def det_from_modp_and_divisor(A,
d,
p,
z_mod,
moduli,
z_so_far=ZZ(1),
N_so_far=ZZ(1)):
"""
This is used for internal purposes for computing determinants
quickly (with the hybrid p-adic / multimodular algorithm).
INPUT:
- A -- a square matrix
- d -- a divisor of the determinant of A
- p -- a prime
- z_mod -- values of det/d (mod ...)
- moduli -- the moduli so far
- z_so_far -- for a modulus p in the list moduli,
(z_so_far mod p) is the determinant of A modulo p.
- N_so_far -- N_so_far is the product over the primes in the list moduli.
OUTPUT:
- A triple (det bound, new z_so_far, new N_so_far).
EXAMPLES::
sage: a = matrix(ZZ, 3, [6, 1, 2, -56, -2, -1, -11, 2, -3])
sage: factor(a.det())
-1 * 13 * 29
sage: d = 13
sage: import sage.matrix.matrix_integer_dense_hnf as matrix_integer_dense_hnf
sage: matrix_integer_dense_hnf.det_from_modp_and_divisor(a, d, 97, [], [])
(-377, -29, 97)
sage: a.det()
-377
"""
tm = verbose("Multimodular stage of det calculation -- using p = %s" % p,
level=2)
z = A.mod(p).det() / d
z = z.lift()
z_mod.append(z)
moduli.append(p)
z = CRT_list([z_so_far, z], [N_so_far, p])
N = N_so_far * p
if z > N // 2:
z -= N
verbose("Finished multimodular det for p = %s" % p, tm, level=2)
return (d * z, z, N)
def discrete_log(a,
base,
ord=None,
bounds=None,
operation='*',
identity=None,
inverse=None,
op=None):
r"""
Totally generic discrete log function.
INPUT:
- ``a`` - group element
- ``base`` - group element (the base)
- ``ord`` - integer (multiple of order of base, or ``None``)
- ``bounds`` - a priori bounds on the log
- ``operation`` - string: '*', '+', 'other'
- ``identity`` - the group's identity
- ``inverse()`` - function of 1 argument ``x`` returning inverse of ``x``
- ``op()`` - function of 2 arguments ``x``, ``y`` returning ``x*y`` in group
``a`` and ``base`` must be elements of some group with identity
given by identity, inverse of ``x`` by ``inverse(x)``, and group
operation on ``x``, ``y`` by ``op(x,y)``.
If operation is '*' or '+' then the other
arguments are provided automatically; otherwise they must be
provided by the caller.
OUTPUT: Returns an integer `n` such that `b^n = a` (or `nb = a`),
assuming that ``ord`` is a multiple of the order of the base `b`.
If ``ord`` is not specified, an attempt is made to compute it.
If no such `n` exists, this function raises a ValueError exception.
.. warning::
If ``x`` has a log method, it is likely to be vastly faster
than using this function. E.g., if ``x`` is an integer modulo
`n`, use its log method instead!
ALGORITHM: Pohlig-Hellman and Baby step giant step.
EXAMPLES::
sage: b = Mod(2,37); a = b^20
sage: discrete_log(a, b)
20
sage: b = Mod(2,997); a = b^20
sage: discrete_log(a, b)
20
sage: K = GF(3^6,'b')
sage: b = K.gen()
sage: a = b^210
sage: discrete_log(a, b, K.order()-1)
210
sage: b = Mod(1,37); x = Mod(2,37)
sage: discrete_log(x, b)
Traceback (most recent call last):
...
ValueError: No discrete log of 2 found to base 1
sage: b = Mod(1,997); x = Mod(2,997)
sage: discrete_log(x, b)
Traceback (most recent call last):
...
ValueError: No discrete log of 2 found to base 1
See :trac:`2356`::
sage: F.<w> = GF(121)
sage: v = w^120
sage: v.log(w)
0
sage: K.<z>=CyclotomicField(230)
sage: w=z^50
sage: discrete_log(w,z)
50
An example where the order is infinite: note that we must give
an upper bound here::
sage: K.<a> = QuadraticField(23)
sage: eps = 5*a-24 # a fundamental unit
sage: eps.multiplicative_order()
+Infinity
sage: eta = eps^100
sage: discrete_log(eta,eps,bounds=(0,1000))
100
In this case we cannot detect negative powers::
sage: eta = eps^(-3)
sage: discrete_log(eta,eps,bounds=(0,100))
Traceback (most recent call last):
...
ValueError: No discrete log of -11515*a - 55224 found to base 5*a - 24
But we can invert the base (and negate the result) instead::
sage: - discrete_log(eta^-1,eps,bounds=(0,100))
-3
An additive example: elliptic curve DLOG::
sage: F=GF(37^2,'a')
sage: E=EllipticCurve(F,[1,1])
sage: F.<a>=GF(37^2,'a')
sage: E=EllipticCurve(F,[1,1])
sage: P=E(25*a + 16 , 15*a + 7 )
sage: P.order()
672
sage: Q=39*P; Q
(36*a + 32 : 5*a + 12 : 1)
sage: discrete_log(Q,P,P.order(),operation='+')
39
An example of big smooth group::
sage: F.<a>=GF(2^63)
sage: g=F.gen()
sage: u=g**123456789
sage: discrete_log(u,g)
123456789
AUTHORS:
- William Stein and David Joyner (2005-01-05)
- John Cremona (2008-02-29) rewrite using ``dict()`` and make generic
"""
if ord is None:
if operation in multiplication_names:
try:
ord = base.multiplicative_order()
except Exception:
ord = base.order()
elif operation in addition_names:
try:
ord = base.additive_order()
except Exception:
ord = base.order()
else:
try:
ord = base.order()
except Exception:
raise ValueError("ord must be specified")
try:
from sage.rings.infinity import Infinity
if ord == +Infinity:
return bsgs(base, a, bounds, operation=operation)
if ord == 1 and a != base:
raise ValueError
f = ord.factor()
l = [0] * len(f)
for i, (pi, ri) in enumerate(f):
for j in range(ri):
if operation in multiplication_names:
c = bsgs(base**(ord // pi),
(a / base**l[i])**(ord // pi**(j + 1)), (0, pi),
operation=operation)
l[i] += c * (pi**j)
elif operation in addition_names:
c = bsgs(base * (ord // pi),
(a - base * l[i]) * (ord // pi**(j + 1)), (0, pi),
operation=operation)
l[i] += c * (pi**j)
from sage.arith.all import CRT_list
return CRT_list(l, [pi**ri for pi, ri in f])
except ValueError:
raise ValueError("No discrete log of %s found to base %s" % (a, base))
