IoTManager/lib/MySensors/core/MySigning.h

/*
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2019 Sensnology AB
 * Full contributor list: https://github.com/mysensors/MySensors/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 */
/**
 * @defgroup MySigninggrpPub Message signing
 * @ingroup publics
 * @{
 *
 * @brief The message signing infrastructure provides message authenticity to users by signing
 * MySensors messages.
 *
 * Signing support created by Patrick "Anticimex" Fallberg.
 *
 * @section MySigninggrptoc Table of contents
 *
 * @ref MySigninggrphowuse <br>
 * @ref MySigningwhitelisting <br>
 * @ref MySigninglimitations <br>
 * @ref MySigningusecases <br>
 * @ref MySigningtechnical <br>
 * @ref MySigninggrpbackground <br>
 * @ref MySigninggrphow <br>
 * @ref MySigninggrpencryption <br>
 *
 * @section MySigninggrphowuse How to use this
 *
 * Before we begin with the details, I just want to emphasize that signing is completely optional
 * and not enabled by default.
 *
 * If you do want the additional security layer signing provides, you pick the backend of your
 * choice in your sketch. Currently, two compatible backends are supported; @ref MY_SIGNING_ATSHA204
 * (hardware backed) and @ref MY_SIGNING_SOFT (software backed). There also exist a simplified
 * variant (@ref MY_SIGNING_SIMPLE_PASSWD) of the software backend which only require one setting
 * to activate.
 *
 * If you use hardware backed signing, then connect the device as follows:
 * @image html MySigning/wiring.png
 * @note The pull-up resistor is optional but recommended.
 * @note If you change the default pin (A3) make sure you use a pin that supports input/output
 *       (ex. A6 & A7 on a Pro Mini are input only pins).
 *
 * To use signing, you need to perform three major steps which are described below.
 *
 * <b>Firstly</b>, you need to make sure to pick a backend to use.
 * @code{.cpp}
 * //#define MY_SIGNING_SOFT
 * //#define MY_SIGNING_SIMPLE_PASSWD
 * #define MY_SIGNING_ATSHA204
 * #include <MySensors.h>
 * ...
 * @endcode
 * Make sure to set the define before the inclusion of MySensors.h.
 * It is ok to mix @ref MY_SIGNING_SOFT and @ref MY_SIGNING_ATSHA204 in a network.
 * They are fully compatible. It is however not recommended to use @ref MY_SIGNING_SOFT on nodes
 * that are publicly accessible (more on that later).
 *
 * If you use @ref MY_SIGNING_SOFT or @ref MY_SIGNING_ATSHA204 you also need to decide if the node
 * (or gateway) in question require messages to be signed in addition to the ability to generate
 * signatures for other nodes.
 * This has to be set by at least one of the nodes in a "pair" or nobody will actually start
 * calculating a signature for a message.
 * Just set the flag @ref MY_SIGNING_REQUEST_SIGNATURES and the node will inform the gateway that it
 * expects the gateway to sign all messages sent to the node. Note that when set in a gateway, the
 * gateway will require ALL nodes in the network to sign messages.
 * If this behaviour is undesired, enable the flag @ref MY_SIGNING_WEAK_SECURITY which will allow
 * the gateway to only require signatures from nodes that in turn require signatures. It will also
 * allow the gateway (and all nodes) to "downgrade" security by clearing the signing/whitelisting
 * requirements (whitelisting is described later on in the @ref MySigningwhitelisting section) in
 * the EEPROM if a node presents itself as not having any security requirements.
 * If @ref MY_SIGNING_WEAK_SECURITY is not set, any node that has presented itself with
 * signing/whitelisting requirements will be permanently marked as such by the receiver
 * (typically the gateway). The only way then to reset/revert this requirement is to clear the
 * EEPROM at the receiver (or disable @ref MY_SIGNING_REQUEST_SIGNATURES, but the preference will be
 * remembered if the request flag is re-enabled before EEPROM is cleared).<br>
 * If you want to have two nodes communicate securely directly with each other, the nodes that
 * require signatures must send a presentation message to all nodes it expect signed messages from
 * (only the gateway is informed automatically). See @ref signerPresentation().<br>
 * A node can have three "states" with respect to signing:
 * 1. Node does not support signing in any way (neither @ref MY_SIGNING_ATSHA204,
 * @ref MY_SIGNING_SOFT nor @ref MY_SIGNING_SIMPLE_PASSWD is set)
 * 2. Node does support signing but don't require messages sent to it to be signed (neither
 * @ref MY_SIGNING_REQUEST_SIGNATURES nor @ref MY_SIGNING_SIMPLE_PASSWD is set)
 * 3. Node does support signing and require messages sent to it to be signed (@ref
 * MY_SIGNING_SOFT or @ref MY_SIGNING_ATSHA204 together with @ref MY_SIGNING_REQUEST_SIGNATURES or
 * @ref MY_SIGNING_SIMPLE_PASSWD are set)
 *
 * <b>Secondly</b>, you need to verify the configuration for the backend.<br>
 * For hardware backed signing it is the pin the device is connected to. In MyConfig.h there are
 * defaults which you might need to adjust to match your personal build. The setting is defined
