add constant-time memcmp()

[skip ci]

src/headers/tomcrypt_misc.h

@@ -39,6 +39,7 @@ int hkdf(int hash_idx,
 #endif  /* LTC_HKDF */
 
 /* ---- MEM routines ---- */
+int mem_neq(const void *a, const void *b, size_t len);
 void zeromem(volatile void *dst, size_t len);
 void burn_stack(unsigned long len);
 

src/misc/mem_neq.c

@@ -0,0 +1,55 @@
+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
+ */
+#include "tomcrypt.h"
+
+/**
+   @file mem_neq.c
+   Compare two blocks of memory for inequality.
+   Steffen Jaeckel
+*/
+
+/**
+   Compare two blocks of memory for inequality.
+
+   The usage is similar to that of standard memcmp(), but you can only test
+   if the memory is equal or not - you can not determine by how much the
+   first different byte differs.
+
+   @param a     The first memory region
+   @param b     The second memory region
+   @param len   The length of the area to compare (octets)
+
+   @return 0 when a and b are equal for len bytes, else they are not equal.
+*/
+int mem_neq(const void *a, const void *b, size_t len)
+{
+   unsigned char ret = 0;
+   const unsigned char* pa;
+   const unsigned char* pb;
+
+   LTC_ARGCHK(a != NULL);
+   LTC_ARGCHK(b != NULL);
+
+   pa = a;
+   pb = b;
+
+   while (len-- > 0) {
+      ret |= *pa ^ *pb;
+      ++pa;
+      ++pb;
+   }
+
+   return ret;
+}
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */