Small cleanups

[shamirs] / shamirssecret.c

diff --git a/shamirssecret.c b/shamirssecret.c
index e3bbe49eb1cbc0ccbd563656b0893c4d3949fb80..fd309f9386ea06d66feeba3d2b7a13c0f0dd5a94 100644 (file)
--- a/shamirssecret.c
+++ b/shamirssecret.c
@@ -1,20 +1,23 @@
 /*
- * Shamir's secret sharing sharing implementation
+ * Shamir's secret sharing implementation
  *
  * Copyright (C) 2013 Matt Corallo <git@bluematt.me>
  *
- * This program is free software; you can redistribute it and/or modify it
- * under the terms and conditions of the GNU General Public License,
- * version 2, as published by the Free Software Foundation.
+ * This file is part of ASSS (Audit-friendly Shamir's Secret Sharing)
  *
- * This program is distributed in the hope it will be useful, but WITHOUT
- * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
- * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
- * more details.
+ * 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.
  *
- * You should have received a copy of the GNU General Public License along with
- * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
- * Place - Suite 330, Boston, MA 02111-1307 USA.
+ * 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/>.
  */
 
 #ifndef IN_KERNEL
@@ -35,7 +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;
 }
@@ -47,7 +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
@@ -111,27 +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
@@ -173,6 +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)
 
@@ -199,7 +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++) {