 * using @ref MY_SIGNING_ATSHA204_PIN.<br>
 * If you use an official MySensors board (like the SenseBender GW) you do not need to set the pin,
 * this is configured automatically by the Arduino board definition files.
 *
 * Similar to picking your backend, this can also be set in your sketch:
 * @code{.cpp}
 * #define MY_SIGNING_ATSHA204
 * #define MY_SIGNING_ATSHA204_PIN 4
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #include <MySensors.h>
 * ...
 * @endcode
 * For the software backed signing backend, an unconnected analog pin is required on boards that
 * does not provide a hardware based random generator unit to set a random seed for the
 * pseudo-random generator.
 * It is important that the pin is floating, or the output of the pseudo-random generator will be
 * predictable, and thus compromise the signatures. The setting is defined using
 * @ref MY_SIGNING_SOFT_RANDOMSEED_PIN. The same configuration possibilities exist as with the other
 * configuration options.
 *
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #include <MySensors.h>
 * ...
 * @endcode
 *
 * An example of a node that require signatures is available in @ref SecureActuator.ino.
 *
 * <b>Thirdly</b>, if you use a signing backend and you don't use @ref MY_SIGNING_SIMPLE_PASSWD, you
 * need to personalize the node.
 *
 * @anchor personalization If you use the ATSHA204A (@ref MY_SIGNING_ATSHA204), before any signing
 * operations can be done, the device needs to be <i>personalized</i>.
 * This can be a daunting process as it involves irreversibly writing configurations to the device,
 * which cannot be undone. I have however tried to simplify the process as much as possibly by
 * creating a helper-sketch specifically for this purpose in @ref SecurityPersonalizer.ino
 * Note that you also need to do personalization for @ref MY_SIGNING_SOFT, but then the values are
 * stored in EEPROM.
 *
 * To personalize a ATSHA204A do the following procedure:
 * 1. Enable @ref GENERATE_KEYS_ATSHA204A<br>
 *    This will lock the ATSHA204A and generate random keys for HMAC (signing) and %AES (encryption).
 *    Copy the keys generated and replace the corresponding definitions under
 *    "User defined key data", specifically @ref MY_HMAC_KEY and @ref MY_AES_KEY.
 * 2. Disable @ref GENERATE_KEYS_ATSHA204A and enable @ref PERSONALIZE_ATSHA204A<br>
 *    This will store the HMAC key to the ATSHA204A and the %AES key to EEPROM. It will also write
 *    a checksum of the personalization data in EEPROM to be able to detect if the data is
 *    altered.<br>
 *    Personalization is now complete.
 *
 * To personalize for software signing do the following procedure:
 * 1. Enable @ref GENERATE_KEYS_SOFT<br>
 *    This will generate random keys for HMAC (signing) and %AES (encryption).
 *    Copy the keys generated and replace the corresponding definitions under
 *    "User defined key data", specifically @ref MY_HMAC_KEY and @ref MY_AES_KEY.
 * 2. Disable @ref GENERATE_KEYS_SOFT and enable @ref PERSONALIZE_SOFT<br>
 *    This will store the HMAC key and the %AES key to EEPROM. It will also write
 *    a checksum of the personalization data in EEPROM to be able to detect if the data is
 *    altered.<br>
 *    Personalization is now complete.
 *
 * If you want to use soft signing and you want to use whitelisting (the ability to revoke/ban
 * compromised nodes in the network) and your target does not provide a unique device ID, you have
 * to generate a unique serial and store it in EEPROM. This can be done by replacing
 * @ref PERSONALIZE_SOFT in step 2 above with @ref PERSONALIZE_SOFT_RANDOM_SERIAL. See the output
 * under "Hardware security peripherals" to determine if this is necessary.
 *
 * When you have personalized your first device after step 2 above, you can run the same sketch on
 * all devices in your network that needs to be personalized in a compatible manner. Pick
 * @ref PERSONALIZE_ATSHA204A or @ref PERSONALIZE_SOFT as needed by the hardware. When the
 * personalization has finished, you just program the sketch you plan to use (with the appropriate
 * signing flags set).
 *
 * If you are using a Raspberry PI-based gateway, personalizaion is done slightly differently:
 * 1. Generate keys, execute @c mysgw with arguments
 *    * To generate HMAC key @verbatim --gen-soft-hmac-key @endverbatim
 *    * To generate %AES key @verbatim --gen-aes-key @endverbatim
 *    * To generate a soft serial number @verbatim --gen-soft-serial @endverbatim
 * 2. Update the gateway config file with the generated keys/valeus
 *    * For HMAC key @verbatim soft_hmac_key=<DATA> @endverbatim
 *    * For %AES key @verbatim aes_key=<DATA> @endverbatim
 *    * For soft serial number @verbatim soft_serial_key=<DATA> @endverbatim
 *
 * You are now set and ready to use message signing in your network.
 * As of now, the following restrictions will be applied to your nodes:
 * * If a node does require signing, any unsigned message sent to the node will be rejected.
 * This also applies to the gateway.
 * * Your radio communication quality is expected to work fine (if any NACK happen on a signing
