- Generalize CryptoInterface.

- Add more HMAC and hash functions to CryptoInterface.

- Add MeshCryptoInterface as a holder of mesh specific crypto functionality.

- Rename broadcastMetadataDelimiter to metadataDelimiter in FloodingMesh since it is not just used for broadcasts, and to save some typing.

libraries/ESP8266WiFiMesh/src/MeshCryptoInterface.h

@@ -0,0 +1,72 @@
+/*
+ * Copyright (C) 2019 Anders Löfgren
+ *
+ * License (MIT license):
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef __MESHCRYPTOINTERFACE_H__
+#define __MESHCRYPTOINTERFACE_H__
+
+#include <Arduino.h>
+#include "CryptoInterface.h"
+
+namespace MeshCryptoInterface 
+{ 
+  /**
+   * There is a constant-time HMAC version available. More constant-time info here: https://www.bearssl.org/constanttime.html
+   * For small messages, it takes substantially longer time to complete than a normal HMAC (5 ms vs 2 ms in a quick benchmark, 
+   * determined by the difference between min and max allowed message length), and it also sets a maximum length that messages can be (1024 bytes by default).
+   * Making the fixed max length variable would defeat the whole purpose of using constant-time, and not making it variable would create the wrong HMAC if message size exceeds the maximum.
+   * 
+   * Also, HMAC is already partially constant-time. Quoting the link above: 
+   * "Hash functions implemented by BearSSL (MD5, SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512) consist in bitwise logical operations and additions on 32-bit or 64-bit words, 
+   * naturally yielding constant-time operations. HMAC is naturally as constant-time as the underlying hash function. The size of the MACed data, and the size of the key, 
+   * may leak, though; only the contents are protected."
+   * 
+   * Thus the non constant-time version is used within the mesh framework instead.
+   */
+  
+  /**
+   * Create a SHA256 HMAC from the message, using the provided hashKey. The result will be hmacLength bytes long and returned as a String in HEX format.
+   * 
+   * @param message The string from which to create the HMAC.
+   * @param hashKey The hash key to use when creating the HMAC.
+   * @param hashKeyLength The length of the hash key in bytes.
+   * @param hmacLength The desired length of the generated HMAC, in bytes. Valid values are 1 to 32. Defaults to CryptoInterface::SHA256_NATURAL_LENGTH.
+   * 
+   * @return A String with the generated HMAC in HEX format.
+   */
+  String createMeshHmac(const String &message, const void *hashKey, const size_t hashKeyLength, const size_t hmacLength = CryptoInterface::SHA256_NATURAL_LENGTH);
+  
+  /**
+   * Verify a SHA256 HMAC which was created from the message using the provided hashKey.
+   * 
+   * @param message The string from which the HMAC was created.
+   * @param messageHmac A string with the generated HMAC in HEX format. Valid messageHmac.length() is 2 to 64.
+   * @param hashKey The hash key to use when creating the HMAC.
+   * @param hashKeyLength The length of the hash key in bytes.
+   * 
+   * @return True if the HMAC is correct. False otherwise.
+   */
+  bool verifyMeshHmac(const String &message, const String &messageHmac, const uint8_t *hashKey, uint8_t hashKeyLength);
+}
+
+#endif