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- /* Copyright (c) 2011-2012, The Tor Project, Inc. */
- /* See LICENSE for licensing information */
- /**
- * \file di_ops.c
- * \brief Functions for data-independent operations.
- **/
- #include "orconfig.h"
- #include "di_ops.h"
- /**
- * Timing-safe version of memcmp. As memcmp, compare the <b>sz</b> bytes at
- * <b>a</b> with the <b>sz</b> bytes at <b>b</b>, and return less than 0 if
- * the bytes at <b>a</b> lexically precede those at <b>b</b>, 0 if the byte
- * ranges are equal, and greater than zero if the bytes at <b>a</b> lexically
- * follow those at <b>b</b>.
- *
- * This implementation differs from memcmp in that its timing behavior is not
- * data-dependent: it should return in the same amount of time regardless of
- * the contents of <b>a</b> and <b>b</b>.
- */
- int
- tor_memcmp(const void *a, const void *b, size_t len)
- {
- const uint8_t *x = a;
- const uint8_t *y = b;
- size_t i = len;
- int retval = 0;
- /* This loop goes from the end of the arrays to the start. At the
- * start of every iteration, before we decrement i, we have set
- * "retval" equal to the result of memcmp(a+i,b+i,len-i). During the
- * loop, we update retval by leaving it unchanged if x[i]==y[i] and
- * setting it to x[i]-y[i] if x[i]!= y[i].
- *
- * The following assumes we are on a system with two's-complement
- * arithmetic. We check for this at configure-time with the check
- * that sets USING_TWOS_COMPLEMENT. If we aren't two's complement, then
- * torint.h will stop compilation with an error.
- */
- while (i--) {
- int v1 = x[i];
- int v2 = y[i];
- int equal_p = v1 ^ v2;
- /* The following sets bits 8 and above of equal_p to 'equal_p ==
- * 0', and thus to v1 == v2. (To see this, note that if v1 ==
- * v2, then v1^v2 == equal_p == 0, so equal_p-1 == -1, which is the
- * same as ~0 on a two's-complement machine. Then note that if
- * v1 != v2, then 0 < v1 ^ v2 < 256, so 0 <= equal_p - 1 < 255.)
- */
- --equal_p;
- equal_p >>= 8;
- /* Thanks to (sign-preserving) arithmetic shift, equal_p is now
- * equal to -(v1 == v2), which is exactly what we need below.
- * (Since we're assuming two's-complement arithmetic, -1 is the
- * same as ~0 (all bits set).)
- *
- * (The result of an arithmetic shift on a negative value is
- * actually implementation-defined in standard C. So how do we
- * get away with assuming it? Easy. We check.) */
- #if ((-60 >> 8) != -1)
- #error "According to cpp, right-shift doesn't perform sign-extension."
- #endif
- #ifndef RSHIFT_DOES_SIGN_EXTEND
- #error "According to configure, right-shift doesn't perform sign-extension."
- #endif
- /* If v1 == v2, equal_p is ~0, so this will leave retval
- * unchanged; otherwise, equal_p is 0, so this will zero it. */
- retval &= equal_p;
- /* If v1 == v2, then this adds 0, and leaves retval unchanged.
- * Otherwise, we just zeroed retval, so this sets it to v1 - v2. */
- retval += (v1 - v2);
- /* There. Now retval is equal to its previous value if v1 == v2, and
- * equal to v1 - v2 if v1 != v2. */
- }
- return retval;
- }
- /**
- * Timing-safe memory comparison. Return true if the <b>sz</b> bytes at
- * <b>a</b> are the same as the <b>sz</b> bytes at <b>b</b>, and 0 otherwise.
- *
- * This implementation differs from !memcmp(a,b,sz) in that its timing
- * behavior is not data-dependent: it should return in the same amount of time
- * regardless of the contents of <b>a</b> and <b>b</b>. It differs from
- * !tor_memcmp(a,b,sz) by being faster.
- */
- int
- tor_memeq(const void *a, const void *b, size_t sz)
- {
- /* Treat a and b as byte ranges. */
- const uint8_t *ba = a, *bb = b;
- uint32_t any_difference = 0;
- while (sz--) {
- /* Set byte_diff to all of those bits that are different in *ba and *bb,
- * and advance both ba and bb. */
- const uint8_t byte_diff = *ba++ ^ *bb++;
- /* Set bits in any_difference if they are set in byte_diff. */
- any_difference |= byte_diff;
- }
- /* Now any_difference is 0 if there are no bits different between
- * a and b, and is nonzero if there are bits different between a
- * and b. Now for paranoia's sake, let's convert it to 0 or 1.
- *
- * (If we say "!any_difference", the compiler might get smart enough
- * to optimize-out our data-independence stuff above.)
- *
- * To unpack:
- *
- * If any_difference == 0:
- * any_difference - 1 == ~0
- * (any_difference - 1) >> 8 == 0x00ffffff
- * 1 & ((any_difference - 1) >> 8) == 1
- *
- * If any_difference != 0:
- * 0 < any_difference < 256, so
- * 0 < any_difference - 1 < 255
- * (any_difference - 1) >> 8 == 0
- * 1 & ((any_difference - 1) >> 8) == 0
- */
- return 1 & ((any_difference - 1) >> 8);
- }
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