X-Git-Url: http://git.bitcoin.ninja/index.cgi?p=shamirs;a=blobdiff_plain;f=shamirssecret.c;h=fd309f9386ea06d66feeba3d2b7a13c0f0dd5a94;hp=b69004884bb889f5f0dc9530d39edd42b76092a8;hb=HEAD;hpb=b6006436d939a7cba7c141f00b4efc638a694d14 diff --git a/shamirssecret.c b/shamirssecret.c index b690048..fd309f9 100644 --- a/shamirssecret.c +++ b/shamirssecret.c @@ -1,20 +1,42 @@ -#define _GNU_SOURCE +/* + * Shamir's secret sharing implementation + * + * Copyright (C) 2013 Matt Corallo + * + * This file is part of ASSS (Audit-friendly Shamir's Secret Sharing) + * + * ASSS is free software: you can redistribute it and/or modify + * it under the terms of the GNU Affero General Public License as + * published by the Free Software Foundation, either version 3 of + * the License, or (at your option) any later version. + * + * ASSS is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU Affero General Public License for more details. + * + * You should have received a copy of the GNU Affero General Public + * License along with ASSS. If not, see + * . + */ -#include -#include +#ifndef IN_KERNEL #include -#include -#include -#include -#include +#define CHECKSTATE(x) assert(x) +#else +#include +#define CHECKSTATE(x) BUG_ON(!(x)) +#endif -#define MAX_LENGTH 1024 -#define ERROREXIT(str...) {fprintf(stderr, str); exit(1);} +#include "shamirssecret.h" + +#ifndef noinline +#define noinline __attribute__((noinline)) +#endif /* * Calculations across the finite field GF(2^8) */ -#define P 256 static uint8_t field_add(uint8_t a, uint8_t b) { return a ^ b; @@ -28,6 +50,9 @@ static uint8_t field_neg(uint8_t a) { return field_sub(0, a); } +//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 +// cache before use to break such attacks. static const uint8_t exp[P] = { 0x01, 0x03, 0x05, 0x0f, 0x11, 0x33, 0x55, 0xff, 0x1a, 0x2e, 0x72, 0x96, 0xa1, 0xf8, 0x13, 0x35, 0x5f, 0xe1, 0x38, 0x48, 0xd8, 0x73, 0x95, 0xa4, 0xf7, 0x02, 0x06, 0x0a, 0x1e, 0x22, 0x66, 0xaa, @@ -65,7 +90,7 @@ static const uint8_t log[P] = { 0x4a, 0xed, 0xde, 0xc5, 0x31, 0xfe, 0x18, 0x0d, 0x63, 0x8c, 0x80, 0xc0, 0xf7, 0x70, 0x07}; // We disable lots of optimizations that result in non-constant runtime (+/- branch delays) -static uint8_t field_mul_ret(uint8_t calc, uint8_t a, uint8_t b) __attribute__((optimize("-O0"))) __attribute__((noinline)); +static uint8_t field_mul_ret(uint8_t calc, uint8_t a, uint8_t b) __attribute__((optimize("-O0"))) noinline; static uint8_t field_mul_ret(uint8_t calc, uint8_t a, uint8_t b) { uint8_t ret, ret2; if (a == 0) @@ -83,26 +108,21 @@ static uint8_t field_mul(uint8_t a, uint8_t b) { } static uint8_t field_invert(uint8_t a) { - assert(a != 0); + CHECKSTATE(a != 0); 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"))) __attribute__((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) { - return field_pow_ret(exp[(log[a] * e) % 255], a, e); + uint8_t ret = exp[(log[a] * e) % 255]; +#ifndef TEST + // 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 ret; } #ifdef TEST @@ -125,24 +145,35 @@ static uint8_t field_mul_calc(uint8_t a, uint8_t b) { static uint8_t field_pow_calc(uint8_t a, uint8_t e) { uint8_t ret = 1; for (uint8_t i = 0; i < e; i++) - ret = field_mul(ret, a); + ret = field_mul_calc(ret, a); return ret; } int main() { // Test inversion with the logarithm tables for (uint16_t