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48 lines
1.5 KiB
C
48 lines
1.5 KiB
C
#include <stdint.h>
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#include <string.h>
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/*************************************************
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* Name: verify
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*
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* Description: Compare two arrays for equality in constant time.
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*
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* Arguments: const unsigned char *a: pointer to first byte array
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* const unsigned char *b: pointer to second byte array
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* size_t len: length of the byte arrays
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*
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* Returns 0 if the byte arrays are equal, 1 otherwise
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**************************************************/
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int verify(const unsigned char *a, const unsigned char *b, size_t len) {
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uint64_t r;
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size_t i;
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r = 0;
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for (i = 0; i < len; i++)
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r |= a[i] ^ b[i];
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r = (-r) >> 63;
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return r;
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}
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/*************************************************
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* Name: cmov
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*
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* Description: Copy len bytes from x to r if b is 1;
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* don't modify x if b is 0. Requires b to be in {0,1};
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* assumes two's complement representation of negative integers.
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* Runs in constant time.
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*
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* Arguments: unsigned char *r: pointer to output byte array
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* const unsigned char *x: pointer to input byte array
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* size_t len: Amount of bytes to be copied
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* unsigned char b: Condition bit; has to be in {0,1}
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**************************************************/
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void cmov(unsigned char *r, const unsigned char *x, size_t len,
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unsigned char b) {
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size_t i;
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b = -b;
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for (i = 0; i < len; i++)
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r[i] ^= b & (x[i] ^ r[i]);
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}
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