
c_{n}=\sum_{i=1}^n {{-b_i\over b_0}c_{n-i}}.
The coefficients of `h(x)` can, under certain conditions on
`f(x)` and `g(x)`, be considered "random" from
certain statistical points of view.
Example: For instance, if
.. math::
f(x)=1,\ \ \ \ g(x)=x^4+x+1,
then
.. math::
h(x)=1+x+x^2+x^3+x^5+x^7+x^8+...\ .
The coefficients of `h` are
.. math::
\begin{array}{c} 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, \\ 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, ...\ . \end{array}
The sequence of `0,1`'s is periodic with period
`P=2^4-1=15` and satisfies Golomb's three randomness
conditions. However, this sequence of period 15 can be "cracked"
(i.e., a procedure to reproduce `g(x)`) by knowing only 8
terms! This is the function of the Berlekamp-Massey algorithm [M]_,
implemented as ``berlekamp_massey.py``.
.. [G] Solomon Golomb, Shift register sequences, Aegean Park Press,
Laguna Hills, Ca, 1967
.. [M] James L. Massey, "Shift-Register Synthesis and BCH Decoding."
IEEE Trans. on Information Theory, vol. 15(1), pp. 122-127, Jan
1969.
AUTHORS:
- Timothy Brock
Created 11-24-2005 by wdj. Last updated 12-02-2005.
"""
import copy
from sage.structure.all import Sequence
from sage.rings.all import is_FiniteField, Integer, PolynomialRing
def lfsr_sequence(key, fill, n):
r"""
This function creates an lfsr sequence.
INPUT:
- ``key`` - a list of finite field elements,
[c_0,c_1,...,c_k].
- ``fill`` - the list of the initial terms of the lfsr
sequence, [x_0,x_1,...,x_k].
- ``n`` - number of terms of the sequence that the
function returns.
OUTPUT: The lfsr sequence defined by
`x_{n+1} = c_kx_n+...+c_0x_{n-k}`, for
`n \leq k`.
EXAMPLES::
sage: F = GF(2); l = F(1); o = F(0)
sage: F = GF(2); S = LaurentSeriesRing(F,'x'); x = S.gen()
sage: fill = [l,l,o,l]; key = [1,o,o,l]; n = 20
sage: L = lfsr_sequence(key,fill,20); L
[1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0]
sage: g = berlekamp_massey(L); g
x^4 + x^3 + 1
sage: (1)/(g.reverse()+O(x^20))
1 + x + x^2 + x^3 + x^5 + x^7 + x^8 + x^11 + x^15 + x^16 + x^17 + x^18 + O(x^20)
sage: (1+x^2)/(g.reverse()+O(x^20))
1 + x + x^4 + x^8 + x^9 + x^10 + x^11 + x^13 + x^15 + x^16 + x^19 + O(x^20)
sage: (1+x^2+x^3)/(g.reverse()+O(x^20))
1 + x + x^3 + x^5 + x^6 + x^9 + x^13 + x^14 + x^15 + x^16 + x^18 + O(x^20)
sage: fill = [l,l,o,l]; key = [l,o,o,o]; n = 20
sage: L = lfsr_sequence(key,fill,20); L
[1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1]
sage: g = berlekamp_massey(L); g
x^4 + 1
sage: (1+x)/(g.reverse()+O(x^20))
1 + x + x^4 + x^5 + x^8 + x^9 + x^12 + x^13 + x^16 + x^17 + O(x^20)
sage: (1+x+x^3)/(g.reverse()+O(x^20))
1 + x + x^3 + x^4 + x^5 + x^7 + x^8 + x^9 + x^11 + x^12 + x^13 + x^15 + x^16 + x^17 + x^19 + O(x^20)
AUTHORS:
- Timothy Brock (2005-11): with code modified from Python
Cookbook, http://aspn.activestate.com/ASPN/Python/Cookbook/
"""
if not isinstance(key, list):
raise TypeError, "key must be a list"
key = Sequence(key)
F = key.universe()
if not is_FiniteField(F):
raise TypeError, "universe of sequence must be a finite field"
s = fill
k = len(fill)
L = []
for i in range(n):
s0 = copy.copy(s)
L.append(s[0])
s = s[1:k]
s.append(sum([key[i]*s0[i] for i in range(k)]))
return L
def lfsr_autocorrelation(L, p, k):
"""
INPUT:
- ``L`` - is a periodic sequence of elements of ZZ or
GF(2). L must have length = p
- ``p`` - the period of L
- ``k`` - k is an integer (0 k p)
OUTPUT: autocorrelation sequence of L
EXAMPLES::
sage: F = GF(2)
sage: o = F(0)
sage: l = F(1)
sage: key = [l,o,o,l]; fill = [l,l,o,l]; n = 20
sage: s = lfsr_sequence(key,fill,n)
sage: lfsr_autocorrelation(s,15,7)
4/15
sage: lfsr_autocorrelation(s,int(15),7)
4/15
AUTHORS:
- Timothy Brock (2006-04-17)
"""
if not isinstance(L, list):
raise TypeError, "L (=%s) must be a list"%L
p = Integer(p)
_p = int(p)
k = int(k)
L0 = L[:_p]
L0 = L0 + L0[:k]
L1 = [int(L0[i])*int(L0[i + k])/p for i in range(_p)]
return sum(L1)
def lfsr_connection_polynomial(s):
"""
INPUT:
- ``s`` - a sequence of elements of a finite field (F)
of even length
OUTPUT:
- ``C(x)`` - the connection polynomial of the minimal
LFSR.
This implements the algorithm in section 3 of J. L. Massey's
article [M]_.
EXAMPLE::
sage: F = GF(2)
sage: F
Finite Field of size 2
sage: o = F(0); l = F(1)
sage: key = [l,o,o,l]; fill = [l,l,o,l]; n = 20
sage: s = lfsr_sequence(key,fill,n); s
[1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0]
sage: lfsr_connection_polynomial(s)
x^4 + x + 1
sage: berlekamp_massey(s)
x^4 + x^3 + 1
Notice that ``berlekamp_massey`` returns the reverse
of the connection polynomial (and is potentially must faster than
this implementation).
AUTHORS:
- Timothy Brock (2006-04-17)
"""
FF = s[0].base_ring()
R = PolynomialRing(FF, "x")
x = R.gen()
C = R(1); B = R(1); m = 1; b = FF(1); L = 0; N = 0
while N < len(s):
if L > 0:
r = min(L+1,C.degree()+1)
d = s[N] + sum([(C.list())[i]*s[N-i] for i in range(1,r)])
if L == 0:
d = s[N]
if d == 0:
m += 1
N += 1
if d > 0:
if 2*L > N:
C = C - d*b**(-1)*x**m*B
m += 1
N += 1
else:
T = C
C = C - d*b**(-1)*x**m*B
L = N + 1 - L
m = 1
b = d
B = T
N += 1
return C