 * related message, it will fail, and enabling signing will put maximum strain on your RF link as
 * maximum sized packets are transmitted in the network). See @ref MySigningTroubleshootinggrp.
 * * All nodes and gateways in a network maintain a table where the signing preferences of all nodes
 * are stored. This is also stored in EEPROM so if a node or gateway reboots, the other nodes does
 * not have to retransmit a signing presentation to the node for the node to start expecting signed
 * messages from other nodes.<br>
 * * By default, the signing preferences are not "downgradeable". That is, any node that at any
 * point in time has indicated a signing requirement will not be able to revert this requirement at
 * the receiving end (except by manual erase of the EEPROM).<br>
 * If you for some reason need to be able to downgrade the security requirements, you can set
 * @ref MY_SIGNING_WEAK_SECURITY at the receiver to allow it to downgrade the security expectations
 * of the node in question.<br>
 * You then need to reset your transmitting node, to force it to transmit updated signing
 * preferences.
 *
 * @section MySigningwhitelisting Whitelisting and node revocation
 *
 * Consider the situation when you have set up your secure topology. We use the remotely operated
 * garage door as an example:
 * * You have a node inside your garage (considered physically inaccessible) that controls your
 *   garage door motor.<br>
 *   This node requires signing since you do not want an unauthorized person sending it orders to
 *   open the door.
 * * You have a keyfob node with a signing backend that uses the same PSK as your door opener node.
 *
 * In this setup, your keyfob can securely transmit messages to your door node since the keyfob will
 * sign the messages it sends and the door node will verify that these were sent from a trusted node
 * (since it used the correct PSK). If the keyfob does not sign the messages, the door node
 * will not accept them. Optionally, your keyfob sends a signed message to your gateway (which
 * require signatures) and the gateway in turn sends a signed message to your garage door.
 *
 * One day your keyfob gets stolen or you lost it or it simply broke down.
 *
 * You now end up with a problem; you need some way of telling your door node that the keyfob in
 * question cannot be trusted any more. You could now repersonalize all your node to switch to a
 * different PSK but this obviously is a hassle. How do you make sure that the "rogue" keyfob can be
 * removed from the "trusted chain"?
 *
 * The answer to this is whitelisting. You let your door node keep a whitelist of all nodes it
 * trusts. If you stop trusting a particular node, you remove it from the nodes whitelist
 * (by uploading a new sketch), and it will no longer be able to communicate signed messages to the
 * door node.
 *
 * In case you want to be able to "whitelist" trusted nodes (in order to be able to revoke them in
 * case they are lost) you also need to take note of the serial number of the ATSHA device or the
 * software value stored in EEPROM. This is unique for each device. The serial number is printed
 * in a copy+paste friendly format by the personalizer for this purpose.<br>
 * The whitelist is stored on the node that require signatures. When a received message is
 * verified, the serial of the sender is looked up in a list stored on the receiving node, and the
 * corresponding serial stored in the list for that sender is then included in the signature
 * verification process. The list is stored as the value of the flag that enables whitelisting,
 * @ref MY_SIGNING_NODE_WHITELISTING.<br>
 *
 * Whitelisting is achieved by 'salting' the signature with some node-unique information known to
 * the receiver. In the case of @ref MY_SIGNING_ATSHA204 this is the unique serial number programmed
 * into the circuit. This unique number is never transmitted over the air in clear text, so Eve will
 * not be able to figure out a "trusted" serial by snooping on the traffic.<br>
 * Instead the value is hashed together with the senders NodeId into the HMAC signature to produce
 * the final signature. The receiver will then take the originating NodeId of the signed message and
 * do the corresponding calculation with the serial it has stored in it's whitelist if it finds a
 * matching entry in it's whitelist.
 *
 * Whitelisting is an optional alternative because it adds some code and configuration options which
 * might not be desirable for every user. So if you want the ability to use whitelists, you need to
 * enable @ref MY_SIGNING_NODE_WHITELISTING. You need to remember that the gateway will remember if
 * a node has presented it with a whitelisting requirement as described above, if you at some point
 * decide to remove the whitelist requirement.<br>
 * The whitelist is provided as value of the flag that enable it as follows (example is a node that
 * require signing as well):
 * @code{.cpp}
 * #define MY_SIGNING_ATSHA204
