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Use only unsigned arithmetic in constant-time operations

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Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
This commit is contained in:
Samuel Neves 2014-10-04 00:13:36 +01:00 committed by Kurt Roeckx
parent d663df2399
commit d2fa182988
1 changed files with 3 additions and 13 deletions
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16
crypto/constant_time_locl.h
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@ -129,17 +129,12 @@ static inline int constant_time_select_int(unsigned int mask, int a, int b);
static inline unsigned int constant_time_msb(unsigned int a) static inline unsigned int constant_time_msb(unsigned int a)
{ {
return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1)); return -(a >> (sizeof(unsigned int) * 8 - 1));
} }
static inline unsigned int constant_time_lt(unsigned int a, unsigned int b) static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
{ {
unsigned int lt; return constant_time_msb(a^((a^b)|((a-b)^b)));
/* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/
lt = ~(a ^ b) & (a - b);
/* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */
lt |= ~a & b;
return constant_time_msb(lt);
} }
static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b) static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
@ -149,12 +144,7 @@ static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
static inline unsigned int constant_time_ge(unsigned int a, unsigned int b) static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
{ {
unsigned int ge; return ~constant_time_lt(a, b);
/* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/
ge = ~((a ^ b) | (a - b));
/* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */
ge |= a & ~b;
return constant_time_msb(ge);
} }
static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b) static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)