r"""
S-Boxes and Their Algebraic Representations
"""
from __future__ import print_function, division
from six.moves import range
from six import integer_types

from sage.combinat.integer_vector import IntegerVectors
from sage.crypto.boolean_function import BooleanFunction
from sage.matrix.constructor import Matrix
from sage.misc.cachefunc import cached_method
from sage.misc.functional import is_even
from sage.misc.misc_c import prod as mul
from sage.modules.free_module_element import vector
from sage.rings.finite_rings.element_base import is_FiniteFieldElement
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
from sage.rings.ideal import FieldIdeal, Ideal
from sage.rings.integer_ring import ZZ
from sage.rings.integer import Integer
from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
from sage.structure.sage_object import SageObject

class SBox(SageObject):
    r"""
    A substitution box or S-box is one of the basic components of
    symmetric key cryptography. In general, an S-box takes ``m`` input
    bits and transforms them into ``n`` output bits. This is called an
    ``mxn`` S-box and is often implemented as a lookup table. These
    S-boxes are carefully chosen to resist linear and differential
    cryptanalysis [He2002]_.

    This module implements an S-box class which allows an algebraic
    treatment and determine various cryptographic properties.

    EXAMPLES:

    We consider the S-box of the block cipher PRESENT [BKLPPRSV2007]_::

        sage: from sage.crypto.sbox import SBox
        sage: S = SBox(12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2); S
        (12, 5, 6, 11, 9, 0, 10, 13, 3, 14, 15, 8, 4, 7, 1, 2)
        sage: S(1)
        5

    Note that by default bits are interpreted in big endian
    order. This is not consistent with the rest of Sage, which has a
    strong bias towards little endian, but is consistent with most
    cryptographic literature::

        sage: S([0,0,0,1])
        [0, 1, 0, 1]

        sage: S = SBox(12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2, big_endian=False)
        sage: S(1)
        5
        sage: S([0,0,0,1])
        [1, 1, 0, 0]


    Now we construct an ``SBox`` object for the 4-bit small scale AES
    S-Box (cf. :mod:`sage.crypto.mq.sr`)::

        sage: sr = mq.SR(1,1,1,4, allow_zero_inversions=True)
        sage: S = SBox([sr.sub_byte(e) for e in list(sr.k)])
        sage: S
        (6, 5, 2, 9, 4, 7, 3, 12, 14, 15, 10, 0, 8, 1, 13, 11)

    AUTHORS:

    - Rusydi H. Makarim (2016-03-31) : added more functions to determine related cryptographic properties
    - Yann Laigle-Chapuy (2009-07-01): improve linear and difference matrix computation
    - Martin R. Albrecht (2008-03-12): initial implementation

    REFERENCES:

    - [He2002]_

    - [BKLPPRSV2007]_

    - [CDL2015]_
    """

    def __init__(self, *args,  **kwargs):
        """
        Construct a substitution box (S-box) for a given lookup table
        `S`.

        INPUT:

        - ``S`` - a finite iterable defining the S-box with integer or
          finite field elements

        - ``big_endian`` - controls whether bits shall be ordered in
          big endian order (default: ``True``)

        EXAMPLES:

        We construct a 3-bit S-box where e.g. the bits (0,0,1) are
        mapped to (1,1,1).::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3); S
            (7, 6, 0, 4, 2, 5, 1, 3)

            sage: S(0)
            7

        TESTS::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox()
            Traceback (most recent call last):
            ...
            TypeError: No lookup table provided.
            sage: S = SBox(1, 2, 3)
            Traceback (most recent call last):
            ...
            TypeError: Lookup table length is not a power of 2.
            sage: S = SBox(5, 6, 0, 3, 4, 2, 1, 2)
            sage: S.n
            3
        """
        if "S" in kwargs:
            S = kwargs["S"]
        elif len(args) == 1:
            S = args[0]
        elif len(args) > 1:
            S = args
        else:
            raise TypeError("No lookup table provided.")

        _S = []
        for e in S:
            if is_FiniteFieldElement(e):
                e = e.polynomial().change_ring(ZZ).subs( e.parent().characteristic() )
            _S.append(e)
        S = _S

        if not ZZ(len(S)).is_power_of(2):
            raise TypeError("Lookup table length is not a power of 2.")
        self._S = S

        self.m = ZZ(len(S)).exact_log(2)
        self.n = ZZ(max(S)).nbits()
        self._F = GF(2)
        self._big_endian = kwargs.get("big_endian",True)

        self.differential_uniformity = self.maximal_difference_probability_absolute

    def _repr_(self):
        """
        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: SBox(7,6,0,4,2,5,1,3) #indirect doctest
            (7, 6, 0, 4, 2, 5, 1, 3)
        """
        return "(" + ", ".join(map(str,list(self))) + ")"

    def __len__(self):
        """
        Return the length of input bit strings.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: len(SBox(7,6,0,4,2,5,1,3))
            3
        """
        return self.m

    def __eq__(self, other):
        """
        S-boxes are considered to be equal if all construction
        parameters match.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: loads(dumps(S)) == S
            True
        """
        return (self._S, self._big_endian) == (other._S, self._big_endian)

    def __ne__(self, other):
        """
        S-boxes are considered to be equal if all construction
        parameters match.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S != S
            False
        """
        return not self.__eq__(other)

    def to_bits(self, x, n=None):
        """
        Return bitstring of length ``n`` for integer ``x``. The
        returned bitstring is guaranteed to have length ``n``.