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #define MY_SIGNING_NODE_WHITELISTING {{.nodeId = GATEWAY_ADDRESS,.serial = {0x09,0x08,0x07,0x06,0x05,0x04,0x03,0x02,0x01}},{.nodeId = 2,.serial = {0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09}}}
 * #include <MySensors.h>
 * ...
 * @endcode
 * In this example, there are two nodes in the whitelist; the gateway, and a separate node that
 * communicates directly with this node (with signed messages). You do not need to do anything
 * special for the sending nodes, apart from making sure they support signing.
 *
 * The "soft" backend of course also support whitelisting. Example:
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #define MY_SIGNING_NODE_WHITELISTING {{.nodeId = GATEWAY_ADDRESS,.serial = {0x09,0x08,0x07,0x06,0x05,0x04,0x03,0x02,0x01}},{.nodeId = 2,.serial = {0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09}}}
 * #include <MySensors.h>
 * ...
 * @endcode
 *
 * For a node that should transmit whitelisted messages but not receive whitelisted messages, you do
 * not need any special configurations:
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * @endcode
 * Remember that you always need to select a signing backend for all nodes that communicate to a
 * node that require whitelisting. Also, note that a node that use whitelisting will not accept
 * messages from nodes that are not present in it's whitelist.
 * And you have to personalize all nodes that use signing with a common HMAC key but different
 * serial numbers (@ref MY_SIGNING_ATSHA204 always has unique serials).
 *
 * @section MySigninglimitations Known limitations
 *
 * Due to the limiting factor of our cheapest Arduino nodes, the use of diversified keys is not
 * implemented. That mean that all nodes in your network share the same PSK (at least the ones that
 * are supposed to exchange signed data). It is important to understand the implications of
 * this, and that is covered in the "Typical use cases" chapter below.<br>
 * Most importantly, if you use @ref MY_SIGNING_SOFT your PSK will be stored in EEPROM and will
 * therefore be accessible by anyone with physical access to your node. Therefore it is <b>NOT</b>
 * recommended to use @ref MY_SIGNING_SOFT on nodes that are placed in a public place or worn on
 * on your person (like a keyfob).<br>
 * Also be reminded that the strength of the signature is inversely proportional to the size of the
 * message. The larger the message, the weaker the signature.
 *
 * @section MySigningusecases Typical use cases
 *
 * "Securely located" in this context mean a node which is not physically publicly accessible.
 * Typically at least your gateway.<br>
 * "Public" in this context mean a node that is located outside your "trusted environment". This
 * includes sensors located outdoors, keyfobs etc.
 *
 * @subsection MySigninglock Securely located lock
 *
 * You have a securely located gateway and a lock somewhere inside your "trusted environment" (e.g.
 * inside your house door, the door to your dungeon or similar).<br>
 * You need to make your node require signed messages but you do not necessarily need to make your
 * gateway require signed messages (unless you are concerned that someone might spoof the lock
 * status of your lock).<br>
 * Configuration example for the secure lock node:<br>
 * @code{.cpp}
 * #define MY_SIGNING_ATSHA204
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #include <MySensors.h>
 * ...
 * @endcode
 * If you do also want your gateway to require signatures from your lock you just enable the same
 * (or similar if using software signing) settings in the gateway.
 *
 * @subsection MySigningpatio Patio motion sensor
 *
 * Your gateway is securely located inside your house, but your motion sensor is located outside
 * your house. You have for some reason elected that this node should sign the messages it send to
 * your gateway. You should lock the data (PSK) in this node then, because if someone were to steal
 * your patio motion sensor, they could rewrite the firmware and spoof your gateway to use it to
 * transmit a correctly signed message to your secure lock inside your house. But if you revoke your
 * gateway (and lock) PSK the outside sensor cannot be used for this anymore. Nor can it be changed
 * in order to do it in the future. You can also use whitelisting to revoke your lost node.<br>
 * This is an unlikely use case because there really is no reason to sign sensor values. If you for
 * some reason want to obfuscate sensor data, encryption is a better alternative.<br>
 * Configuration example for a motion sensor:<br>
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #include <MySensors.h>
 * ...
 * @endcode
 *
 * The gateway needs to be configured with a whitelist (and it has to have an entry for all nodes
 * that send and/or require signed messages):<br>
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #define MY_SIGNING_NODE_WHITELISTING {{.nodeId = MOTION_SENSOR_ID,.serial = {0x12,0x34,0x56,0x78,0x90,0x12,0x34,0x56,0x78}}}
 * #include <MySensors.h>
 * ...
 * @endcode

