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Time-constant P256.scalarAdd and P256.scalarSub

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This commit is contained in:
Olivier Chéron 2017-11-21 19:25:41 +01:00
parent f4be05eb2e
commit 3253501166
2 changed files with 28 additions and 21 deletions
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29
Crypto/PubKey/ECC/P256.hs
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@ -222,34 +222,21 @@ scalarIsZero s = unsafeDoIO $ withScalar s $ \d -> do
result <- ccryptonite_p256_is_zero d
return $ result /= 0
scalarNeedReducing :: Ptr P256Scalar -> IO Bool
scalarNeedReducing d = do
c <- ccryptonite_p256_cmp d ccryptonite_SECP256r1_n
return (c >= 0)
-- | Perform addition between two scalars
--
-- > a + b
scalarAdd :: Scalar -> Scalar -> Scalar
scalarAdd a b =
withNewScalarFreeze $ \d -> withScalar a $ \pa -> withScalar b $ \pb -> do
carry <- ccryptonite_p256_add pa pb d
when (carry /= 0) $ void $ ccryptonite_p256_sub d ccryptonite_SECP256r1_n d
needReducing <- scalarNeedReducing d
when needReducing $ do
ccryptonite_p256_mod ccryptonite_SECP256r1_n d d
withNewScalarFreeze $ \d -> withScalar a $ \pa -> withScalar b $ \pb ->
ccryptonite_p256e_modadd ccryptonite_SECP256r1_n pa pb d
-- | Perform subtraction between two scalars
--
-- > a - b
scalarSub :: Scalar -> Scalar -> Scalar
scalarSub a b =
withNewScalarFreeze $ \d -> withScalar a $ \pa -> withScalar b $ \pb -> do
borrow <- ccryptonite_p256_sub pa pb d
when (borrow /= 0) $ void $ ccryptonite_p256_add d ccryptonite_SECP256r1_n d
--needReducing <- scalarNeedReducing d
--when needReducing $ do
-- ccryptonite_p256_mod ccryptonite_SECP256r1_n d d
withNewScalarFreeze $ \d -> withScalar a $ \pa -> withScalar b $ \pb ->
ccryptonite_p256e_modsub ccryptonite_SECP256r1_n pa pb d
-- | Give the inverse of the scalar
--
@ -352,12 +339,12 @@ foreign import ccall "cryptonite_p256_is_zero"
ccryptonite_p256_is_zero :: Ptr P256Scalar -> IO CInt
foreign import ccall "cryptonite_p256_clear"
ccryptonite_p256_clear :: Ptr P256Scalar -> IO ()
foreign import ccall "cryptonite_p256_add"
ccryptonite_p256_add :: Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> IO CInt
foreign import ccall "cryptonite_p256e_modadd"
ccryptonite_p256e_modadd :: Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> IO ()
foreign import ccall "cryptonite_p256_add_d"
ccryptonite_p256_add_d :: Ptr P256Scalar -> P256Digit -> Ptr P256Scalar -> IO CInt
foreign import ccall "cryptonite_p256_sub"
ccryptonite_p256_sub :: Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> IO CInt
foreign import ccall "cryptonite_p256e_modsub"
ccryptonite_p256e_modsub :: Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> Ptr P256Scalar -> IO ()
foreign import ccall "cryptonite_p256_cmp"
ccryptonite_p256_cmp :: Ptr P256Scalar -> Ptr P256Scalar -> IO CInt
foreign import ccall "cryptonite_p256_mod"

20
cbits/p256/p256.c
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@ -386,3 +386,23 @@ void cryptonite_p256_to_bin(const cryptonite_p256_int* src, uint8_t dst[P256_NBY
p += 4;
}
}
/*
"p256e" functions are not part of the original source
*/
// c = a + b mod MOD
void cryptonite_p256e_modadd(const cryptonite_p256_int* MOD, const cryptonite_p256_int* a, const cryptonite_p256_int* b, cryptonite_p256_int* c) {
int carry = cryptonite_p256_add(a, b, c);
// same as cryptonite_p256_mod, but with top = carry
addM(MOD, 0, P256_DIGITS(c), subM(MOD, carry, P256_DIGITS(c), -1));
}
// c = a - b mod MOD
void cryptonite_p256e_modsub(const cryptonite_p256_int* MOD, const cryptonite_p256_int* a, const cryptonite_p256_int* b, cryptonite_p256_int* c) {
int borrow = cryptonite_p256_sub(a, b, c);
// use borrow as mask in order to make difference positive when necessary
addM(MOD, 0, P256_DIGITS(c), borrow);
}