        INPUT:

        - ``x`` - an integer

        - ``n`` - bit length (optional)

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.to_bits(6)
            [1, 1, 0]

            sage: S.to_bits( S(6) )
            [0, 0, 1]

            sage: S( S.to_bits( 6 ) )
            [0, 0, 1]
        """
        if n is None and self.m == self.n:
            n = self.n

        if self._big_endian:
            swp = lambda x: list(reversed(x))
        else:
            swp = lambda x: x
        return swp(self._rpad([self._F(_) for _ in ZZ(x).digits(2)], n))

    def from_bits(self, x, n=None):
        """
        Return integer for bitstring ``x`` of length ``n``.

        INPUT:

        - ``x`` - a bitstring

        - ``n`` - bit length (optional)

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.from_bits( [1,1,0])
            6

            sage: S( S.from_bits( [1,1,0] ) )
            1
            sage: S.from_bits( S( [1,1,0] ) )
            1
        """
        if n is None and self.m == self.n:
            n = self.m

        if self._big_endian:
            swp = lambda x: list(reversed(x))
        else:
            swp = lambda x: x

        return ZZ( [ZZ(_) for _ in self._rpad(swp(x), n)], 2)

    def _rpad(self,x, n=None):
        """
        Right pads ``x`` such that ``len(x) == n``.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S._rpad([1,1])
            [1, 1, 0]
        """
        if n is None and self.m == self.n:
            n = self.n
        return  x + [self._F(0)]*(n-len(x))

    def __call__(self, X):
        """
        Apply substitution to ``X``.

        If ``X`` is a list, it is interpreted as a sequence of bits
        depending on the bit order of this S-box.

        INPUT:

        - ``X`` - either an integer, a tuple of `\GF{2}` elements of
          length ``len(self)`` or a finite field element in
          `\GF{2^n}`. As a last resort this function tries to convert
          ``X`` to an integer.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([7,6,0,4,2,5,1,3])
            sage: S(7)
            3

            sage: S((0,2,3))
            [0, 1, 1]

            sage: S[0]
            7

            sage: S[(0,0,1)]
            [1, 1, 0]

            sage: k.<a> = GF(2^3)
            sage: S(a^2)
            a

            sage: S(QQ(3))
            4

            sage: S([1]*10^6)
            Traceback (most recent call last):
            ...
            TypeError: Cannot apply SBox to provided element.

            sage: S(1/2)
            Traceback (most recent call last):
            ...
            TypeError: Cannot apply SBox to 1/2.

            sage: S = SBox(3, 0, 1, 3, 1, 0, 2, 2)
            sage: S(0)
            3
            sage: S([0,0,0])
            [1, 1]
        """
        if isinstance(X, integer_types + (Integer,)):
            return self._S[ZZ(X)]

        try:
            from sage.modules.free_module_element import vector
            K = X.parent()
            if K.order() == 2**self.n:
                X = vector(X)
            else:
                raise TypeError
            if not self._big_endian:
                X = list(reversed(X))
            else:
                X = list(X)
            X = ZZ([ZZ(_) for _ in X], 2)
            out =  self.to_bits(self._S[X], self.n)
            if self._big_endian:
                out = list(reversed(out))
            return K(vector(GF(2),out))
        except (AttributeError, TypeError):
            pass

        try:
            if len(X) == self.m:
                if self._big_endian:
                    X = list(reversed(X))
                X = ZZ([ZZ(_) for _ in X], 2)
                out =  self._S[X]
                return self.to_bits(out,self.n)
        except TypeError:
            pass

        try:
            return self._S[ZZ(X)]
        except TypeError:
            pass

        if len(str(X)) > 50:
            raise TypeError("Cannot apply SBox to provided element.")
        else:
            raise TypeError("Cannot apply SBox to %s."%(X,))

    def __getitem__(self, X):
        """
        See  :meth:`SBox.__call__`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([7,6,0,4,2,5,1,3])
            sage: S[7]
            3
        """
        return self(X)

    def is_permutation(self):
        r"""
        Return ``True`` if this S-Box is a permutation.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.is_permutation()
            True

            sage: S = SBox(3,2,0,0,2,1,1,3)
            sage: S.is_permutation()
            False
        """
        if self.m != self.n:
            return False
        return len(set([self(i) for i in range(2**self.m)])) == 2**self.m

    def __iter__(self):
        """
        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: [e for e in S]
            [7, 6, 0, 4, 2, 5, 1, 3]
        """
        for i in range(2**self.m):
            yield self(i)

    def difference_distribution_matrix(self):
        """
        Return difference distribution matrix ``A`` for this S-box.

        The rows of ``A`` encode the differences ``Delta I`` of the
        input and the columns encode the difference ``Delta O`` for
        the output. The bits are ordered according to the endianess of
        this S-box. The value at ``A[Delta I,Delta O]`` encodes how
        often ``Delta O`` is the actual output difference given
        ``Delta I`` as input difference.

        See [He2002]_ for an introduction to differential
        cryptanalysis.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.difference_distribution_matrix()
            [8 0 0 0 0 0 0 0]
            [0 2 2 0 2 0 0 2]
            [0 0 2 2 0 0 2 2]
            [0 2 0 2 2 0 2 0]
            [0 2 0 2 0 2 0 2]
            [0 0 2 2 2 2 0 0]
            [0 2 2 0 0 2 2 0]
            [0 0 0 0 2 2 2 2]
        """
        m = self.m
        n = self.n

        nrows = 1<<m
        ncols = 1<<n

        A = Matrix(ZZ, nrows, ncols)

        for i in range(nrows):
            si = self(i)
            for di in range(nrows):
                A[ di , si^self(i^di)] += 1
        return A

    def maximal_difference_probability_absolute(self):
        """
        Return the difference probability of the difference with the
        highest probability in absolute terms, i.e. how often it
        occurs in total.

