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

scripts/timeattack.c

@@ -196,6 +196,12 @@ CALLBACK(attack_memeq4, bool,
 	return !m;
 }
 
+CALLBACK(attack_memeq5, bool,
+	u_char *subj, u_char *data, size_t len)
+{
+	return memeq_const(subj, data, len);
+}
+
 static bool attack_memeq(char *name, u_int iterations, u_int distance)
 {
 	struct {
@@ -206,6 +212,7 @@ static bool attack_memeq(char *name, u_int iterations, u_int distance)
 		{ "memeq2", attack_memeq2 },
 		{ "memeq3", attack_memeq3 },
 		{ "memeq4", attack_memeq4 },
+		{ "memeq5", attack_memeq5 },
 	};
 	u_char exp[16];
 	int i;
@@ -350,7 +357,7 @@ int main(int argc, char *argv[])
 	if (argc < 3)
 	{
 		fprintf(stderr, "usage: %s <attack> <iterations> <distance>\n", argv[0]);
-		fprintf(stderr, "  <attack>: memeq[1-4] / aead / signer\n");
+		fprintf(stderr, "  <attack>: memeq[1-5] / 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]);

src/libstrongswan/tests/suites/test_utils.c

@@ -306,6 +306,48 @@ START_TEST(test_memxor_aligned)
 }
 END_TEST
 
+/*******************************************************************************
+ * memeq/const
+ */
+
+static struct {
+	char *a;
+	char *b;
+	size_t n;
+	bool res;
+} memeq_data[] = {
+	{NULL, NULL, 0, TRUE},
+	{"a", "b", 0, TRUE},
+	{"", "", 1, TRUE},
+	{"abcdefgh", "abcdefgh", 8, TRUE},
+	{"a", "b", 1, FALSE},
+	{"A", "a", 1, FALSE},
+	{"\0a", "\0b", 2, FALSE},
+	{"abc", "abd", 3, FALSE},
+	{"abc", "dbd", 3, FALSE},
+	{"abcdefgh", "abcdffgh", 8, FALSE},
+	{"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz",
+	 "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz", 52, TRUE},
+	{"abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz",
+	 "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyy", 52, FALSE},
+	{"bbcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz",
+	 "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz", 52, FALSE},
+};
+
+START_TEST(test_memeq)
+{
+	ck_assert(memeq(memeq_data[_i].a, memeq_data[_i].b,
+					memeq_data[_i].n) == memeq_data[_i].res);
+}
+END_TEST
+
+START_TEST(test_memeq_const)
+{
+	ck_assert(memeq_const(memeq_data[_i].a, memeq_data[_i].b,
+						  memeq_data[_i].n) == memeq_data[_i].res);
+}
+END_TEST
+
 /*******************************************************************************
  * memstr
  */
@@ -779,6 +821,11 @@ Suite *utils_suite_create()
 	tcase_add_test(tc, test_memxor_aligned);
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("memeq");
+	tcase_add_loop_test(tc, test_memeq, 0, countof(memeq_data));
+	tcase_add_loop_test(tc, test_memeq_const, 0, countof(memeq_data));
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("memstr");
 	tcase_add_loop_test(tc, test_memstr, 0, countof(memstr_data));
 	suite_add_tcase(s, tc);

src/libstrongswan/utils/utils.c

@@ -109,6 +109,25 @@ void memwipe_noinline(void *ptr, size_t n)
 	memwipe_inline(ptr, n);
 }
 
+/**
+ * Described in header.
+ */
+bool memeq_const(const void *x, const void *y, size_t len)
+{
+	const u_char *a, *b;
+	u_int bad = 0;
+	size_t i;
+
+	a = (const u_char*)x;
+	b = (const u_char*)y;
+
+	for (i = 0; i < len; i++)
+	{
+		bad |= a[i] != b[i];
+	}
+	return !bad;
+}
+
 /**
  * Described in header.
  */

src/libstrongswan/utils/utils.h

@@ -184,6 +184,11 @@ static inline bool memeq(const void *x, const void *y, size_t len)
 	return memcmp(x, y, len) == 0;
 }
 
+/**
+ * Same as memeq(), but with a constant runtime, safe for cryptographic use.
+ */
+bool memeq_const(const void *x, const void *y, size_t len);
+
 /**
  * Calling memcpy() with NULL pointers, even with n == 0, results in undefined
  * behavior according to the C standard.  This version is guaranteed to not