 * @subsection MySigningkeyfob Keyfob for garage door opener
 *
 * Perhaps the most typical usecase for signed messages. Your keyfob should be totally locked down.
 * If the garage door opener is secured (and it should be) it can be unlocked. That way, if you
 * loose your keyfob, you can revoke the PSK in both the opener and your gateway,
 * thus rendering the keyfob useless without having to replace your nodes. You can also use
 * whitelisting to revoke your lost keyfob.<br>
 * Configuration example for the keyfob (keyfob will only transmit to another node and not receive
 * anything):<br>
 * @code{.cpp}
 * #define MY_SIGNING_ATSHA204
 * #include <MySensors.h>
 * ...
 * @endcode
 *
 * Configuration example for the door controller node (should require signing from anyone who wants
 * to control it):<br>
 * @code{.cpp}
 * #define MY_SIGNING_SOFT
 * #define MY_SIGNING_SOFT_RANDOMSEED_PIN 7
 * #define MY_SIGNING_REQUEST_SIGNATURES
 * #define MY_SIGNING_NODE_WHITELISTING {{.nodeId = GATEWAY_ADDRESS,.serial = {0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88}},{.nodeId = KEYFOB_ID,.serial = {<FROM ATSHA ON KEYFOB>}}}
 * #include <MySensors.h>
 * ...
 * @endcode
 *
 * @subsection MySigningsketches Relevant sketches
 *
 * - @ref SecureActuator.ino
 * - @ref SecurityPersonalizer.ino
 *
 * @section MySigningtechnical The technical stuff
 *
 * The following sequence diagram illustrate how messages are passed in a MySensors network with
 * respect to signing:
 * @image html MySigning/signingsequence.png
 *
 * None of this activity is “visible” to you (as the sensor sketch implementor). All you need to do
 * is to set your preferences in your sketch and personalize accordingly.
 * That is enough to enable protection from both Eve and Mallory in your network
 * although if you do not also enable encryption, Eve can eavesdrop, but not do anything about,
 * your messages (except possibly preventing them from arriving).
 *
 * How are the messages actually affected by the signing?<br>
 * The following illustration shows what part of the message is signed, and where the signature is
 * stored:
 * @image html MySigning/signingillustrated1.png
 *
 * The first byte of the header is not covered by the signature, because in the network, this byte
 * is used to track hops in the network and therefore might change if the message is passing a relay
 * node. So it cannot be part of the signature, or the signature would be invalid when it arrives to
 * its destination. The signature also carries a byte with a signing identifier to prevent false
 * results from accidental mixing of incompatible signing backends in the network. Thus, the maximum
 * size for a payload is 29-7 bytes. Larger payloads are not possible to sign at the moment. Another
 * thing to consider is that the strength of the signature is inversely proportional to the payload
 * size.
 *
 * As for the software backend, it turns out that the ATSHA does not do “vanilla” HMAC processing.
 * Fortunately, Atmel has documented exactly how the circuit processes the data and hashes thus
 * making it possible to generate signatures that are identical to signatures generated by the
 * circuit.
 *
 * The signatures are calculates in the following way:
 * @image html MySigning/signingillustrated2.png
 *
 * Exactly how this is done can be reviewed in the source for the software backend
 * (MySigningAtsha204Soft.cpp) and the ATSHA204A
 * <a href="http://www.atmel.com/Images/Atmel-8885-CryptoAuth-ATSHA204A-Datasheet.pdf">datasheet
 * </a>. In the MySensors protocol, the following internal messagetypes handles signature
 * requirements and nonce requests:<br>
 * @ref I_SIGNING_PRESENTATION <br>
 * @ref I_NONCE_REQUEST <br>
 * @ref I_NONCE_RESPONSE <br>
 *
 * Also, the version field in the header has been reduced from 3 to 2 bits in order to fit a single
 * bit to indicate that a message is signed.
 *
 * @section MySigninggrpbackground Background and concepts
 *
 * Suppose two participants, Alice and Bob, wants to exchange a message. Alice sends a message to
 * Bob. In MySensors “language” Alice could be a gateway and Bob an actuator (light switch,
 * electronic lock, etc). But to be generic, we will substitute the term “gateway” with Alice and a
 * “node” with Bob (although the reverse relationship is also supported).
 *
 * Alice sends a message to Bob. This message can be heard by anyone who wants to listen (and also
 * by anyone that is within “hearing” distance). Normally, this is perhaps not a big issue. Nothing
 * Alice says to Bob may be secret or sensitive in any way. However, sometimes (or perhaps always)
 * Bob want to be sure that the message Bob receives actually came from Alice. In cryptography,
 * this is known as <i>authenticity</i>. Bob needs some way of determining that the message is
 * authentic from Alice, when Bob receives it. This prevents an eavesdropper, Eve, to trick Bob into