        Equivalently, this is equal to the differential uniformity
        of this S-Box.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.maximal_difference_probability_absolute()
            2

        .. note::

          This code is mainly called internally.
        """
        A = self.difference_distribution_matrix().__copy__()
        A[0,0] = 0
        return max(map(abs, A.list()))

    def maximal_difference_probability(self):
        r"""
        Return the difference probability of the difference with the
        highest probability in the range between 0.0 and 1.0
        indicating 0\% or 100\% respectively.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.maximal_difference_probability()
            0.25
        """
        return self.maximal_difference_probability_absolute()/(2.0**self.n)

    @cached_method
    def linear_approximation_matrix(self):
        """
        Return linear approximation matrix ``A`` for this S-box.

        Let ``i_b`` be the ``b``-th bit of ``i`` and ``o_b`` the
        ``b``-th bit of ``o``. Then ``v = A[i,o]`` encodes the bias of
        the equation ``sum( i_b * x_i ) = sum( o_b * y_i )`` if
        ``x_i`` and ``y_i`` represent the input and output variables
        of the S-box.

        See [He2002]_ for an introduction to linear cryptanalysis.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.linear_approximation_matrix()
            [ 4  0  0  0  0  0  0  0]
            [ 0  0  0  0  2  2  2 -2]
            [ 0  0 -2 -2 -2  2  0  0]
            [ 0  0 -2  2  0  0 -2 -2]
            [ 0  2  0  2 -2  0  2  0]
            [ 0 -2  0  2  0  2  0  2]
            [ 0 -2 -2  0  0 -2  2  0]
            [ 0 -2  2  0 -2  0  0 -2]

        According to this matrix the first bit of the input is equal
        to the third bit of the output 6 out of 8 times::

            sage: for i in srange(8): print(S.to_bits(i)[0] == S.to_bits(S(i))[2])
            False
            True
            True
            True
            False
            True
            True
            True
        """
        m = self.m
        n = self.n

        nrows = 1<<m
        ncols = 1<<n

        B = BooleanFunction(self.m)
        L = []
        for j in range(ncols):
            for i in range(nrows):
                B[i] = ZZ(self(i)&j).popcount()
            L.append(B.walsh_hadamard_transform())

        A = Matrix(ZZ, ncols, nrows, L)
        A = -A.transpose()/2
        A.set_immutable()

        return A

    def maximal_linear_bias_absolute(self):
        """
        Return maximal linear bias, i.e. how often the linear
        approximation with the highest bias is true or false minus
        `2^{n-1}`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.maximal_linear_bias_absolute()
            2
        """
        A = self.linear_approximation_matrix().__copy__()
        A[0,0] = 0
        return max(map(abs, A.list()))

    def maximal_linear_bias_relative(self):
        """
        Return maximal bias of all linear approximations of this
        S-box.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.maximal_linear_bias_relative()
            0.25
        """
        return self.maximal_linear_bias_absolute()/(2.0**self.m)

    def ring(self):
        """
        Create, return and cache a polynomial ring for S-box
        polynomials.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.ring()
            Multivariate Polynomial Ring in x0, x1, x2, y0, y1, y2 over Finite Field of size 2
        """
        try:
            return self._ring
        except AttributeError:
            pass

        m = self.m
        n = self.n

        X = range(m)
        Y = range(n)
        self._ring = PolynomialRing(self._F, m+n, ["x%d"%i for i in X] + ["y%d"%i for i in Y])
        return self._ring

    def solutions(self, X=None, Y=None):
        """
        Return a dictionary of solutions to this S-box.

        INPUT:

        - ``X`` - input variables (default: ``None``)

        - ``Y`` - output variables (default: ``None``)

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([7,6,0,4,2,5,1,3])
            sage: F = S.polynomials()
            sage: s = S.solutions()
            sage: any(f.subs(_s) for f in F for _s in s)
            False
        """
        if X is None and Y is None:
            P = self.ring()
            gens = P.gens()
        else:
            P = X[0].parent()
            gens = X + Y

        m = self.m
        n = self.n

        solutions = []
        for i in range(1<<m):
            solution = self.to_bits(i,m) + self( self.to_bits(i,m) )
            solutions.append( dict(zip(gens, solution)) )

        return solutions

    def polynomials(self, X=None, Y=None, degree=2, groebner=False):
        """
        Return a list of polynomials satisfying this S-box.

        First, a simple linear fitting is performed for the given
        ``degree`` (cf. for example [BC2003]_). If ``groebner=True`` a
        Groebner basis is also computed for the result of that
        process.

        INPUT:

        - ``X`` - input variables

        - ``Y`` - output variables

        - ``degree`` - integer > 0 (default: ``2``)

        - ``groebner`` - calculate a reduced Groebner basis of the
          spanning polynomials to obtain more polynomials (default:
          ``False``)

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: P = S.ring()

        By default, this method returns an indirect representation::

            sage: S.polynomials()
            [x0*x2 + x1 + y1 + 1,
             x0*x1 + x1 + x2 + y0 + y1 + y2 + 1,
             x0*y1 + x0 + x2 + y0 + y2,
             x0*y0 + x0*y2 + x1 + x2 + y0 + y1 + y2 + 1,
             x1*x2 + x0 + x1 + x2 + y2 + 1,
             x0*y0 + x1*y0 + x0 + x2 + y1 + y2,
             x0*y0 + x1*y1 + x1 + y1 + 1,
             x1*y2 + x1 + x2 + y0 + y1 + y2 + 1,
             x0*y0 + x2*y0 + x1 + x2 + y1 + 1,
             x2*y1 + x0 + y1 + y2,
             x2*y2 + x1 + y1 + 1,
             y0*y1 + x0 + x2 + y0 + y1 + y2,
             y0*y2 + x1 + x2 + y0 + y1 + 1,
             y1*y2 + x2 + y0]

