Shamira Changeset - 32a0e0fcabd0

Changeset - 32a0e0fcabd0

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Laman - 5 years ago 2020-05-03 18:28:16

optimized the reconstruction operation

6 files changed with 55 insertions and 32 deletions:

readme.md

src/shamira/condensed.py

src/shamira/core.py

src/shamira/gf256.py

src/shamira/tests/test_gf256.py

src/shamira/tests/test_shamira.py

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readme.md

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@@ @@ -7,38 +7,38 @@ Outputs the shares as hexadecimal, Base3 @@
 ## Installation and usage ##
 Can be run straight from the cloned repository by executing the package with `python -m shamira` or simply installed with `python setup.py build`, `python setup.py install`. Then imported in your code with `import shamira` or run from the command line with `shamira`.
 ## Performance ##
-As it is, the code is not very fast. Splitting takes _n_ evaluations of a polynomial of order _k_ over Galois field 256, leading to _O(n\*k)_ finite field multiplications. Reconstruction of the constant parameters during joining similarly takes _O(k\*k)_ multiplications.
+As it is, the code is not very fast. Let's assume we have a secret of length _m_. For each byte, the splitting takes _n_ evaluations of a polynomial of order _k_ over Galois field 256, leading to _O(n\*k\*m)_ finite field multiplications. Reconstruction of the constant parameters during joining takes _O(k\*k + k\*m)_ multiplications.
 Benchmark results, all values mean _seconds per byte_ of the secret length:
 <table>
     <tr>
         <th>k / n parameters</th>
         <th>Split</th>
         <th>Join</th>
     </tr>
     <tr>
         <td>2 / 3 (a Raspberry Pi 3)</td>
         <td>7.99e-05</td>
         <td>0.000428</td>
         <td>8.32e-05</td>
         <td>0.000435</td>
     </tr>
     <tr>
         <td>2 / 3 (a laptop)</td>
         <td>1e-05</td>
         <td>6.7e-05</td>
         <td>1.09e-05</td>
         <td>7.17e-05</td>
     </tr>
     <tr>
         <td>254 / 254 (a Raspberry Pi 3)</td>
         <td>0.417</td>
         <td>0.471</td>
         <td>0.414</td>
         <td>0.0314</td>
     </tr>
     <tr>
         <td>254 / 254 (a laptop)</td>
         <td>0.0431</td>
         <td>0.0542</td>
         <td>0.0435</td>
         <td>0.00347</td>
     </tr>
 </table>
 While the speeds are not awful, for longer secrets I recommend encrypting them with a random key of your choice and splitting only the key. Anyway, you can run your own benchmark with `shamira benchmark`

src/shamira/condensed.py

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@@ @@ -24,13 +24,13 @@ L = [None]*256 # logarithms @@
 acc = 1
 for i in range(256):
 	E[i] = acc
 	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
+INV = [E[255-L[i]] if i!=0 else None for i in range(256)]  # multiplicative inverse
 def get_constant_coef(*points):
 	"""Compute constant polynomial coefficient given the points.
 	See https://en.wikipedia.org/wiki/Shamir's_Secret_Sharing#Computationally_Efficient_Approach"""
@@ @@ -39,13 +39,13 @@ def get_constant_coef(*points): @@
 	for i in range(k):
 		(x, y) = points[i]
 		prod = 1
 		for j in range(k):
 			if i==j: continue
 			(xj, yj) = points[j]
-			prod = gfmul(prod, (gfmul(xj, inv[xj^x])))
+			prod = gfmul(prod, (gfmul(xj, INV[xj^x])))
 		res ^= gfmul(y, prod)
 	return res
 ###
 import base64

src/shamira/core.py

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@@ @@ -40,17 +40,19 @@ def reconstruct_raw(*shares): @@
 	:param shares: (((int) i, (bytes) share), ...)
 	:return: (bytes) reconstructed secret. Too few shares return garbage."""
 	if len({x for (x, _) in shares}) < len(shares):
 		raise MalformedShare("Found a non-unique share. Please check your inputs.")
 	secret_len = len(shares[0][1])
 	(xs, payloads) = zip(*shares)
 	secret_len = len(payloads[0])
 	res = [None]*secret_len
 	weights = gf256.compute_weights(xs)
 	for i in range(secret_len):
 		points = [(x, s[i]) for (x, s) in shares]
 		res[i] = (gf256.get_constant_coef(*points))
 		ys = [s[i] for s in payloads]
 		res[i] = (gf256.get_constant_coef(weights, ys))
 	return bytes(res)
 def generate(secret, k, n, encoding="b32", label="", omit_k_n=False):
 	"""Wraps generate_raw().

