Rename tor_mem_is_zero to fast_mem_is_zero()

For memeq and friends, "tor_" indicates constant-time and "fast_"
indicates optimized.  I'm fine with leaving the constant-time
"safe_mem_is_zero" with its current name, but the "tor_" prefix on
the current optimized version is misleading.

Also, make the tor_digest*_is_zero() uniformly constant-time, and
add a fast_digest*_is_zero() version to use as needed.

A later commit in this branch will fix all the users of
tor_mem_is_zero().

Closes ticket 30309.

1 files changed, 3 insertions, 6 deletions

diff --git a/src/lib/string/util_string.c b/src/lib/string/util_string.c
index 0c4e399008..f5061a11d2 100644
--- a/src/lib/string/util_string.c
+++ b/src/lib/string/util_string.c
@@ -71,7 +71,7 @@ tor_memstr(const void *haystack, size_t hlen, const char *needle)
 
 /** Return true iff the 'len' bytes at 'mem' are all zero. */
 int
-tor_mem_is_zero(const char *mem, size_t len)
+fast_mem_is_zero(const char *mem, size_t len)
 {
   static const char ZERO[] = {
     0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0,
@@ -95,17 +95,14 @@ tor_mem_is_zero(const char *mem, size_t len)
 int
 tor_digest_is_zero(const char *digest)
 {
-  static const uint8_t ZERO_DIGEST[] = {
-    0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0
-  };
-  return tor_memeq(digest, ZERO_DIGEST, DIGEST_LEN);
+  return safe_mem_is_zero(digest, DIGEST_LEN);
 }
 
 /** Return true iff the DIGEST256_LEN bytes in digest are all zero. */
 int
 tor_digest256_is_zero(const char *digest)
 {
-  return tor_mem_is_zero(digest, DIGEST256_LEN);
+  return safe_mem_is_zero(digest, DIGEST256_LEN);
 }
 
 /** Remove from the string <b>s</b> every character which appears in