 * thinking it was Alice that sent a message Eve in fact transmitted. Bob also needs to know how to
 * determine if the message has been repeated. Eve could record a message sent by Alice that Bob
 * accepted and then send the same message again. Eve could also in some way prevent Bob from
 * receiving the message and delay it in order to permit the message to arrive to Bob at a time Eve
 * chooses, instead of Alice. Such an attack is known as a <b>replay attack</b>.<br>
 * <i>Authenticity</i> permits Bob to determine if Alice is the true sender of a message.
 * @image html MySigning/alicenfriends.png
 *
 * It can also be interesting for Bob to know that the message Alice sent has not been tampered with
 * in any way. This is the <i>integrity</i> of the message. We now introduce Mallory, who could be
 * intercepting the communication between Alice and Bob and replace some parts of the message but
 * keeping the parts that authenticate the message. That way, Bob still trusts Alice to be the
 * source, but the contents of the message was not the content Alice sent. Bob needs to be able to
 * determine that the contents of the message was not altered after Alice sent it.<br>
 * Mallory would in this case be a <b>man-in-the-middle</b> attacker.<br>
 * <i>Integrity</i> permits Bob to verify that the messages received from Alice has not been
 * tampered with.<br>
 * This is achieved by adding a <i>signature</i> to the message, which Bob can inspect to validate
 * that Alice is the author.
 * @image html MySigning/alicenfriends2.png
 *
 * The signing scheme used, needs to address both these attack scenarios. Neither Eve nor Mallory
 * must be permitted to interfere with the message exchange between Alice and Bob.
 *
 * The key challenge to implementing a secure signing scheme is to ensure that every signature is
 * different, even if the message is not. If not, <b>replay attacks</b> would be very hard to
 * prevent.<br>
 * One way of doing this is to increment some counter on the sender side and include it in the
 * signature. This is however predictable.<br>
 * A better option would be to introduce a random number to the signature. That way, it is
 * impossible to predict what the signature will be. The problem is, that also makes it impossible
 * for the receiver (Bob) to verify that the signature is valid.<br>
 * A solution to this is to let Bob generate the random number, keep it in memory and send it to
 * Alice. Alice can then use the random number in the signature calculation and send the signed
 * message back to Bob who can validate the signature with the random number used.
 * This random number is in cryptography known as a
 * <a href="https://en.wikipedia.org/wiki/Cryptographic_nonce">nonce</a> or
 * <a href="https://en.wikipedia.org/wiki/Salt_%28cryptography%29">salt</a>.
 *
 * However, Mallory might be eavesdropping on the communication and snoop up the nonce in order to
 * generate a new valid signature for a different message. To counter this, both Alice and Bob keep
 * a secret that only they know. This secret is never transmitted over the air,
 * nor is it revealed to anybody. This secret is known as a
 * <a href="https://en.wikipedia.org/wiki/Pre-shared_key"> pre-shared key</a> (PSK).
 *
 * If Eve or Mallory are really sophisticated, he/she might use a <b>delayed replay attack</b>.
 * This can be done by allowing Bob to transmit a nonce to Alice. But when Alice transmits the
 * uniquely signed message, Mallory prevents Bob from receiving it, to a point when Mallory
 * decides Bob should receive it. An example of such an attack is described
 * <a href="http://spencerwhyte.blogspot.se/2014/03/delay-attack-jam-intercept-and-replay.html">
 * here</a>.<br>
 * This needs to be addressed as well, and one way of doing this is to have Bob keep track of time
 * between a transmitted nonce and a signed message to verify. If Bob is asked for a nonce, Bob
 * knows that a signed message is going to arrive “soon”. Bob can then decide that if the signed
 * message does not arrive within a predefined timeframe, Bob throws away the generated nonce and
 * thus makes it impossible to verify the message if it arrives late.
 *
 * The flow can be described like this:
 * @image html MySigning/alicenbob.png
 * The benefits for MySensors to support this are obvious. Nobody wants others to be able to control
 * or manipulate any actuators in their home.
 *
 * @section MySigninggrphow How this is done
 *
 * There exist many forms of message signature solutions to combat Eve and Mallory.<br>
 * Most of these solutions are quite complex in term of computations, so I elected to use an
 * algorithm that an external circuit is able to process. This has the added benefit of protecting
 * any keys and intermediate data used for calculating the signature so that even if someone were to
