From: Matt Corallo Date: Fri, 29 Sep 2017 17:13:53 +0000 (-0400) Subject: Small cleanups X-Git-Url: http://git.bitcoin.ninja/?a=commitdiff_plain;h=HEAD;p=shamirs Small cleanups --- diff --git a/main.c b/main.c index af5a7d7..f79f074 100644 --- a/main.c +++ b/main.c @@ -58,8 +58,6 @@ static void derive_missing_part(uint8_t total_shares, uint8_t shares_required, b // shares, because more shares could be added arbitrarily, any x should not be // able to rule out any possible secrets) and try each possible q, making sure // that each q gives us a new possibility for the secret. - bool impossible_secrets[P]; - memset(impossible_secrets, 0, sizeof(impossible_secrets)); for (uint16_t final_x = 1; final_x < P; final_x++) { bool x_already_used = false; for (uint8_t j = 0; j < shares_required; j++) { diff --git a/shamirssecret.c b/shamirssecret.c index f5ca5c5..fd309f9 100644 --- a/shamirssecret.c +++ b/shamirssecret.c @@ -38,7 +38,6 @@ * Calculations across the finite field GF(2^8) */ -#ifndef TEST static uint8_t field_add(uint8_t a, uint8_t b) { return a ^ b; } @@ -50,7 +49,6 @@ static uint8_t field_sub(uint8_t a, uint8_t b) { static uint8_t field_neg(uint8_t a) { return field_sub(0, a); } -#endif //TODO: Using static tables will very likely create side-channel attacks when measuring cache hits // Because these are fairly small tables, we can probably get them loaded mostly/fully into @@ -114,27 +112,17 @@ static uint8_t field_invert(uint8_t a) { return exp[0xff - log[a]]; // log[1] == 0xff } -// We disable lots of optimizations that result in non-constant runtime (+/- branch delays) -static uint8_t field_pow_ret(uint8_t calc, uint8_t a, uint8_t e) __attribute__((optimize("-O0"))) noinline; -static uint8_t field_pow_ret(uint8_t calc, uint8_t a, uint8_t e) { - uint8_t ret, ret2; - if (a == 0) - ret2 = 0; - else - ret2 = calc; - if (e == 0) - ret = 1; - else - ret = ret2; - return ret; -} static uint8_t field_pow(uint8_t a, uint8_t e) { + uint8_t ret = exp[(log[a] * e) % 255]; #ifndef TEST - // Although this function works for a==0, its not trivially obvious why, - // and since we never call with a==0, we just assert a != 0 (except when testing) + // We only work for a == 0 by branching (below), but since we + // never call with a==0, we just assert a != 0 (except when testing) CHECKSTATE(a != 0); +#else + if (a == 0 && e != 0) + ret = 0; #endif - return field_pow_ret(exp[(log[a] * e) % 255], a, e); + return ret; } #ifdef TEST @@ -176,6 +164,17 @@ int main() { for (uint16_t j = 0; j < P; j++) CHECKSTATE(field_pow(i, j) == field_pow_calc(i, j)); } + + // Test invertibility of add/negate/subtract + for (uint16_t i = 0; i < P; i++) { + CHECKSTATE(field_neg(field_neg(i)) == i); + // Test add/sub commutativity + for (uint16_t j = 0; j < P; j++) { + CHECKSTATE(field_add(i, j) == field_add(j, i)); + CHECKSTATE(field_add(i, field_neg(j)) == field_sub(i, j)); + CHECKSTATE(field_add(field_neg(j), i) == field_sub(i, j)); + } + } } #endif // defined(TEST) @@ -202,7 +201,7 @@ uint8_t calculateQ(uint8_t coefficients[], uint8_t shares_required, uint8_t x) { * Derives the secret given a set of shares_required points (x and q coordinates) */ uint8_t calculateSecret(uint8_t x[], uint8_t q[], uint8_t shares_required) { - // Calculate the x^0 term using a derivation of the forumula at + // Calculate the x^0 term using a derivation of the formula at // http://en.wikipedia.org/wiki/Lagrange_polynomial#Example_2 uint8_t ret = 0, i, j; for (i = 0; i < shares_required; i++) {