        We can get a direct representation by computing a
        lexicographical Groebner basis with respect to the right
        variable ordering, i.e. a variable ordering where the output
        bits are greater than the input bits::

            sage: P.<y0,y1,y2,x0,x1,x2> = PolynomialRing(GF(2),6,order='lex')
            sage: S.polynomials([x0,x1,x2],[y0,y1,y2], groebner=True)
            [y0 + x0*x1 + x0*x2 + x0 + x1*x2 + x1 + 1,
             y1 + x0*x2 + x1 + 1,
             y2 + x0 + x1*x2 + x1 + x2 + 1]
        """
        def nterms(nvars, deg):
            """
            Return the number of monomials possible up to a given
            degree.

            INPUT:

            - ``nvars`` - number of variables

            - ``deg`` - degree

            TESTS::

                sage: from sage.crypto.sbox import SBox
                sage: S = SBox(7,6,0,4,2,5,1,3)
                sage: F = S.polynomials(degree=3) # indirect doctest
            """
            total = 1
            divisor = 1
            var_choices = 1

            for d in range(1, deg+1):
                var_choices *= (nvars - d + 1)
                divisor *= d
                total += var_choices/divisor
            return total

        m = self.m
        n = self.n
        F = self._F

        if X is None and Y is None:
            P = self.ring()
            X = P.gens()[:m]
            Y = P.gens()[m:]
        else:
            P = X[0].parent()

        gens = X+Y

        bits = []
        for i in range(1<<m):
            bits.append( self.to_bits(i,m) + self(self.to_bits(i,m)) )

        ncols = (1<<m)+1

        A = Matrix(P, nterms(m+n, degree), ncols)

        exponents = []
        for d in range(degree+1):
            exponents += IntegerVectors(d, max_length=m+n, min_length=m+n, min_part=0, max_part=1).list()

        row = 0
        for exponent in exponents:
            A[row,ncols-1] = mul([gens[i]**exponent[i] for i in range(len(exponent))])
            for col in range(1<<m):
                A[row,col] = mul([bits[col][i] for i in range(len(exponent)) if exponent[i]])
            row +=1

        for c in range(ncols):
            A[0,c] = 1

        RR = A.echelon_form(algorithm='row_reduction')

        # extract spanning stet
        gens = (RR.column(ncols-1)[1<<m:]).list()

        if not groebner:
            return gens

        FI = set(FieldIdeal(P).gens())
        I = Ideal(gens + list(FI))
        gb = I.groebner_basis()

        gens = []
        for f in gb:
            if f not in FI: # filter out field equations
                gens.append(f)
        return gens

    def interpolation_polynomial(self, k=None):
        r"""
        Return a univariate polynomial over an extension field
        representing this S-box.

        If ``m`` is the input length of this S-box then the extension
        field is of degree ``m``.

        If the output length does not match the input length then a
        ``TypeError`` is raised.

        INPUT:

        - ``k`` - an instance of `\GF{2^m}` (default: ``None``)

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: f = S.interpolation_polynomial()
            sage: f
            x^6 + a*x^5 + (a + 1)*x^4 + (a^2 + a + 1)*x^3
              + (a^2 + 1)*x^2 + (a + 1)*x + a^2 + a + 1

            sage: a = f.base_ring().gen()

            sage: f(0), S(0)
            (a^2 + a + 1, 7)

            sage: f(a^2 + 1), S(5)
            (a^2 + 1, 5)
        """
        if self.m != self.n:
            raise TypeError("Lagrange interpolation only supported if self.m == self.n.")

        if k is None:
            k = GF(2**self.m,'a')
        l = []
        for i in range(2**self.m):
            i = self.to_bits(i, self.m)
            o = self(i)
            if self._big_endian:
                i = reversed(i)
                o = reversed(o)
            l.append( (k(vector(i)), k(vector(o))) )

        P = PolynomialRing(k,'x')
        return P.lagrange_polynomial(l)

    def cnf(self, xi=None, yi=None, format=None):
        """
        Return a representation of this S-Box in conjunctive normal
        form.

        This function examines the truth tables for each output bit of
        the S-Box and thus has complexity `n * 2^m` for an ``m x n``
        S-Box.

        INPUT:

        - ``xi`` - indices for the input variables (default: ``1...m``)

        - ``yi`` - indices for the output variables (default: ``m+1 ... m+n``)

        - ``format`` - output format, see below (default: ``None``)

        FORMATS:

        - ``None`` - return a list of tuples of integers where each
          tuple represents a clause, the absolute value of an integer
          represents a variable and the sign of an integer indicates
          inversion.

        - ``symbolic`` - a string that can be parsed by the
          ``SymbolicLogic`` package.

        - ``dimacs`` - a string in DIMACS format which is the gold
          standard for SAT-solver input (cf. http://www.satlib.org/).

        - ``dimacs_headless`` - a string in DIMACS format, but without
          the header. This is useful for concatenation of outputs.

        EXAMPLES:

        We give a very small example to explain the output format::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(1,2,0,3); S
            (1, 2, 0, 3)
            sage: cnf = S.cnf(); cnf
            [(1, 2, -3),  (1, 2, 4),
             (1, -2, 3),  (1, -2, -4),
             (-1, 2, -3), (-1, 2, -4),
             (-1, -2, 3), (-1, -2, 4)]

        This output completely describes the S-Box. For instance, we
        can check that ``S([0,1]) -> [1,0]`` satisfies every clause if
        the first input bit corresponds to the index ``1`` and the
        last output bit corresponds to the index ``3`` in the
        output.

