added support for shamir key split secret size greater than 16 bytes

lib/Crypto/Protocol/SecretSharing.py

@@ -36,6 +36,7 @@
 from Crypto.Util.number import long_to_bytes, bytes_to_long
 from Crypto.Random import get_random_bytes as rng
 
+SHAMIR_BLOCK_SIZE = 16
 
 def _mult_gf2(f1, f2):
     """Multiply two polynomials in GF(2)"""
@@ -276,3 +277,44 @@ def combine(shares, ssss=False):
                     denominator *= x_j + x_m
             result += y_j * numerator * denominator.inverse()
         return result.encode()
+
+    @staticmethod
+    def split_large(k, n, secret, ssss=False):
+        """
+        Wrapper for Shamir.split()
+        when len(key) > SHAMIR_BLOCK_SIZE (16)
+        """
+        if isinstance(secret, bytes) is not True:
+            raise TypeError("Secret must be bytes")
+        if len(secret) % 16 != 0:
+            raise ValueError("Secret size must be in 16 byte increments")
+
+        blocks = len(secret) // SHAMIR_BLOCK_SIZE
+        shares = [b'' for _ in range(n)]
+        for i in range(blocks):
+            block_shares = Shamir.split(k, n,
+                    secret[i*SHAMIR_BLOCK_SIZE:(i+1)*SHAMIR_BLOCK_SIZE], ssss)
+            for j in range(n):
+                shares[j] += block_shares[j][1]
+        return [(i+1,shares[i]) for i in range(n)]
+
+    @staticmethod
+    def combine_large(shares, ssss=False):
+        """
+        Wrapper for Shamir.combine()
+        when len(key) > SHAMIR_BLOCK_SIZE (16)
+        """
+        share_len = len(shares[0][1])
+        for share in shares:
+            if len(share[1]) % 16 != 0:
+                raise ValueError(f"Share #{share[0]} is not in 16 byte increments")
+            if len(share[1]) != share_len:
+                raise ValueError("Share sizes are inconsistant")
+        blocks = share_len // SHAMIR_BLOCK_SIZE
+        result = b''
+        for i in range(blocks):
+            block_shares = [
+                    (int(idx), share[i*SHAMIR_BLOCK_SIZE:(i+1)*SHAMIR_BLOCK_SIZE]) 
+                for idx, share in shares]
+            result += Shamir.combine(block_shares, ssss)
+        return result

lib/Crypto/Protocol/SecretSharing.pyi

@@ -19,4 +19,8 @@ class Shamir(object):
     def split(k: int, n: int, secret: bytes, ssss: Optional[bool]) -> List[Tuple[int, bytes]]: ...
     @staticmethod
     def combine(shares: List[Tuple[int, bytes]], ssss: Optional[bool]) -> bytes: ...
+    @staticmethod
+    def split_large(k: int, n: int, secret: bytes, ssss: Optional[bool]) -> List[Tuple[int, bytes]]: ...
+    @staticmethod
+    def combine_large(shares: List[Tuple[int, bytes]], ssss: Optional[bool]) -> bytes: ...
 

lib/Crypto/SelfTest/Protocol/test_SecretSharing.py

@@ -209,6 +209,15 @@ def test2(self):
              "11-065d0082c80b1aea18f4abe0c49df72e",
              "12-84a09430c1d20ea9f388f3123c3733a3"),
         )
+        test_vectors_large = (
+            # combine key and shares to test larger than 16 byte key
+            (2, test_vectors[0][1] + test_vectors[1][1],
+                '1-'+test_vectors[0][2][2:] + test_vectors[1][2][2:],
+                '2-'+test_vectors[0][3][2:] + test_vectors[1][3][2:],
+                '3-'+test_vectors[0][4][2:] + test_vectors[1][4][2:],
+                '4-'+test_vectors[0][5][2:] + test_vectors[1][5][2:],
+            ),
+        )
 
         def get_share(p):
             pos = p.find('-')
@@ -225,6 +234,17 @@ def get_share(p):
                 result = Shamir.combine(shares, True)
                 self.assertEqual(secret, result)
 
+        for tv in test_vectors_large:
+            k = tv[0]
+            secret = unhexlify(tv[1])
+            max_perms = 10
+            for perm, shares_idx in enumerate(permutations(range(2, len(tv)), k)):
+                if perm > max_perms:
+                    break
+                shares = [ get_share(tv[x]) for x in shares_idx ]
+                result = Shamir.combine_large(shares, True)
+                self.assertEqual(secret, result)
+
     def test3(self):
         # Loopback split/recombine
         secret = unhexlify(b("000102030405060708090a0b0c0d0e0f"))
@@ -253,6 +273,21 @@ def test5(self):
         shares = Shamir.split(2, 3, secret)
         self.assertRaises(ValueError, Shamir.combine, (shares[0], shares[0]))
 
+    def test6(self):
+        # Test key size greater than 16 bytes
+        secret = unhexlify(b("000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f"))
+        shares = Shamir.split_large(2, 3, secret)
+
+        secret2 = Shamir.combine_large(shares[:2])
+        self.assertEqual(secret, secret2)
+
+    def test7(self):
+        secret = unhexlify(b("000102030405060708090a0b0c0d0e0f0001020304050607"))
+        self.assertRaises(ValueError, Shamir.split_large, 2, 3, secret)
+
+    def test8(self):
+        secret = 123456
+        self.assertRaises(TypeError, Shamir.split_large, 2, 3, secret)
 
 def get_tests(config={}):
     tests = []