diff options
Diffstat (limited to 'crypto/ecc.c')
| -rw-r--r-- | crypto/ecc.c | 31 |
1 files changed, 20 insertions, 11 deletions
diff --git a/crypto/ecc.c b/crypto/ecc.c index 6cf9a945fc6c..f97ff234086f 100644 --- a/crypto/ecc.c +++ b/crypto/ecc.c @@ -402,14 +402,26 @@ static uint128_t mul_64_64(u64 left, u64 right) return result; } -static uint128_t add_128_128(uint128_t a, uint128_t b) +/* Calculate addition with overflow checking. Returns true on wrap-around, + * false otherwise. + */ +static bool check_add_128_128_overflow(uint128_t *result, uint128_t a, + uint128_t b) { - uint128_t result; + bool carry; - result.m_low = a.m_low + b.m_low; - result.m_high = a.m_high + b.m_high + (result.m_low < a.m_low); + result->m_low = a.m_low + b.m_low; + carry = (result->m_low < a.m_low); - return result; + result->m_high = a.m_high + b.m_high + carry; + + /* Using constant-time bitwise arithmetic to prevent timing + * side-channels. + */ + carry = (result->m_high < a.m_high) | + ((result->m_high == a.m_high) & carry); + + return carry; } static void vli_mult(u64 *result, const u64 *left, const u64 *right, @@ -434,9 +446,7 @@ static void vli_mult(u64 *result, const u64 *left, const u64 *right, uint128_t product; product = mul_64_64(left[i], right[k - i]); - - r01 = add_128_128(r01, product); - r2 += (r01.m_high < product.m_high); + r2 += check_add_128_128_overflow(&r01, r01, product); } result[k] = r01.m_low; @@ -459,7 +469,7 @@ static void vli_umult(u64 *result, const u64 *left, u32 right, uint128_t product; product = mul_64_64(left[k], right); - r01 = add_128_128(r01, product); + check_add_128_128_overflow(&r01, r01, product); /* no carry */ result[k] = r01.m_low; r01.m_low = r01.m_high; @@ -496,8 +506,7 @@ static void vli_square(u64 *result, const u64 *left, unsigned int ndigits) product.m_low <<= 1; } - r01 = add_128_128(r01, product); - r2 += (r01.m_high < product.m_high); + r2 += check_add_128_128_overflow(&r01, r01, product); } result[k] = r01.m_low; |