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

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