-#define _GNU_SOURCE
+/*
+ * Shamir's secret sharing implementation
+ *
+ * Copyright (C) 2013 Matt Corallo <git@bluematt.me>
+ *
+ * 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
+ * <http://www.gnu.org/licenses/>.
+ */
-#include <stdint.h>
-#include <stdio.h>
+#ifndef IN_KERNEL
#include <assert.h>
-#include <string.h>
-#include <unistd.h>
-#include <stdlib.h>
-#include <sys/mman.h>
+#define CHECKSTATE(x) assert(x)
+#else
+#include <linux/bug.h>
+#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
+#ifndef TEST
static uint8_t field_add(uint8_t a, uint8_t b) {
return a ^ 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
+// 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,
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)
}
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) __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)
#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)
- assert(a != 0);
+ CHECKSTATE(a != 0);
#endif
return field_pow_ret(exp[(log[a] * e) % 255], a, e);
}
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 < 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));
}
}
#endif // defined(TEST)
* 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) {
+/**
+ * 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
// 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]));
}
return ret;
}
-
-
-
-int main(int argc, char* argv[]) {
- assert(mlockall(MCL_CURRENT | MCL_FUTURE) == 0);
-
- char split = 0;
- uint8_t total_shares = 0, shares_required = 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
- total_shares = t;
- break;
- }
- case 'k': {
- int t = atoi(optarg);
- if (t <= 0 || t >= P)
- ERROREXIT("n must be > 0 and < %u\n", P)
- else
- shares_required = 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 <total shares> -k <shares required> -i <input file> -o <output file path base>\n");
- printf("Combine usage: -c -k <shares provided == shares required> <-f <share>>*k -o <output file>\n");
- exit(0);
- break;
- default:
- ERROREXIT("getopt failed?\n")
- }
- if (!(split & 0x2))
- ERROREXIT("Must specify one of -c, -s or -?\n")
- split &= 0x1;
-
- if (argc != optind)
- ERROREXIT("Invalid argument\n")
-
- if (split) {
- if (!total_shares || !shares_required)
- ERROREXIT("n and k must be set.\n")
-
- if (shares_required > total_shares)
- ERROREXIT("k must be <= n\n")
-
- if (files_count != 0 || !in_file || !out_file_param)
- ERROREXIT("Must specify -i <input file> and -o <output file path base> 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];
-
- 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[shares_required], D[total_shares][secret_length];
-
- for (uint32_t i = 0; i < secret_length; i++) {
- a[0] = secret[i];
-
- for (uint8_t j = 1; j < shares_required; j++)
- assert(fread(&a[j], sizeof(uint8_t), 1, random) == 1);
- for (uint8_t j = 0; j < total_shares; j++)
- D[j][i] = calculateQ(a, shares_required, j+1);
-
- if (i % 32 == 0 && i != 0)
- printf("Finished processing %u bytes.\n", i);
- }
-
- char out_file_name_buf[strlen(out_file_param) + 4];
- strcpy(out_file_name_buf, out_file_param);
- for (uint8_t i = 0; i < total_shares; 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");*/
-
- // Clear sensitive data (No, GCC 4.7.2 is currently not optimizing this out)
- memset(secret, 0, sizeof(uint8_t)*secret_length);
- memset(a, 0, sizeof(uint8_t)*shares_required);
- memset(in_file, 0, strlen(in_file));
-
- fclose(random);
- } else {
- if (!shares_required)
- ERROREXIT("k must be set.\n")
-
- if (files_count != shares_required || in_file || !out_file_param)
- ERROREXIT("Must not specify -i and must specify -o and exactly k -f <input file>s in combine mode.\n")
-
- uint8_t x[shares_required], q[shares_required];
- FILE* files_fps[shares_required];
-
- for (uint8_t i = 0; i < shares_required; 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];
-
- uint32_t i = 0;
- while (fread(&q[0], sizeof(uint8_t), 1, files_fps[0]) == 1) {
- for (uint8_t j = 1; j < shares_required; 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, shares_required);
- }
- 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 < shares_required; i++)
- fclose(files_fps[i]);
-
- // Clear sensitive data (No, GCC 4.7.2 is currently not optimizing this out)
- memset(secret, 0, sizeof(uint8_t)*i);
- memset(q, 0, sizeof(uint8_t)*shares_required);
- memset(out_file_param, 0, strlen(out_file_param));
- for (uint8_t i = 0; i < shares_required; i++)
- memset(files[i], 0, strlen(files[i]));
- memset(x, 0, sizeof(uint8_t)*shares_required);
- }
-
- return 0;
-}
#endif // !defined(TEST)
projects / shamirs / blobdiff