 * actually steal a sensor and disassembled it, they would not be able to extract the keys and other
 * information from the device.<br>
 * A common scheme for message signing (authenticity and integrity) is implemented using
 * <a href="http://en.wikipedia.org/wiki/Hash-based_message_authentication_code">HMAC</a> which in
 * combination with a strong <a href="http://en.wikipedia.org/wiki/Hash_function"> hash function</a>
 * provides a very strong level of protection.<br>
 * The <a href="http://www.atmel.com/devices/ATSHA204A.aspx">Atmel ATSHA204A</a> is a low-cost,
 * low-voltage/current circuit that provides HMAC calculation capabilities with SHA256 hashing which
 * is a (currently) virtually unbreakable combination. If SHA256 were to be hacked, a certain
 * <a href="http://en.wikipedia.org/wiki/Bitcoin">cryptocurrency</a> would immediately be rendered
 * worthless.<br>
 * The ATSHA device also contain a random number generator (RNG) which enables the generation of a
 * good nonce, as in, <i>non-predictable</i>.<br>
 * As I acknowledge that some might not want to use an additional external circuit, I have also
 * implemented a software version of the ATSHA device, capable of generating the same signatures as
 * the ATSHA device does. Because it is pure-software however, it does not provide as good nonces
 * (it uses the <a href="http://arduino.cc/en/reference/random">Arduino pseudo-random generator</a>)
 * and the HMAC key is stored in SW and is therefore readable if the memory is dumped. It also
 * naturally claims more flash space due to the more complex software. But for indoor
 * sensors/actuators this might be good enough for most people.
 *
 * @section MySigninggrpencryption Why encryption is not part of this
 *
 * Well, some could be uncomfortable with somebody being able to snoop temperatures, motion or the
 * state changes of locks in the environment.
 * Signing does <b>not</b> address these issues. Encryption is needed to prevent this.<br>
 * It is my personal standpoint that encryption should not be part of the MySensors “protocol”. The
 * reason is that a gateway and a node does not really care about messages being readable or not by
 * “others”. It makes more sense that such guarantees are provided by the underlying transmission
 * layer (RF solution in this case). It is the information transmitted over the air that needs to be
 * secret (if user so desires). The “trust” level on the other hand needs to go all the way into the
 * sketches (who might have different requirements of trust depending on the message participant),
 * and for this reason, it is more important (and less complicated) to ensure authenticity and
 * <i>integrity</i> at protocol-level as message contents is still readable throughout the protocol
 * stack. But as soon as the message leaves the “stack” it can be scramble into “garbage” when
 * transmitted over the air and then reassembled by a receiving node before being fed in “the clear”
 * up the stack at the receiving end.
 *
 * There are methods and possibilities to provide encryption also in software, but if this is done,
 * it is my recommendation that this is done after integrity- and authentication information has
 * been provided to the message (if this is desired). Integrity and authentication is of course not
 * mandatory and some might be happy with only having encryption to cover their need for security.
 * I, however, have only focused on <i>integrity</i> and <i>authenticity</i> while at the same time
 * keeping the current message routing mechanisms intact and therefore leave the matter of
 * <i>secrecy</i> to be implemented in the “physical” transport layer. With the <i>integrity</i> and
 * <i>authenticity</i> handled in the protocol it ought to be enough for a simple encryption
 * (nonce-less %AES with a PSK for instance) on the message as it is sent to the RF backend. Atmel
 * does provide such circuits as well but I have not investigated the matter further as it given the
 * current size of the ethernet gateway sketch is close to the size limit on an Arduino Nano, so it
 * will be difficult to fit this into some existing gateway designs.<br>
 * Also it is worth to consider that the state of a lock can just as readily be determined by simply
 * looking at the door in question or attempting to open it, so obfuscating this information will
 * not necessarily deter an attacker in any way.<br>
 * Nevertheless, I do acknowledge that people find the fact that all information is sent “in the
 * clear” even if it require some technical effort for an intruder to obtain and inspect this
 * information. So I do encourage the use of encrypting transport layers.<br>
 * This is however not covered by this implementation.<br>
 * This might change in the future as more powerful platforms emerge which permit more complex
 * security schemes and better hardware acceleration.
 */
/** @}*/

