Shamira Changeset - 0957647049ef

Changeset - 0957647049ef

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Laman - 4 years ago 2020-12-18 12:13:06

merged FFT

6 files changed with 197 insertions and 8 deletions:

src/shamira/core.py

src/shamira/fft.py

src/shamira/gf256.py

src/shamira/tests/test_fft.py

src/shamira/tests/test_gf256.py

src/shamira/tests/test_shamira.py

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src/shamira/core.py

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@@ @@ -3,39 +3,45 @@ @@
 import os
 import re
 import base64
 import binascii
 from . import gf256
 from . import fft
 class SException(Exception): pass
 class InvalidParams(SException): pass
 class DetectionException(SException): pass
 class DecodingException(SException): pass
 class MalformedShare(SException): pass
 def compute_x(n):
 	return fft.precompute_x(fft.ceil_size(n))[:n]
 def _share_byte(secret_b, k, n):
 	if not k<=n<255:
 		raise InvalidParams("Failed k<=n<255, k={0}, n={1}".format(k, n))
 	# we might be concerned with zero coefficients degenerating our polynomial, but there's no reason - we still need k shares to determine it is the case
 	coefs = [int(b) for b in os.urandom(k-1)]+[int(secret_b)]
 	points = [gf256.evaluate(coefs, i) for i in range(1, n+1)]
 	return points
 	# we might be concerned with zero coefficients degenerating our polynomial,
 	# but there's no reason - we still need k shares to determine it is the case
 	coefs = [int(secret_b)]+[int(b) for b in os.urandom(k-1)]
 	return fft.evaluate(coefs, n)
 def generate_raw(secret, k, n):
 	"""Splits secret into shares.
 	:param secret: (bytes)
 	:param k: number of shares necessary for secret recovery. 1 <= k <= n
 	:param n: (int) number of shares generated. 1 <= n < 255
 	:return: [(i, (bytes) share), ...]"""
 	xs = compute_x(n)
 	shares = [_share_byte(b, k, n) for b in secret]
-	return [(i+1, bytes([s[i] for s in shares])) for i in range(n)]
+	return [(xi, bytes([s[i] for s in shares])) for (i, xi) in enumerate(xs)]
 def reconstruct_raw(*shares):
 	"""Tries to recover the secret from its shares.
 	:param shares: (((int) i, (bytes) share), ...)

src/shamira/fft.py

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@@ new file 100644 @@
 import math
 import cmath
 import itertools
 from .gf256 import gfmul, gfpow
 # divisors of 255 and their factors in natural numbers
 DIVISORS = [3, 5, 15, 17, 51, 85, 255]
 FACTORS = {3: [3], 5: [5], 15: [3, 5], 17: [17], 51: [3, 17], 85: [5, 17], 255: [3, 5, 17]}
 # values of n-th square roots in GF256
 SQUARE_ROOTS = {3: 189, 5: 12, 15: 225, 17: 53, 51: 51, 85: 15, 255: 3}
 def ceil_size(n):
 	assert n <= DIVISORS[-1]
 	for (i, ni) in enumerate(DIVISORS):
 		if ni >= n:
 			break
 	return ni
 def precompute_x(n):
 	"""Return a geometric sequence [1, w, w**2, ..., w**(n-1)], where w**n==1.
 	This can be done only for certain values of n."""
 	assert n in SQUARE_ROOTS, n
 	w = SQUARE_ROOTS[n]  # primitive N-th square root of 1
 	return list(itertools.accumulate([1]+[w]*(n-1), gfmul))
 def complex_dft(p):
 	"""Quadratic formula from the definition. The basic case in complex numbers."""
 	N = len(p)
 	w = cmath.exp(-2*math.pi*1j/N)  # primitive N-th square root of 1
 	y = [0]*N
 	for k in range(N):
 		xk = w**k
 		for n in range(N):
 			y[k] += p[n] * xk**n
 	return y
 def dft(p):
 	"""Quadratic formula from the definition. In GF256."""
 	N = len(p)
 	x = precompute_x(N)
 	y = [0]*N
 	for k in range(N):
 		for n in range(N):
 			y[k] ^= gfmul(p[n], gfpow(x[k], n))
 	return y
 def compute_inverse(N1, N2):
 	for i in range(N2):
 		if N1*i % N2 == 1:
 			return i
 	raise ValueError("Failed to find an inverse to {0} mod {1}.".format(N1, N2))
 def prime_fft(p, divisors, basic_dft=dft):
 	"""https://en.wikipedia.org/wiki/Prime-factor_FFT_algorithm"""
 	if len(divisors) == 1:
 		return basic_dft(p)
 	N = len(p)
 	N1 = divisors[0]
 	N2 = N//N1
 	N1_inv = compute_inverse(N1, N2)
 	N2_inv = compute_inverse(N2, N1)
 	ys = []
 	for n1 in range(N1):  # compute rows
 		p_ = [p[(n2*N1+n1*N2) % N] for n2 in range(N2)]
 		ys.append(prime_fft(p_, divisors[1:], basic_dft))
 	for k2 in range(N2):  # compute cols
 		p_ = [row[k2] for row in ys]
 		y_ = basic_dft(p_)
 		for (yi, row) in zip(y_, ys):  # update col
 			row[k2] = yi
 	# remap and output
 	res = [0]*N
 	for k1 in range(N1):
 		for k2 in range(N2):
 			res[(k1*N2*N2_inv+k2*N1*N1_inv) % N] = ys[k1][k2]
 	return res
 def evaluate(coefs, n):
 	ni = ceil_size(n)
 	extended_coefs = coefs + [0]*(ni-len(coefs))
 	ys = prime_fft(extended_coefs, FACTORS[ni])
 	return ys[:n]

