+#define _GNU_SOURCE
+
+#include <stdint.h>
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+
+#define MAX_LENGTH 1024
+
+/*
+ * Calculations across the finite field GF(p)
+ * Lots of side-channel attacks in here
+ */
+/*
+const uint8_t p = 251; // Largest 8-bit prime
+static uint8_t field_add(uint8_t a, uint8_t b) {
+ assert(a < p && b < p);
+ return (((uint16_t)a) + ((uint16_t)b)) % p;
+}
+
+static uint8_t field_sub(uint8_t a, uint8_t b) {
+ assert(a < p && b < p);
+ return (((uint16_t)p) + ((uint16_t)a) - ((uint16_t)b)) % p;
+}
+
+static uint8_t field_mul(uint8_t a, uint8_t b) {
+ assert(a < p && b < p);
+ return (((uint16_t)a) * ((uint16_t)b)) % p;
+}
+
+static uint8_t field_pow(uint8_t a, uint8_t e) {
+ assert(a < p);
+ uint8_t ret = 1;
+ for (uint8_t i = 0; i < e; i++) {
+ ret = field_mul(ret, a);
+ }
+ return ret;
+}
+
+static uint8_t field_neg(uint8_t a) {
+ assert(a < p);
+ if (a == 0)
+ return 0;
+ return p - a;
+}
+
+static uint8_t field_invert(uint8_t a) {
+ // Brute force, yay!
+ assert(a < p);
+ for (uint8_t i = 0; i < p; i++) {
+ if (field_mul(i, a) == 1)
+ return i;
+ }
+ assert(0);
+}*/
+
+
+
+/*
+ * Calculations across the finite field GF(2^8)
+ */
+const uint16_t p = 256;
+static uint8_t field_add(uint8_t a, uint8_t b) {
+ return a ^ b;
+}
+
+static uint8_t field_sub(uint8_t a, uint8_t b) {
+ return a ^ b;
+}
+
+static uint8_t field_mul(uint8_t a, uint8_t b) {
+ // TODO side-channel attacks here?
+ uint8_t ret = 0;
+ uint8_t counter;
+ uint8_t carry;
+ for (counter = 0; counter < 8; counter++) {
+ if (b & 1)
+ ret ^= a;
+ carry = (a & 0x80);
+ a <<= 1;
+ if (carry)
+ a ^= 0x1b; /* what x^8 is modulo x^8 + x^4 + x^3 + x + 1 */
+ b >>= 1;
+ }
+ return ret;
+}
+
+// WARNING: Do not use if e is secret (potential side-channel attacks)
+static uint8_t field_pow(uint8_t a, uint8_t e) {
+ // TODO: This could be sped up pretty trivially
+ uint8_t ret = 1;
+ for (uint8_t i = 0; i < e; i++)
+ ret = field_mul(ret, a);
+ return ret;
+}
+
+static uint8_t field_neg(uint8_t a) {
+ return field_sub(0, a);
+}
+
+static const uint8_t inverse[] = { // Multiplicative inverse of each element in the field
+ 0xff, // 0 has no inverse
+ 0x01, 0x8d, 0xf6, 0xcb, 0x52, 0x7b, 0xd1, 0xe8, 0x4f, 0x29, 0xc0, 0xb0, 0xe1, 0xe5, 0xc7, 0x74,
+ 0xb4, 0xaa, 0x4b, 0x99, 0x2b, 0x60, 0x5f, 0x58, 0x3f, 0xfd, 0xcc, 0xff, 0x40, 0xee, 0xb2, 0x3a,
+ 0x6e, 0x5a, 0xf1, 0x55, 0x4d, 0xa8, 0xc9, 0xc1, 0x0a, 0x98, 0x15, 0x30, 0x44, 0xa2, 0xc2, 0x2c,
