utils: Add a constant time chunk_equals() variant for cryptographic purposes

scripts/timeattack.c

@@ -235,6 +235,48 @@ static bool attack_memeq(char *name, u_int iterations, u_int distance)
 	return FALSE;
 }
 
+CALLBACK(attack_chunk1, bool,
+	u_char *subj, u_char *data, size_t len)
+{
+	return chunk_equals(chunk_create(subj, len), chunk_create(data, len));
+}
+
+CALLBACK(attack_chunk2, bool,
+	u_char *subj, u_char *data, size_t len)
+{
+	return chunk_equals_const(chunk_create(subj, len), chunk_create(data, len));
+}
+
+static bool attack_chunk(char *name, u_int iterations, u_int distance)
+{
+	struct {
+		char *name;
+		attackfn_t fn;
+	} attacks[] = {
+		{ "chunk1", attack_chunk1 },
+		{ "chunk2", attack_chunk2 },
+	};
+	u_char exp[16];
+	int i;
+
+	srandom(time(NULL));
+	for (i = 0; i < sizeof(exp); i++)
+	{
+		exp[i] = random();
+	}
+	fprintf(stderr, "attacking %b\n", exp, sizeof(exp));
+
+	for (i = 0; i < countof(attacks); i++)
+	{
+		if (streq(name, attacks[i].name))
+		{
+			return timeattack(attacks[i].fn, exp, sizeof(exp),
+							  iterations, distance);
+		}
+	}
+	return FALSE;
+}
+
 CALLBACK(attack_aead, bool,
 	aead_t *aead, u_char *data, size_t len)
 {
@@ -357,7 +399,7 @@ int main(int argc, char *argv[])
 	if (argc < 3)
 	{
 		fprintf(stderr, "usage: %s <attack> <iterations> <distance>\n", argv[0]);
-		fprintf(stderr, "  <attack>: memeq[1-5] / aead / signer\n");
+		fprintf(stderr, "  <attack>: memeq[1-5] / chunk[1-2] / aead / signer\n");
 		fprintf(stderr, "  <iterations>: number of invocations * 1000\n");
 		fprintf(stderr, "  <distance>: time difference in ns for a hit\n");
 		fprintf(stderr, "  example: %s memeq1 100 500\n", argv[0]);
@@ -368,5 +410,9 @@ int main(int argc, char *argv[])
 	{
 		return !attack_memeq(argv[1], atoi(argv[2]), atoi(argv[3]));
 	}
+	if (strpfx(argv[1], "chunk"))
+	{
+		return !attack_chunk(argv[1], atoi(argv[2]), atoi(argv[3]));
+	}
 	return !attack_transform(argv[1], atoi(argv[2]), atoi(argv[3]));
 }

src/libstrongswan/tests/suites/test_chunk.c

@@ -60,6 +60,32 @@ START_TEST(test_chunk_equals)
 }
 END_TEST
 
+/*******************************************************************************
+ * equals_const
+ */
+
+START_TEST(test_chunk_equals_const)
+{
+	chunk_t chunk = chunk_from_str("chunk");
+	chunk_t chunk_a, chunk_b;
+
+	chunk_a = chunk_empty;
+	chunk_b = chunk_empty;
+	ck_assert(!chunk_equals_const(chunk_a, chunk_b));
+
+	chunk_a = chunk;
+	ck_assert(!chunk_equals_const(chunk_a, chunk_b));
+	chunk_b = chunk;
+	ck_assert(chunk_equals_const(chunk_a, chunk_b));
+
+	chunk_b = chunk_from_str("asdf");
+	ck_assert(!chunk_equals_const(chunk_a, chunk_b));
+
+	chunk_b = chunk_from_str("chunk");
+	ck_assert(chunk_equals_const(chunk_a, chunk_b));
+}
+END_TEST
+
 /*******************************************************************************
  * chunk_compare test
  */
@@ -1013,6 +1039,7 @@ Suite *chunk_suite_create()
 
 	tc = tcase_create("equals");
 	tcase_add_test(tc, test_chunk_equals);
+	tcase_add_test(tc, test_chunk_equals_const);
 	suite_add_tcase(s, tc);
 
 	tc = tcase_create("chunk_compare");

src/libstrongswan/utils/chunk.h

@@ -309,6 +309,19 @@ static inline bool chunk_equals(chunk_t a, chunk_t b)
 			a.len == b.len && memeq(a.ptr, b.ptr, a.len);
 }
 
+/**
+ * Compare two chunks for equality, constant time for cryptographic purposes.
+ *
+ * Note that this function is constant time only for chunks with the same
+ * length, i.e. it does not protect against guessing the length of one of the
+ * chunks.
+ */
+static inline bool chunk_equals_const(chunk_t a, chunk_t b)
+{
+	return a.ptr != NULL  && b.ptr != NULL &&
+			a.len == b.len && memeq_const(a.ptr, b.ptr, a.len);
+}
+
 /**
  * Compare two chunks (given as pointers) for equality (useful as callback),
  * NULL chunks are never equal.