src/shamira/gf256.py

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@@ @@ -26,22 +26,34 @@ for i in range(256): @@
 	L[acc] = i
 	acc = _gfmul(acc, g)
 L[1] = 0
 INV = [E[255-L[i]] if i!=0 else None for i in range(256)]  # multiplicative inverse
 @cache
 def gfmul(a, b):
 	"""Fast multiplication. Basic multiplication is expensive. a*b==g**(log(a)+log(b))"""
 	assert 0<=a<=255, 0<=b<=255
 	if a==0 or b==0: return 0
 	t = L[a]+L[b]
 	if t>255: t -= 255
 	return E[t]
 def gfpow(x, k):
 	"""Compute x**k."""
 	i = 1
 	res = 1
 	while i <= k:
 		if k&i:
 			res = gfmul(res, x)
 		x = gfmul(x, x)
 		i <<= 1
 	return res
 def evaluate(coefs, x):
 	"""Evaluate polynomial's value at x.
 	:param coefs: [an, ..., a1, a0]."""
 	res = 0

src/shamira/tests/test_fft.py

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@@ new file 100644 @@
 # GNU GPLv3, see LICENSE
 import random
 import functools
 import operator
 from unittest import TestCase
 from .. import gf256
 from ..fft import *
 def batch_evaluate(coefs, xs):
 	return [gf256.evaluate(coefs, x) for x in xs]
 class TestFFT(TestCase):
 	def test_complex_dft(self):
 		self.assertEqual(complex_dft([0]), [0+0j])
 		self.assertEqual(complex_dft([1]), [1+0j])
 		self.assertEqual(complex_dft([2]), [2+0j])
 		all(self.assertAlmostEqual(a, b) for (a, b) in zip(complex_dft([3, 1]), [4+0j, 2+0j]))
 		all(self.assertAlmostEqual(a, b) for (a, b) in zip(complex_dft([3, 1, 4]), [8+0j, 0.5+2.59807621j, 0.5-2.59807621j]))
 		all(self.assertAlmostEqual(a, b) for (a, b) in zip(complex_dft([3, 1, 4, 1]), [9+0j, -1+0j, 5+0j, -1+0j]))
 		all(self.assertAlmostEqual(a, b) for (a, b) in zip(
 			complex_dft([3, 1, 4, 1, 5]),
 			[14+0j, 0.80901699+2.04087031j, -0.30901699+5.20431056j, -0.30901699-5.20431056j, 0.80901699-2.04087031j]
 		))
 	def test_complex_prime_fft(self):
 		random.seed(1918)
 		for divisors in [[3], [2, 3], [3, 5], [3, 5, 17], [2, 3, 5, 7, 11]]:
 			n = functools.reduce(operator.mul, divisors)
 			coefficients = [random.randint(-128, 127) for i in range(n)]
 			a = prime_fft(coefficients, divisors, complex_dft)
 			b = complex_dft(coefficients)
 			all(self.assertAlmostEqual(ai, bi) for (ai, bi) in zip(a, b))
 	def test_finite_dft(self):
 		random.seed(1918)
 		x = {i: precompute_x(i) for i in [3, 5, 15, 17]}  # all sets of xs
 		for n in [3, 5, 15, 17]:
 			coefficients = [random.randint(0, 255) for i in range(n)]
 			self.assertEqual(
 				dft(coefficients),
 				batch_evaluate(coefficients[::-1], x[n])
+			)
