begin work on crypto

src/mesh/CryptoEngine.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <Arduino.h>
+
+/**
+ * see docs/software/crypto.md for details.
+ *
+ * The NONCE is constructed by concatenating:
+ * a 32 bit sending node number + a 64 bit packet number + a 32 bit block counter (starts at zero)
+ */
+
+class CryptoEngine
+{
+  public:
+    /**
+     * Set the key used for encrypt, decrypt.
+     *
+     * As a special case: If all bytes are zero, we assume _no encryption_ and send all data in cleartext.
+     *
+     * @param numBytes must be 32 for now (AES256)
+     */
+    void setKey(size_t numBytes, const uint8_t *bytes);
+
+    /**
+     * Encrypt a packet
+     *
+     * @param bytes is updated in place
+     */
+    void encrypt(uint32_t fromNode, uint64_t packetNum, size_t numBytes, uint8_t *bytes);
+    void decrypt(uint32_t fromNode, uint64_t packetNum, size_t numBytes, uint8_t *bytes);
+};