        We can convert this representation to the DIMACS format::

            sage: print(S.cnf(format='dimacs'))
            p cnf 4 8
            1 2 -3 0
            1 2 4 0
            1 -2 3 0
            1 -2 -4 0
            -1 2 -3 0
            -1 2 -4 0
            -1 -2 3 0
            -1 -2 4 0

        For concatenation we can strip the header::

            sage: print(S.cnf(format='dimacs_headless'))
            1 2 -3 0
            1 2 4 0
            1 -2 3 0
            1 -2 -4 0
            -1 2 -3 0
            -1 2 -4 0
            -1 -2 3 0
            -1 -2 4 0

        This might be helpful in combination with the ``xi`` and
        ``yi`` parameter to assign indices manually::

            sage: print(S.cnf(xi=[10,20],yi=[30,40], format='dimacs_headless'))
            10 20 -30 0
            10 20 40 0
            10 -20 30 0
            10 -20 -40 0
            -10 20 -30 0
            -10 20 -40 0
            -10 -20 30 0
            -10 -20 40 0

        We can also return a string which is parse-able by the
        ``SymbolicLogic`` package::

            sage: log = SymbolicLogic()
            sage: s = log.statement(S.cnf(format='symbolic'))
            sage: log.truthtable(s)[1:]
            [['False', 'False', 'False', 'False', 'False'],
             ['False', 'False', 'False', 'True', 'False'],
             ['False', 'False', 'True', 'False', 'False'],
             ['False', 'False', 'True', 'True', 'True'],
             ['False', 'True', 'False', 'False', 'True'],
             ['False', 'True', 'False', 'True', 'True'],
             ['False', 'True', 'True', 'False', 'True'],
             ['False', 'True', 'True', 'True', 'True'],
             ['True', 'False', 'False', 'False', 'True'],
             ['True', 'False', 'False', 'True', 'True'],
             ['True', 'False', 'True', 'False', 'True'],
             ['True', 'False', 'True', 'True', 'True'],
             ['True', 'True', 'False', 'False', 'True'],
             ['True', 'True', 'False', 'True', 'True'],
             ['True', 'True', 'True', 'False', 'True'],
             ['True', 'True', 'True', 'True', 'True']]


        This function respects endianness of the S-Box::

            sage: S = SBox(1,2,0,3, big_endian=False); S
            (1, 2, 0, 3)
            sage: cnf = S.cnf(); cnf
            [(1, 2, -4), (1, 2, 3),
             (-1, 2, 4), (-1, 2, -3),
             (1, -2, -4), (1, -2, -3),
             (-1, -2, 4), (-1, -2, 3)]

        S-Boxes with m!=n also work:

            sage: o = list(range(8)) + list(range(8))
            sage: shuffle(o)
            sage: S = SBox(o)
            sage: S.is_permutation()
            False

            sage: len(S.cnf()) == 3*2^4
            True


        TESTS:

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(1,2,0,3, big_endian=False)
            sage: S.cnf([1000,1001,1002], [2000,2001,2002])
            Traceback (most recent call last):
            ...
            TypeError: first arg required to have length 2, got 3 instead.
        """
        m, n = self.m, self.n

        if xi is None:
            xi = [i+1 for i in range(m)]

        if yi is None:
            yi = [m+i+1 for i in range(n)]

        if len(xi) != m:
            raise TypeError("first arg required to have length %d, got %d instead."%(m,len(xi)))

        if len(yi) != n:
            raise TypeError("second arg required to have length %d, got %d instead."%(n,len(yi)))

        output_bits = range(n)
        if not self._big_endian:
            output_bits = list(reversed(output_bits))

        C = [] # the set of clauses
        for e in range(2**m):
            x = self.to_bits(e, m)
            y = self(x) # evaluate at x
            for output_bit in output_bits: # consider each bit
                clause = [(-1)**(int(v)) * i for v,i in zip(x, xi)]
                clause.append( (-1)**(1-int(y[output_bit])) *  yi[output_bit] )
                C.append(tuple(clause))

        if format is None:
            return C
        elif format == 'symbolic':
            gd = self.ring().gens()
            formula = []
            for clause in C:
                clause = "|".join([str(gd[abs(v)-1]).replace("-","~") for v in clause])
                formula.append("("+clause+")")
            return " & ".join(formula)

        elif format.startswith('dimacs'):
            if format == "dimacs_headless":
                header = ""
            else:
                header = "p cnf %d %d\n"%(m+n,len(C))
            values = " 0\n".join([" ".join(map(str,line)) for line in C])
            return header + values + " 0\n"
        else:
            raise ValueError("Format '%s' not supported."%(format,))

    def component_function(self, b):
        r"""
        Return a Boolean function corresponding to the component function
        `b \cdot S(x)`.

        If `S` is an `m \times n` S-Box, then `b \in \GF{2}^n` and
        `\cdot` denotes dot product of two vectors.

        INPUT:

        - ``b`` -- either an integer or a tuple of `\GF{2}` elements of
          length ``self.n``

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([7,6,0,4,2,5,1,3])
            sage: f3 = S.component_function(3)
            sage: f3.algebraic_normal_form()
            x0*x1 + x0*x2 + x0 + x2

            sage: f5 = S.component_function([1, 0, 1])
            sage: f5.algebraic_normal_form()
            x0*x2 + x0 + x1*x2
        """
        m = self.m
        n = self.n
        ret = BooleanFunction(m)

        if isinstance(b, integer_types + (Integer,)):
            b = vector(GF(2), self.to_bits(b, n))
        elif len(b) == n:
            b = vector(GF(2), b)
        else:
            raise TypeError("cannot compute component function using parameter %s"%(b,))

        for x in range(1<<m):
            ret[x] = bool(b.dot_product(vector(GF(2), self.to_bits(self(x), n))))
        return ret

    def nonlinearity(self):
        """
        Return the nonlinearity of this S-Box.

