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/*
* Compare two fixed-size regions of memory, in a crypto-safe way,
* i.e. without timing or cache side channels indicating anything
* about what the answer was or where the first difference (if any)
* might have been.
*/
#include "defs.h"
#include "misc.h"
bool smemeq(const void *av, const void *bv, size_t len)
{
const unsigned char *a = (const unsigned char *)av;
const unsigned char *b = (const unsigned char *)bv;
unsigned val = 0;
while (len-- > 0) {
val |= *a++ ^ *b++;
}
/* Now val is 0 iff we want to return 1, and in the range
* 0x01..0xFF iff we want to return 0. So subtracting from 0x100
* will clear bit 8 iff we want to return 0, and leave it set iff
* we want to return 1, so then we can just shift down. */
return (0x100 - val) >> 8;
}
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