Commit f07a5125 authored by Nick Mathewson's avatar Nick Mathewson 🥔
Browse files

Implement a constant-time safe_mem_is_zero.

parent 92d6a83e

src/common/di_ops.c

+17 −0
Original line number Diff line number Diff line
@@ -203,3 +203,20 @@ dimap_search(const di_digest256_map_t *map, const uint8_t *key, 
  return (void *)result;

}



/**

 * Return true iff the <b>sz</b> bytes at <b>mem</b> are all zero. Runs in

 * time independent of the contents of <b>mem</b>.

 */

int

safe_mem_is_zero(const void *mem, size_t sz)

{

  uint32_t total = 0;

  const uint8_t *ptr = mem;



  while (sz--) {

    total |= *ptr++;

  }



  return 1 & ((total - 1) >> 8);

}

src/common/di_ops.h

+2 −0
Original line number Diff line number Diff line
@@ -27,6 +27,8 @@ int tor_memeq(const void *a, const void *b, size_t sz); 
#define fast_memeq(a,b,c)  (0==memcmp((a),(b),(c)))

#define fast_memneq(a,b,c) (0!=memcmp((a),(b),(c)))



int safe_mem_is_zero(const void *mem, size_t sz);



/** A type for a map from DIGEST256_LEN-byte blobs to void*, such that

 * data lookups take an amount of time proportional only to the size

 * of the map, and not to the position or presence of the item in the map.

src/test/test_util.c

+10 −0
Original line number Diff line number Diff line
@@ -2843,6 +2843,16 @@ test_util_di_ops(void) 
    test_eq(neq1, !eq1);

  }



  tt_int_op(1, ==, safe_mem_is_zero("", 0));

  tt_int_op(1, ==, safe_mem_is_zero("", 1));

  tt_int_op(0, ==, safe_mem_is_zero("a", 1));

  tt_int_op(0, ==, safe_mem_is_zero("a", 2));

  tt_int_op(0, ==, safe_mem_is_zero("\0a", 2));

  tt_int_op(1, ==, safe_mem_is_zero("\0\0a", 2));

  tt_int_op(1, ==, safe_mem_is_zero("\0\0\0\0\0\0\0\0", 8));

  tt_int_op(1, ==, safe_mem_is_zero("\0\0\0\0\0\0\0\0a", 8));

  tt_int_op(0, ==, safe_mem_is_zero("\0\0\0\0\0\0\0\0a", 9));



 done:

  ;

}