+ 0x45, 0x92, 0x6c, 0xf3, 0x39, 0x66, 0x42, 0xf2, 0x35, 0x20, 0x6f, 0x77, 0xbb, 0x59, 0x19, 0x1d,
+ 0xfe, 0x37, 0x67, 0x2d, 0x31, 0xf5, 0x69, 0xa7, 0x64, 0xab, 0x13, 0x54, 0x25, 0xe9, 0x09, 0xed,
+ 0x5c, 0x05, 0xca, 0x4c, 0x24, 0x87, 0xbf, 0x18, 0x3e, 0x22, 0xf0, 0x51, 0xec, 0x61, 0x17, 0x16,
+ 0x5e, 0xaf, 0xd3, 0x49, 0xa6, 0x36, 0x43, 0xf4, 0x47, 0x91, 0xdf, 0x33, 0x93, 0x21, 0x3b, 0x79,
+ 0xb7, 0x97, 0x85, 0x10, 0xb5, 0xba, 0x3c, 0xb6, 0x70, 0xd0, 0x06, 0xa1, 0xfa, 0x81, 0x82, 0x83,
+ 0x7e, 0x7f, 0x80, 0x96, 0x73, 0xbe, 0x56, 0x9b, 0x9e, 0x95, 0xd9, 0xf7, 0x02, 0xb9, 0xa4, 0xde,
+ 0x6a, 0x32, 0x6d, 0xd8, 0x8a, 0x84, 0x72, 0x2a, 0x14, 0x9f, 0x88, 0xf9, 0xdc, 0x89, 0x9a, 0xfb,
+ 0x7c, 0x2e, 0xc3, 0x8f, 0xb8, 0x65, 0x48, 0x26, 0xc8, 0x12, 0x4a, 0xce, 0xe7, 0xd2, 0x62, 0x0c,
+ 0xe0, 0x1f, 0xef, 0x11, 0x75, 0x78, 0x71, 0xa5, 0x8e, 0x76, 0x3d, 0xbd, 0xbc, 0x86, 0x57, 0x0b,
+ 0x28, 0x2f, 0xa3, 0xda, 0xd4, 0xe4, 0x0f, 0xa9, 0x27, 0x53, 0x04, 0x1b, 0xfc, 0xac, 0xe6, 0x7a,
+ 0x07, 0xae, 0x63, 0xc5, 0xdb, 0xe2, 0xea, 0x94, 0x8b, 0xc4, 0xd5, 0x9d, 0xf8, 0x90, 0x6b, 0xb1,
+ 0x0d, 0xd6, 0xeb, 0xc6, 0x0e, 0xcf, 0xad, 0x08, 0x4e, 0xd7, 0xe3, 0x5d, 0x50, 0x1e, 0xb3, 0x5b,
+ 0x23, 0x38, 0x34, 0x68, 0x46, 0x03, 0x8c, 0xdd, 0x9c, 0x7d, 0xa0, 0xcd, 0x1a, 0x41, 0x1c};
+static uint8_t field_invert(uint8_t a) {
+ assert(a != 0);
+ return inverse[a];
+}
+
+
+
+/*
+ * Calculations across the polynomial q
+ */
+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)));
+ }
+ 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
+ // http://en.wikipedia.org/wiki/Lagrange_polynomial#Example_2
+ uint8_t ret = 0;
+ for (uint8_t i = 0; i < k; i++) {
+ uint8_t temp = q[i];
+ for (uint8_t j = 0; j < k; j++) {
+ if (i == j)
+ continue;
+ temp = field_mul(temp, field_neg(x[j]));
+ temp = field_mul(temp, field_invert(field_sub(x[i], x[j])));
+ }
+ ret = field_add(ret, temp);
+ }
+ return ret;
+}
+
+#define ERROREXIT(str...) {fprintf(stderr, str); exit(1);}
+
+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 <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 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 <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];
+ 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++) {
+ do
+ assert(fread(&a[i][j], sizeof(uint8_t), 1, random) == 1);
+ while (a[i][j] >= p);
+ }
+ 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 <input file>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;
+}
projects / shamirs / commitdiff