        The nonlinearity of an S-Box is defined as the minimum nonlinearity
        of all its component functions.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = mq.SR(1,4,4,8).sbox()
            sage: S.nonlinearity()
            112
        """
        m = self.m
        return (1 << (m-1)) - self.maximal_linear_bias_absolute()

    def linearity(self):
        """
        Return the linearity of this S-Box.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = mq.SR(1, 4, 4, 8).sbox()
            sage: S.linearity()
            32
        """
        return self.maximal_linear_bias_absolute() << 1

    def is_apn(self):
        r"""
        Return ``True`` if this S-Box is an almost perfect nonlinear (APN)
        function.

        An `m \times m` S-Box `S` is called almost perfect nonlinear if for
        every nonzero `\alpha \in \GF{2}^m` and every
        `\beta \in \GF{2}^m`, the equation
        `S(x) \oplus S(x \oplus \alpha) = \beta` has 0 or 2 solutions.
        Equivalently, the differential uniformity of `S` is equal to 2.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0,1,3,6,7,4,5,2])
            sage: S.is_apn()
            True
            sage: S.differential_uniformity()
            2
        """
        if self.m != self.n:
            raise TypeError("APN function is only defined for self.m == self.n")
        return self.differential_uniformity() == 2

    def differential_branch_number(self):
        r"""
        Return differential branch number of this S-Box.

        The differential branch number of an S-Box `S` is defined as

        .. MATH::

            \min_{v, w \neq v} \{ \mathrm{wt}(v \oplus w) + \mathrm{wt}(S(v) \oplus S(w)) \}

        where `\mathrm{wt}(x)` denotes the Hamming weight of vector `x`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2])
            sage: S.differential_branch_number()
            3
        """
        m = self.m
        n = self.n
        ret = (1<<m) + (1<<n)

        for a in range(1<<m):
            for b in range(1<<n):
                if (a != b):
                    x = a ^ b
                    y = self(a) ^ self(b)
                    w = ZZ(x).popcount() + ZZ(y).popcount()
                    if w < ret:
                        ret = w
        return ret

    def linear_branch_number(self):
        r"""
        Return linear branch number of this S-Box.

        The linear branch number of an S-Box `S` is defined as

        .. MATH::

            \min_{\substack{\alpha \neq 0, \beta \\ \mathrm{LAM}(\alpha, \beta) \neq 0}}
                \{ \mathrm{wt}(\alpha) + \mathrm{wt}(\beta) \}

        where `\mathrm{LAM}(\alpha, \beta)` is the entry at row `\alpha` and
        column `\beta` of linear approximation matrix correspond to this
        S-Box. The `\mathrm{wt}(x)` denotes the Hamming weight of `x`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2])
            sage: S.linear_branch_number()
            2
        """
        m = self.m
        n = self.n
        ret = (1<<m) + (1<<n)
        lat = self.linear_approximation_matrix()

        for a in range(1, 1<<m):
            for b in range(1<<n):
                if lat[a,b] != 0:
                    w = ZZ(a).popcount() + ZZ(b).popcount()
                    if w < ret:
                        ret = w
        return ret

    @cached_method
    def autocorrelation_matrix(self):
        r"""
        Return autocorrelation matrix correspond to this S-Box.

        for an `m \times n` S-Box `S`, its autocorrelation matrix entry at
        row `a \in \GF{2}^m` and column `b \in \GF{2}^n`
        (considering their integer representation) is defined as:

        .. MATH::

            \sum_{x \in \GF{2}^m} (-1)^{b \cdot S(x) \oplus b \cdot S(x \oplus a)}

        Equivalently, the columns `b` of autocorrelation matrix correspond to
        the autocorrelation spectrum of component function `b \cdot S(x)`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(7,6,0,4,2,5,1,3)
            sage: S.autocorrelation_matrix()
            [ 8  8  8  8  8  8  8  8]
            [ 8  0  0  0  0  0  0 -8]
            [ 8  0 -8  0  0  0  0  0]
            [ 8  0  0  0  0 -8  0  0]
            [ 8 -8  0  0  0  0  0  0]
            [ 8  0  0  0  0  0 -8  0]
            [ 8  0  0 -8  0  0  0  0]
            [ 8  0  0  0 -8  0  0  0]
        """
        from sage.combinat.matrices.hadamard_matrix import hadamard_matrix

        n = self.n
        A = self.difference_distribution_matrix() * hadamard_matrix(1<<n)
        A.set_immutable()

        return A

    def linear_structures(self):
        r"""
        Return a list of 3-valued tuple `(b, \alpha, c)` such that `\alpha` is
        a `c`-linear structure of the component function `b \cdot S(x)`.

        A Boolean function `f : \GF{2}^m \mapsto \GF{2}` is said
        to have a `c`-linear structure if there exists a nonzero `\alpha` such
        that `f(x) \oplus f(x \oplus \alpha)` is a constant function `c`.

        An `m \times n` S-Box `S` has a linear structure if there exists a
        component function `b \cdot S(x)` that has a linear structure.

        The three valued tuple `(b, \alpha, c)` shows that `\alpha` is a
        `c`-linear structure of the component function `b \cdot S(x)`. This
        implies that for all output differences `\beta` of the S-Box
        correspond to input difference `\alpha`, we have `b \cdot \beta = c`.

