/* crypto/constant_time_locl.h */
-/*
+/*-
* Utilities for constant-time cryptography.
*
* Author: Emilia Kasper (emilia@openssl.org)
extern "C" {
#endif
-/*
- * The following methods return a bitmask of all ones (0xff...f) for true
+/*-
+ * The boolean methods return a bitmask of all ones (0xff...f) for true
* and 0 for false. This is useful for choosing a value based on the result
* of a conditional in constant time. For example,
*
* can be written as
*
* unsigned int lt = constant_time_lt(a, b);
- * c = a & lt | b & ~lt;
+ * c = constant_time_select(lt, a, b);
*/
/*
static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
/* Convenience method for getting an 8-bit mask. */
static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);
+/* Signed integers. */
+static inline unsigned int constant_time_eq_int(int a, int b);
+/* Convenience method for getting an 8-bit mask. */
+static inline unsigned char constant_time_eq_int_8(int a, int b);
+
+
+/*-
+ * Returns (mask & a) | (~mask & b).
+ *
+ * When |mask| is all 1s or all 0s (as returned by the methods above),
+ * the select methods return either |a| (if |mask| is nonzero) or |b|
+ * (if |mask| is zero).
+ */
+static inline unsigned int constant_time_select(unsigned int mask,
+ unsigned int a, unsigned int b);
+/* Convenience method for unsigned chars. */
+static inline unsigned char constant_time_select_8(unsigned char mask,
+ unsigned char a, unsigned char b);
+/* Convenience method for signed integers. */
+static inline int constant_time_select_int(unsigned int mask, int a, int b);
static inline unsigned int constant_time_msb(unsigned int a)
{
- return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
+ return 0-(a >> (sizeof(a) * 8 - 1));
}
static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
{
- unsigned int lt;
- /* 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);
+ return constant_time_msb(a^((a^b)|((a-b)^b)));
}
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)
{
- unsigned int ge;
- /* 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);
+ return ~constant_time_lt(a, b);
}
static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
return (unsigned char)(constant_time_eq(a, b));
}
+static inline unsigned int constant_time_eq_int(int a, int b)
+ {
+ return constant_time_eq((unsigned)(a), (unsigned)(b));
+ }
+
+static inline unsigned char constant_time_eq_int_8(int a, int b)
+ {
+ return constant_time_eq_8((unsigned)(a), (unsigned)(b));
+ }
+
+static inline unsigned int constant_time_select(unsigned int mask,
+ unsigned int a, unsigned int b)
+ {
+ return (mask & a) | (~mask & b);
+ }
+
+static inline unsigned char constant_time_select_8(unsigned char mask,
+ unsigned char a, unsigned char b)
+ {
+ return (unsigned char)(constant_time_select(mask, a, b));
+ }
+
+static inline int constant_time_select_int(unsigned int mask, int a, int b)
+ {
+ return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
+ }
+
#ifdef __cplusplus
}
#endif