From 3ae1abe53e6f43219904cd2860d51c7e34dc3821 Mon Sep 17 00:00:00 2001
From: Rhys Weatherley <rhys.weatherley@gmail.com>
Date: Tue, 31 Mar 2015 10:13:33 +1000
Subject: [PATCH] Secure memory block compares

---
 libraries/Crypto/Crypto.cpp | 24 ++++++++++++++++++++++++
 libraries/Crypto/Crypto.h   |  2 ++
 2 files changed, 26 insertions(+)

diff --git a/libraries/Crypto/Crypto.cpp b/libraries/Crypto/Crypto.cpp
index 28295258..8088db8a 100644
--- a/libraries/Crypto/Crypto.cpp
+++ b/libraries/Crypto/Crypto.cpp
@@ -53,3 +53,27 @@ void clean(void *dest, size_t size)
  * Unlike memset(), this function attempts to prevent the compiler
  * from optimizing away the variable clear.
  */
+
+/**
+ * \brief Compares two memory blocks for equality.
+ *
+ * \param data1 Points to the first memory block.
+ * \param data2 Points to the second memory block.
+ * \param len The size of the memory blocks in bytes.
+ *
+ * Unlike memcmp(), this function attempts to compare the two memory blocks
+ * in a way that will not reveal the contents in the instruction timing.
+ * In particular, this function will not stop early if a byte is different.
+ * It will instead continue onto the end of the array.
+ */
+bool secure_compare(const void *data1, const void *data2, size_t len)
+{
+    uint8_t result = 0;
+    const uint8_t *d1 = (const uint8_t *)data1;
+    const uint8_t *d2 = (const uint8_t *)data2;
+    while (len > 0) {
+        result |= (*d1++ ^ *d2++);
+        --len;
+    }
+    return (bool)((((uint16_t)0x0100) - result) >> 8);
+}
diff --git a/libraries/Crypto/Crypto.h b/libraries/Crypto/Crypto.h
index b4ba2446..98d8d9fc 100644
--- a/libraries/Crypto/Crypto.h
+++ b/libraries/Crypto/Crypto.h
@@ -34,4 +34,6 @@ inline void clean(T &var)
     clean(&var, sizeof(T));
 }
 
+bool secure_compare(const void *data1, const void *data2, size_t len);
+
 #endif