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Make secp256k1_scalar_add_bit conditional; make secp256k1_scalar_split_lambda_var constant time

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This has the effect of making `secp256k1_scalar_mul_shift_var` constant
time in both input scalars. Keep the _var name because it is NOT constant
time in the shift amount.

As used in `secp256k1_scalar_split_lambda_var`, the shift is always
the constant 272, so this function becomes constant time, and it
loses the `_var` suffix.
This commit is contained in:
Andrew Poelstra
2015-05-22 11:51:51 -05:00
parent 91c0ce95ca
commit ed35d43a0c
7 changed files with 18 additions and 16 deletions
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6
src/scalar.h
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@@ -42,8 +42,8 @@ static void secp256k1_scalar_get_b32(unsigned char *bin, const secp256k1_scalar_
/** Add two scalars together (modulo the group order). Returns whether it overflowed. */
static int secp256k1_scalar_add(secp256k1_scalar_t *r, const secp256k1_scalar_t *a, const secp256k1_scalar_t *b);
/** Add a power of two to a scalar. The result is not allowed to overflow. */
static void secp256k1_scalar_add_bit(secp256k1_scalar_t *r, unsigned int bit);
/** Conditionally add a power of two to a scalar. The result is not allowed to overflow. */
static void secp256k1_scalar_cadd_bit(secp256k1_scalar_t *r, unsigned int bit, int flag);
/** Multiply two scalars (modulo the group order). */
static void secp256k1_scalar_mul(secp256k1_scalar_t *r, const secp256k1_scalar_t *a, const secp256k1_scalar_t *b);
@@ -95,7 +95,7 @@ static int secp256k1_scalar_eq(const secp256k1_scalar_t *a, const secp256k1_scal
/** Find r1 and r2 such that r1+r2*2^128 = a. */
static void secp256k1_scalar_split_128(secp256k1_scalar_t *r1, secp256k1_scalar_t *r2, const secp256k1_scalar_t *a);
/** Find r1 and r2 such that r1+r2*lambda = a, and r1 and r2 are maximum 128 bits long (see secp256k1_gej_mul_lambda). */
static void secp256k1_scalar_split_lambda_var(secp256k1_scalar_t *r1, secp256k1_scalar_t *r2, const secp256k1_scalar_t *a);
static void secp256k1_scalar_split_lambda(secp256k1_scalar_t *r1, secp256k1_scalar_t *r2, const secp256k1_scalar_t *a);
#endif
/** Multiply a and b (without taking the modulus!), divide by 2**shift, and round to the nearest integer. Shift must be at least 256. */