        .. SEEALSO::

            :meth:`is_linear_structure`,
            :meth:`has_linear_structure`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0,1,3,6,7,4,5,2])
            sage: S.linear_structures()
            [(1, 1, 1), (2, 2, 1), (3, 3, 1), (4, 4, 1), (5, 5, 1), (6, 6, 1), (7, 7, 1)]
        """
        n = self.n
        m = self.m
        act = self.autocorrelation_matrix()
        ret = []

        for j in range(1, 1<<n):
            for i in range(1, 1<<m):
                if (abs(act[i,j]) == (1<<m)):
                    c = ((1 - (act[i][j] >> self.m)) >> 1)
                    ret.append((j, i, c))
        return ret

    def has_linear_structure(self):
        """
        Return ``True`` if there exists a nonzero component function of this
        S-Box that has a linear structure.

        .. SEEALSO::

            :meth:`is_linear_structure`,
            :meth:`linear_structures`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2)
            sage: S.has_linear_structure()
            True
        """
        return any(self.component_function(i).has_linear_structure() for i in range(1, 1<<self.n))

    def is_linear_structure(self, a, b):
        r"""
        Return ``True`` if `a` is a linear structure of the component function
        `b \cdot S(x)` where S is this `m \times n` S-Box.

        INPUT:

        - ``a`` -- either an integer or a tuple of `\GF{2}` elements of
          length equal to the input size of SBox
        - ``b`` -- either an integer or a tuple of `\GF{2}` elements of
          length equal to the output size of SBox

        .. SEEALSO::

            :meth:`linear_structures`,
            :meth:`has_linear_structure`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2)
            sage: S.component_function(1).autocorrelation()
            (16, -16, 0, 0, 0, 0, 0, 0, -16, 16, 0, 0, 0, 0, 0, 0)
            sage: S.is_linear_structure(1, 1)
            True
            sage: S.is_linear_structure([1, 0, 0, 1], [0, 0, 0, 1])
            True
            sage: S.is_linear_structure([0, 1, 1, 1], 1)
            False
        """
        return self.component_function(b).is_linear_structure(a)

    def max_degree(self):
        """
        Return the maximal algebraic degree of all its component functions.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2])
            sage: S.max_degree()
            3
        """
        n = self.n
        ret = 0

        for i in range(n):
            deg_Si = self.component_function(1<<i).algebraic_normal_form().degree()
            if deg_Si > ret:
                ret = deg_Si
        return ret

    def min_degree(self):
        """
        Return the minimal algebraic degree of all its component functions.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2])
            sage: S.min_degree()
            2
        """
        n = self.n
        ret = self.m

        for b in range(1, 1<<n):
            deg_bS = self.component_function(b).algebraic_normal_form().degree()
            if deg_bS < ret:
                ret = deg_bS
        return ret

    def is_balanced(self):
        r"""
        Return ``True`` if this S-Box is balanced.

        An S-Box is balanced if all its component functions are balanced.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2])
            sage: S.is_balanced()
            True
        """
        n = self.n

        for b in range(1, 1<<n):
            bS = self.component_function(b)
            if not bS.is_balanced():
                return False
        return True

    def is_almost_bent(self):
        r"""
        Return ``True`` if this S-Box is an almost bent (AB) function.

        An `m \times m` S-Box `S`, for `m` odd, is called almost bent if its
        nonlinearity is equal to `2^{m-1} - 2^{(m-1)/2}`.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0,1,3,6,7,4,5,2])
            sage: S.is_almost_bent()
            True
        """
        if self.m != self.n:
            raise TypeError("almost bent function only exists for self.m == self.n")

        m = self.m

        if is_even(m):
            return False

        return self.nonlinearity() == 2**(m-1) - 2**((m-1)//2)

    def fixed_points(self):
        """
        Return a list of all fixed points of this S-Box.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0,1,3,6,7,4,5,2])
            sage: S.fixed_points()
            [0, 1]
        """
        m = self.m
        return [i for i in range(1<<m) if i == self(i)]

    def inverse(self):
        """
        Return the inverse of this S-Box.

        Note that the S-Box must be invertible, otherwise it will raise
        a ``TypeError``.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0, 1, 3, 6, 7, 4, 5, 2])
            sage: Sinv = S.inverse()
            sage: [Sinv(S(i)) for i in range(8)]
            [0, 1, 2, 3, 4, 5, 6, 7]
        """
        if not self.is_permutation():
            raise TypeError("S-Box must be a permutation")

        m = self.m
        L = [self(i) for i in range(1<<m)]
        return SBox([L.index(i) for i in range(1<<m)], big_endian=self._big_endian)

    def is_monomial_function(self):
        r"""
        Return ``True`` if this S-Box is a monomial/power function.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([0,1,3,6,7,4,5,2])
            sage: S.is_monomial_function()
            False
            sage: S.interpolation_polynomial()
            (a + 1)*x^6 + (a^2 + a + 1)*x^5 + (a^2 + 1)*x^3

            sage: S = SBox(0,1,5,6,7,2,3,4)
            sage: S.is_monomial_function()
            True
            sage: S.interpolation_polynomial()
            x^6
        """
        return self.interpolation_polynomial().is_monomial()

    def is_plateaued(self):
        r"""
        Return ``True`` if this S-Box is plateaued, i.e. for all nonzero
        `b \in \mathbb{F}_2^n` the Boolean function `b \cdot S(x)`
        is plateaued.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox(0, 3, 1, 2, 4, 6, 7, 5)
            sage: S.is_plateaued()
            True
        """
        n = self.n