 	def test_finite_prime_fft(self):
 		random.seed(1918)
 		for divisors in [[3], [3, 5], [3, 17], [5, 17], [3, 5, 17]]:
 			n = functools.reduce(operator.mul, divisors)
 			coefficients = [random.randint(0, 255) for i in range(n)]
 			a = prime_fft(coefficients, divisors)
 			b = dft(coefficients)
 			all(self.assertAlmostEqual(ai, bi) for (ai, bi) in zip(a, b))

src/shamira/tests/test_gf256.py

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@@ @@ -19,12 +19,32 @@ class TestGF256(TestCase): @@
 	def test_gfmul(self):
 		for a in range(256):
 			for b in range(256):
 				self.assertEqual(_gfmul(a, b), gfmul(a, b))
 	def test_gfpow(self):
 		self.assertEqual(gfpow(0, 0), 1)
 		for i in range(1, 256):
 			self.assertEqual(gfpow(i, 0), 1)
 			self.assertEqual(gfpow(i, 1), i)
 			self.assertEqual(gfpow(0, i), 0)
 			self.assertEqual(gfpow(1, i), 1)
 			self.assertEqual(gfpow(i, 256), i)
 			self.assertEqual(gfpow(i, 2), gfmul(i, i))
 		random.seed(1918)
 		for i in range(256):
 			j = random.randint(2, 255)
 			k = random.randint(3, 255)
 			y = 1
 			for m in range(k):
 				y = gfmul(y, j)
 			self.assertEqual(gfpow(j, k), y)
 	def test_evaluate(self):
 		for x in range(256):
 			(a0, a1, a2, a3) = (x, x>>1, x>>2, x>>3)
 			self.assertEqual(evaluate([17], x), 17)  # constant polynomial
 			self.assertEqual(evaluate([a3, a2, a1, a0], 0), x)  # any polynomial at 0
 			self.assertEqual(evaluate([a3, a2, a1, a0], 1), a0^a1^a2^a3)  # polynomial at 1 == sum of coefficients

src/shamira/tests/test_shamira.py

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@@ @@ -2,13 +2,13 @@ @@
 import os
 import random
 from unittest import TestCase
 from .. import *
 from .. import gf256
 from ..core import encode, decode,detect_encoding, _share_byte
+from ..core import encode, decode, detect_encoding, _share_byte, compute_x
 class TestShamira(TestCase):
 	_urandom = os.urandom
 	@classmethod
@@ @@ -26,13 +26,13 @@ class TestShamira(TestCase): @@
 		with self.assertRaises(InvalidParams):  # too many shares
 			_share_byte(b"a", 5, 255)
 		with self.assertRaises(ValueError):  # not castable to int
 			_share_byte("x", 2, 3)
 		ys = _share_byte(ord(b"a"), 2, 3)
-		xs = list(range(1, 4))
+		xs = compute_x(3)
 		weights = gf256.compute_weights(xs)
 		self.assertEqual(gf256.get_constant_coef(weights, ys), ord(b"a"))
 		weights = gf256.compute_weights(xs[:2])
 		self.assertEqual(gf256.get_constant_coef(weights, ys[:2]), ord(b"a"))

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