commit f07a5125cb5bb9ee5968ded163cfdd73e5ad028c Author: Nick Mathewson nickm@torproject.org Date: Tue Dec 25 22:22:07 2012 -0500

Implement a constant-time safe_mem_is_zero. --- src/common/di_ops.c | 17 +++++++++++++++++ src/common/di_ops.h | 2 ++ src/test/test_util.c | 10 ++++++++++ 3 files changed, 29 insertions(+), 0 deletions(-)

diff --git a/src/common/di_ops.c b/src/common/di_ops.c index b73a3cc..e299e31 100644 --- a/src/common/di_ops.c +++ b/src/common/di_ops.c @@ -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); +} + diff --git a/src/common/di_ops.h b/src/common/di_ops.h index a86f56c..1444828 100644 --- a/src/common/di_ops.h +++ b/src/common/di_ops.h @@ -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. diff --git a/src/test/test_util.c b/src/test/test_util.c index 04ca42d..f949c6d 100644 --- a/src/test/test_util.c +++ b/src/test/test_util.c @@ -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: ; }