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bn.c: constant time

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NIIBE Yutaka
2014-01-15 22:05:18 +09:00
parent b35765d58b
commit 38f8b91335
2 changed files with 19 additions and 20 deletions
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5
ChangeLog
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@@ -1,3 +1,8 @@
2014-01-15 Niibe Yutaka <gniibe@fsij.org>
* src/bn.c (bn256_is_zero, bn256_is_ge, bn256_cmp): Computation
should be constant time.
2013-12-25 Niibe Yutaka <gniibe@fsij.org> 2013-12-25 Niibe Yutaka <gniibe@fsij.org>
* VERSION: 1.1.1. * VERSION: 1.1.1.

34
src/bn.c
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@@ -1,7 +1,7 @@
/* /*
* bn.c -- 256-bit (and 512-bit) bignum calculation * bn.c -- 256-bit (and 512-bit) bignum calculation
* *
* Copyright (C) 2011, 2013 Free Software Initiative of Japan * Copyright (C) 2011, 2013, 2014 Free Software Initiative of Japan
* Author: NIIBE Yutaka <gniibe@fsij.org> * Author: NIIBE Yutaka <gniibe@fsij.org>
* *
* This file is a part of Gnuk, a GnuPG USB Token implementation. * This file is a part of Gnuk, a GnuPG USB Token implementation.
@@ -278,12 +278,12 @@ int
bn256_is_zero (const bn256 *X) bn256_is_zero (const bn256 *X)
{ {
int i; int i;
int r = 1;
for (i = 0; i < BN256_WORDS; i++) for (i = 0; i < BN256_WORDS; i++)
if (X->words[i] != 0) r &= (X->words[i] == 0);
return 0;
return 1; return r;
} }
int int
@@ -295,30 +295,24 @@ bn256_is_even (const bn256 *X)
int int
bn256_is_ge (const bn256 *A, const bn256 *B) bn256_is_ge (const bn256 *A, const bn256 *B)
{ {
int i; uint32_t borrow;
bn256 tmp[1];
for (i = BN256_WORDS - 1; i >= 0; i--) borrow = bn256_sub (tmp, A, B);
if (A->words[i] > B->words[i]) return borrow == 0;
return 1;
else if (A->words[i] < B->words[i])
return 0;
return 1;
} }
int int
bn256_cmp (const bn256 *A, const bn256 *B) bn256_cmp (const bn256 *A, const bn256 *B)
{ {
int i; uint32_t borrow;
int is_zero;
bn256 tmp[1];
for (i = BN256_WORDS - 1; i >= 0; i--) borrow = bn256_sub (tmp, A, B);
if (A->words[i] > B->words[i]) is_zero = bn256_is_zero (tmp);
return 1; return is_zero ? 0 : (borrow ? -1 : 1);
else if (A->words[i] < B->words[i])
return -1;
return 0;
} }