src/shamira/gf256.py

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 # GNU GPLv3, see LICENSE
 """Arithmetic operations on Galois Field 2**8. See https://en.wikipedia.org/wiki/Finite_field_arithmetic"""
 from functools import reduce
 import operator
 def _gfmul(a, b):
 	"""Basic multiplication. Russian peasant algorithm."""
 	res = 0
 	while a and b:
 		if b&1: res ^= a
@@ @@ -44,21 +47,28 @@ def evaluate(coefs, x): @@
 	for a in coefs:
 		res ^= gfmul(a, xk)
 		xk = gfmul(xk, x)
 	return res
-def get_constant_coef(*points):
+def get_constant_coef(weights, y_coords):
 	"""Compute constant polynomial coefficient given the points.
 	See https://en.wikipedia.org/wiki/Shamir's_Secret_Sharing#Computationally_Efficient_Approach"""
 	k = len(points)
 	res = 0
 	for i in range(k):
 		(x, y) = points[i]
 		prod = 1
 		for j in range(k):
 			if i==j: continue
 			(xj, yj) = points[j]
 			prod = gfmul(prod, (gfmul(xj, INV[xj^x])))
 		res ^= gfmul(y, prod)
 	return reduce(
 		operator.xor,
 		map(lambda ab: gfmul(*ab), zip(weights, y_coords))
+	)
 def compute_weights(x_coords):
 	assert x_coords
 	res = [
 			reduce(
 				gfmul,
 				(gfmul(xj, INV[xj^xi]) for xj in x_coords if xi!=xj),
 			) for xi in x_coords
+	]
 	return res

src/shamira/tests/test_gf256.py

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@@ @@ -27,13 +27,15 @@ class TestGF256(TestCase): @@
 			(a0, a1, a2, a3) = (x, x>>1, x>>2, x>>3)
 			self.assertEqual(evaluate([17], x), 17)  # constant polynomial
 			self.assertEqual(evaluate([a0, a1, a2, a3], 0), x)  # any polynomial at 0
 			self.assertEqual(evaluate([a0, a1, a2, a3], 1), a0^a1^a2^a3)  # polynomial at 1 == sum of coefficients
 	def test_get_constant_coef(self):
 		self.assertEqual(get_constant_coef((1, 1), (2, 2), (3, 3)), 0)
 		weights = compute_weights((1, 2, 3))
 		ys = (1, 2, 3)
 		self.assertEqual(get_constant_coef(weights, ys), 0)
 		random.seed(17)
 		random_matches = 0
 		for i in range(10):
 			k = random.randint(2, 255)
@@ @@ -52,11 +54,14 @@ class TestGF256(TestCase): @@
 		self.assertLess(random_matches, 2)  # with a chance (255/256)**10=0.96 there should be no match
 	def check_coefs_match(self, k, m):
 		coefs = [random.randint(0, 255) for i in range(k)]
 		points = [(j, evaluate(coefs, j)) for j in range(1, 256)]
 		random.shuffle(points)
 		return (get_constant_coef(*points[:m]), coefs[0])
 		(xs, ys) = zip(*points[:m])
 		weights = compute_weights(xs)
 		return (get_constant_coef(weights, ys), coefs[0])
 if __name__=='__main__':
 	unittest.main()

src/shamira/tests/test_shamira.py

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@@ @@ -25,17 +25,23 @@ class TestShamira(TestCase): @@
 			_share_byte(b"a", 5, 4)
 		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)
 		vals = _share_byte(ord(b"a"), 2, 3)
 		points = list(zip(range(1, 256), vals))
 		self.assertEqual(gf256.get_constant_coef(*points), ord(b"a"))
 		self.assertEqual(gf256.get_constant_coef(*points[:2]), ord(b"a"))
 		self.assertNotEqual(gf256.get_constant_coef(*points[:1]), ord(b"a"))  # underdetermined => random
 		ys = _share_byte(ord(b"a"), 2, 3)
 		xs = list(range(1, 4))
 		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"))
 		weights = gf256.compute_weights(xs[:1])
 		self.assertNotEqual(gf256.get_constant_coef(weights, ys[:1]), ord(b"a"))  # underdetermined => random
 	def test_generate_reconstruct_raw(self):
 		for (k, n) in [(2, 3), (254, 254)]:
 			shares = generate_raw(b"abcd", k, n)
 			random.shuffle(shares)
 			self.assertEqual(reconstruct_raw(*shares[:k]), b"abcd")

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