i = 1; i < P; i++) - assert(field_mul_calc(i, field_invert(i)) == 1); + CHECKSTATE(field_mul_calc(i, field_invert(i)) == 1); // Test multiplication with the logarithm tables - for (uint16_t i = 0; i < 2; i++) { + for (uint16_t i = 0; i < P; i++) { for (uint16_t j = 0; j < P; j++) - assert(field_mul(i, j) == field_mul_calc(i, j)); + CHECKSTATE(field_mul(i, j) == field_mul_calc(i, j)); } // Test exponentiation with the logarithm tables for (uint16_t i = 0; i < P; i++) { for (uint16_t j = 0; j < P; j++) - assert(field_pow(i, j) == field_pow_calc(i, 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) @@ -153,22 +184,29 @@ int main() { * Calculations across the polynomial q */ #ifndef TEST -static uint8_t calculateQ(uint8_t a[], uint8_t k, uint8_t x) { - assert(x != 0); // q(0) == secret, though so does a[0] - uint8_t ret = a[0]; - for (uint8_t i = 1; i < k; i++) { - ret = field_add(ret, field_mul(a[i], field_pow(x, i))); +/** + * Calculates the Y coordinate that the point with the given X + * coefficients[0] == secret, the rest are random values + */ +uint8_t calculateQ(uint8_t coefficients[], uint8_t shares_required, uint8_t x) { + uint8_t ret = coefficients[0], i; + CHECKSTATE(x != 0); // q(0) == secret, though so does a[0] + for (i = 1; i < shares_required; i++) { + ret = field_add(ret, field_mul(coefficients[i], field_pow(x, i))); } return ret; } -uint8_t calculateSecret(uint8_t x[], uint8_t q[], uint8_t k) { - // Calculate the x^0 term using a derivation of the forumula at +/** + * 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 formula at // http://en.wikipedia.org/wiki/Lagrange_polynomial#Example_2 - uint8_t ret = 0; - for (uint8_t i = 0; i < k; i++) { + uint8_t ret = 0, i, j; + for (i = 0; i < shares_required; i++) { uint8_t temp = q[i]; - for (uint8_t j = 0; j < k; j++) { + for (j = 0; j < shares_required; j++) { if (i == j) continue; temp = field_mul(temp, field_neg(x[j])); @@ -178,179 +216,4 @@ uint8_t calculateSecret(uint8_t x[], uint8_t q[], uint8_t k) { } return ret; } - - - -int main(int argc, char* argv[]) { - assert(mlockall(MCL_CURRENT | MCL_FUTURE) == 0); - - char split = 0; - uint8_t n = 0, k = 0; - char* files[P]; uint8_t files_count = 0; - char *in_file = (void*)0, *out_file_param = (void*)0; - - int i; - while((i = getopt(argc, argv, "scn:k:f:o:i:h?")) != -1) - switch(i) { - case 's': - if ((split & 0x2) && !(split & 0x1)) - ERROREXIT("-s (split) and -c (combine) are mutually exclusive\n") - else - split = (0x2 | 0x1); - break; - case 'c': - if ((split & 0x2) && (split & 0x1)) - ERROREXIT("-s (split) and -c (combine) are mutually exclusive\n") - else - split = 0x2; - break; - case 'n': { - int t = atoi(optarg); - if (t <= 0 || t >= P) - ERROREXIT("n must be > 0 and < %u\n", P) - else - n = t; - break; - } - case 'k': { - int t = atoi(optarg); - if (t <= 0 || t >= P) - ERROREXIT("n must be > 0 and < %u\n", P) - else - k = t; - break; - } - case 'i': - in_file = optarg; - break; - case 'o': - out_file_param = optarg; - break; - case 'f': - if (files_count >= P-1) - ERROREXIT("May only specify up to %u files\n", P-1) - files[files_count++] = optarg; - break; - case 'h': - case '?': - printf("Split usage: -s -n -k -i -o \n"); - printf("Combine usage: -c -k <-f >*k -o \n"); - exit(0); - break; - default: - ERROREXIT("getopt failed?