        for b in range(1, 1<<n):
            bS = self.component_function(b)
            if not bS.is_plateaued():
                return False
        return True

    def is_bent(self):
        r"""
        Return ``True`` if this S-Box is bent, i.e. its nonlinearity
        is equal to `2^{m-1} - 2^{m/2 - 1}` where `m` is the input size
        of the S-Box.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: R.<x> = GF(2**2, 'a')[]
            sage: base = R.base_ring()
            sage: a = base.gen()
            sage: G = a * x^2 + 1
            sage: S = SBox([G(x * y**(14)) for x in sorted(base) for y in sorted(base)])
            sage: S.is_bent()
            True
            sage: S.nonlinearity()
            6
            sage: S.linear_approximation_matrix()
            [ 8 -2  2 -2]
            [ 0 -2  2 -2]
            [ 0 -2  2 -2]
            [ 0 -2  2 -2]
            [ 0 -2  2 -2]
            [ 0 -2 -2  2]
            [ 0  2  2  2]
            [ 0  2 -2 -2]
            [ 0 -2  2 -2]
            [ 0  2 -2 -2]
            [ 0 -2 -2  2]
            [ 0  2  2  2]
            [ 0 -2  2 -2]
            [ 0  2  2  2]
            [ 0  2 -2 -2]
            [ 0 -2 -2  2]
        """
        m = self.m
        n = self.n

        if not is_even(m) or n > m//2:
            return False

        return self.nonlinearity() == 2**(m-1) - 2**(m//2 - 1)

    def is_involution(self):
        r"""
        Return ``True`` if this S-Box is an involution, i.e. the inverse S-Box
        is equal itself.

        EXAMPLES::

            sage: from sage.crypto.sbox import SBox
            sage: S = SBox([x**254 for x in sorted(GF(2**8))])
            sage: S.is_involution()
            True
        """
        return self == self.inverse()

def feistel_construction(*args):
    r"""
    Return an S-Box constructed by Feistel structure using smaller S-Boxes in
    ``args``. The number of round in the construction is equal to the number of
    S-Boxes provided as input. For more results concerning the differential
    uniformity and the nonlinearity of S-Boxes constructed by Feistel structures
    see [CDL2015]_ .

    INPUT:

    - ``args`` - a finite iterable SBox objects

    EXAMPLES:

    Suppose we construct an `8 \times 8` S-Box with 3-round Feistel construction
    from the S-Box of PRESENT::

        sage: from sage.crypto.sbox import SBox
        sage: s = SBox(12,5,6,11,9,0,10,13,3,14,15,8,4,7,1,2)
        sage: from sage.crypto.sbox import feistel_construction
        sage: S = feistel_construction(s, s, s)

    The properties of the constructed S-Box can be easily examined::

        sage: S.nonlinearity()
        96
        sage: S.differential_branch_number()
        2
        sage: S.linear_branch_number()
        2
    """
    if len(args) == 1:
        if isinstance(args[0], SBox):
            sboxes = [args[0]]
        else:
            sboxes = args[0]
    elif len(args) > 1:
        sboxes = args
    else:
        raise TypeError("No input provided")

    for sb in sboxes:
        if not isinstance(sb, SBox):
            raise TypeError("All input must be an instance of SBox object")

    b = sboxes[0].m
    m = 2*b

    def substitute(x):
        mask = (1<<b) - 1
        xl = (x>>b) & mask
        xr = x & mask
        for sb in sboxes:
            xl, xr = sb(xl) ^ xr, xl
        return (xl<<b) | xr

    return SBox([substitute(i) for i in range(1<<m)])

def misty_construction(*args):
    r"""
    Return an S-Box constructed by MISTY structure using smaller S-Boxes in
    ``args``. The number of round in the construction is equal to the number of
    S-Boxes provided as input. For further result related to the nonlinearity
    and differential uniformity of the constructed S-Box one may consult [CDL2015]_.

    INPUT:

    - ``args`` - a finite iterable SBox objects

    EXAMPLES:

    We construct an `8 \times 8` S-Box using 3-round MISTY structure with the following
    `4 \times 4` S-Boxes `S1, S2, S3` (see Example 2 in [CDL2015]_)::

        sage: from sage.crypto.sbox import SBox
        sage: S1 = SBox([0x4,0x0,0x1,0xF,0x2,0xB,0x6,0x7,0x3,0x9,0xA,0x5,0xC,0xD,0xE,0x8])
        sage: S2 = SBox([0x0,0x0,0x0,0x1,0x0,0xA,0x8,0x3,0x0,0x8,0x2,0xB,0x4,0x6,0xE,0xD])
        sage: S3 = SBox([0x0,0x7,0xB,0xD,0x4,0x1,0xB,0xF,0x1,0x2,0xC,0xE,0xD,0xC,0x5,0x5])
        sage: from sage.crypto.sbox import misty_construction
        sage: S = misty_construction(S1, S2, S3)
        sage: S.differential_uniformity()
        8
        sage: S.linearity()
        64
    """
    if len(args) == 1:
        if isinstance(args[0], SBox):
            sboxes = [args[0]]
        else:
            sboxes = args[0]
    elif len(args) > 1:
        sboxes = args
    else:
        raise TypeError("No input provided")

    for sb in sboxes:
        if not isinstance(sb, SBox):
            raise TypeError("All input must be an instance of SBox object")

    b = sboxes[0].m
    m = 2*b

    def substitute(x):
        mask = (1<<b) - 1
        xl = (x>>b) & mask
        xr = x & mask
        for sb in sboxes:
            xl, xr = sb(xr) ^ xl, xl
        return (xl<<b) | xr

    return SBox([substitute(i) for i in range(1<<m)])