\n") - } - if (!(split & 0x2)) - ERROREXIT("Must specify either -c or -s\n") - split &= 0x1; - - if (argc != optind) - ERROREXIT("Invalid argument\n") - - if (split) { - if (!n || !k) - ERROREXIT("n and k must be set.\n") - - if (k > n) - ERROREXIT("k must be <= n\n") - - if (files_count != 0 || !in_file || !out_file_param) - ERROREXIT("Must specify -i and -o but not -f in split mode.\n") - - FILE* random = fopen("/dev/random", "r"); - assert(random); - FILE* secret_file = fopen(in_file, "r"); - if (!secret_file) - ERROREXIT("Could not open %s for reading.\n", in_file) - - uint8_t secret[MAX_LENGTH]; - memset(secret, 0, MAX_LENGTH*sizeof(uint8_t)); - - size_t secret_length = fread(secret, 1, MAX_LENGTH*sizeof(uint8_t), secret_file); - if (secret_length == 0) - ERROREXIT("Error reading secret\n") - if (fread(secret, 1, 1, secret_file) > 0) - ERROREXIT("Secret may not be longer than %u\n", MAX_LENGTH) - fclose(secret_file); - printf("Using secret of length %lu\n", secret_length); - - uint8_t a[secret_length][k], D[n][secret_length]; - - for (uint8_t i = 0; i < secret_length; i++) { - a[i][0] = secret[i]; - - for (uint8_t j = 1; j < k; j++) - assert(fread(&a[i][j], sizeof(uint8_t), 1, random) == 1); - for (uint8_t j = 0; j < n; j++) - D[j][i] = calculateQ(a[i], k, j+1); - } - - char out_file_name_buf[strlen(out_file_param) + 4]; - strcpy(out_file_name_buf, out_file_param); - for (uint8_t i = 0; i < n; i++) { - /*printf("%u-", i); - for (uint8_t j = 0; j < secret_length; j++) - printf("%02x", D[i][j]); - printf("\n");*/ - - sprintf(((char*)out_file_name_buf) + strlen(out_file_param), "%u", i); - FILE* out_file = fopen(out_file_name_buf, "w+"); - if (!out_file) - ERROREXIT("Could not open output file %s\n", out_file_name_buf) - - uint8_t x = i+1; - if (fwrite(&x, sizeof(uint8_t), 1, out_file) != 1) - ERROREXIT("Could not write 1 byte to %s\n", out_file_name_buf) - - if (fwrite(D[i], 1, secret_length, out_file) != secret_length) - ERROREXIT("Could not write %lu bytes to %s\n", secret_length, out_file_name_buf) - - fclose(out_file); - } - /*printf("secret = "); - for (uint8_t i = 0; i < secret_length; i++) - printf("%02x", secret[i]); - printf("\n");*/ - - fclose(random); - } else { - if (!k) - ERROREXIT("k must be set.\n") - - if (files_count != k || in_file || !out_file_param) - ERROREXIT("Must not specify -i and must specify -o and exactly k -f s in combine mode.\n") - - uint8_t x[k], q[k]; - FILE* files_fps[k]; - - for (uint8_t i = 0; i < k; i++) { - files_fps[i] = fopen(files[i], "r"); - if (!files_fps[i]) - ERROREXIT("Couldn't open file %s for reading.\n", files[i]) - if (fread(&x[i], sizeof(uint8_t), 1, files_fps[i]) != 1) - ERROREXIT("Couldn't read the x byte of %s\n", files[i]) - } - - uint8_t secret[MAX_LENGTH]; - - uint8_t i = 0; - while (fread(&q[0], sizeof(uint8_t), 1, files_fps[0]) == 1) { - for (uint8_t j = 1; j < k; j++) { - if (fread(&q[j], sizeof(uint8_t), 1, files_fps[j]) != 1) - ERROREXIT("Couldn't read next byte from %s\n", files[j]) - } - secret[i++] = calculateSecret(x, q, k); - } - printf("Got secret of length %u\n", i); - - FILE* out_file = fopen(out_file_param, "w+"); - fwrite(secret, sizeof(uint8_t), i, out_file); - fclose(out_file); - - for (uint8_t i = 0; i < k; i++) - fclose(files_fps[i]); - } - - return 0; -} #endif // !defined(TEST)