/**
 * @defgroup MySigninggrp MySigning
 * @ingroup internals
 * @{
 *
 * @brief API declaration for MySigning signing backend
 *
 * @see MySigninggrpPub
 */
/**
 * @file MySigning.h
 *
 * @brief API declaration for MySigning signing backend
 */
#ifndef MySigning_h
#define MySigning_h

#include "MySensorsCore.h"
#include "drivers/ATSHA204/ATSHA204.h"

#ifdef MY_SIGNING_NODE_WHITELISTING
typedef struct {
	uint8_t nodeId;                   /**< @brief The ID of the node */
	uint8_t serial[SHA204_SERIAL_SZ]; /**< @brief Node specific serial number */
} whitelist_entry_t;
#endif

/** @brief Helper macro to determine the number of elements in a array */
#define NUM_OF(x) (sizeof(x)/sizeof(x[0]))

/**
 * @brief Initializes signing infrastructure and associated backend.
 *
 * This function makes sure that the internal states of the signing infrastructure
 * is set to a known initial state.
 * \n@b Usage: This function should be called before any signing related operations take place.
 */
void signerInit(void);

/**
 * @brief Does signing specific presentation for a node.
 *
 * This function makes sure any signing related presentation info is shared with the other part.
 * The presentation of the gateways signing preferences is done in @ref signerProcessInternal().
 * \n@b Usage: This function should be called by the presentation routine of the MySensors library.
 * You only need to call this directly from a sketch to set up a node to node signed message exchange.
 * If you do call this directly from a sketch, and you at some point change your sketch to go from
 * requiring signing to not requiring signatures, you need to present this change to the node at least
 * once, so it can update its requirements tables accordingly. Or it will keep believing that this node
 * require signatures and attempt to send signed messages to it.
 *
 * @param msg Message buffer to use.
 * @param destination Node ID of the destination.
 */
void signerPresentation(MyMessage &msg, uint8_t destination);

/**
 * @brief Manages internal signing message handshaking.
 *
 * This function takes care of signing related message handshaking such as nonce exchange.
 * \n@b Usage: This function should be called by the incoming message handler before any further message
 * processing is performed on internal messages. This function should only be called for @ref C_INTERNAL class
 * messages.
 *
 * @param msg Message buffer to process.
 * @returns @c true if caller should stop further message processing.
 */
bool signerProcessInternal(MyMessage &msg);

/**
 * @brief Check timeout of verification session.
 *
 * Nonce will be purged if it takes too long for a signed message to be sent to the receiver.
 * \n@b Usage: This function should be called on regular intervals, typically within some process loop.
 *
 * @returns @c true if session is still valid.
 */
bool signerCheckTimer(void);

/**
 * @brief Get nonce from provided message and store for signing operations.
 *
 * Returns @c false if subsystem is busy processing an ongoing signing operation.<br>
 * Returns @c false if signing identifier found in message is not supported by the used backend.<br>
 * If successful, this marks the start of a signing operation at the sending side so
 * implementation is expected to do any necessary initializations within this call.
 * \n@b Usage: This function is typically called as action when receiving a @ref I_NONCE_RESPONSE
 * message.
 *
 * @param msg The message to get the nonce from.
 * @returns @c true if successful, else @c false.
 */
bool signerPutNonce(MyMessage &msg);

/**
 * @brief Signs provided message. All remaining space in message payload buffer is used for
 * signing identifier and signature.
 *
 * Nonce used for signature calculation is the one generated previously within @ref signerProcessInternal().<br>
 * Nonce will be cleared when this function is called to prevent re-use of nonce.<br>
 * Returns @c false if subsystem is busy processing an ongoing signing operation.<br>
 * Returns @c false if not two bytes or more of free payload space is left in provided message.<br>
 * This ends a signing operation at the sending side so implementation is expected to do any
 * deinitializations and enter a power saving state within this call.
 * \n@b Usage: This function is typically called as action when receiving a @ref I_NONCE_RESPONSE
 * message and after @ref signerPutNonce() has successfully been executed.
 *
 * @param msg The message to sign.
 * @returns @c true if successful, else @c false.
*/
bool signerSignMsg(MyMessage &msg);

/**
 * @brief Verifies signature in provided message.
 *
 * Nonce used for verification is the one previously set using @ref signerPutNonce().<br>
 * Nonce will be cleared when this function is called to prevent re-use of nonce.<br>
 * Returns @c false if subsystem is busy processing an ongoing signing operation.<br>
 * Returns @c false if signing identifier found in message is not supported by the used backend.<br>
 * This ends a signing operation at the receiving side so implementation is expected to do any
 * deinitializations and enter a power saving state within this call.
 * \n@b Usage: This function is typically called when receiving a message that is flagged as signed
 * and @ref MY_SIGNING_REQUEST_SIGNATURES is activated.
 *
 * @param msg The message to verify.
 * @returns @c true if successful, else @c false.
 */
bool signerVerifyMsg(MyMessage &msg);

/**
 * @brief Do a timing neutral memory comparison.
 *
 * The function behaves similar to memcmp with the difference that it will
 * always use the same number of instructions for a given number of bytes,
 * no matter how the two buffers differ and the response is either 0 or -1.
 *
 * @param a First buffer for comparison.
 * @param b Second buffer for comparison.
 * @param sz The number of bytes to compare.
 * @returns 0 if buffers match, -1 if they do not.
 */
int signerMemcmp(const void* a, const void* b, size_t sz);

#endif
/** @}*/

/**
 * @defgroup MySigningDebugMessages Signing related debug messages
 * @ingroup MySigninggrpPub
 * @{
 *
 * @brief Explanation of the abstract signing related debug messages
 *
 * MySigning-related log messages, format: [!]SYSTEM:SUB SYSTEM:MESSAGE
 * - [!] Exclamation mark is prepended in case of error or warning
 * - SYSTEM:
 *  - <b>SGN</b> messages emitted by MySigning
 * - SUB SYSTEMS:
 *  - SGN:<b>INI</b>	from @ref signerInit
 *  - SGN:<b>PER</b>	from @ref signerInit
 *  - SGN:<b>PRE</b>	from @ref signerPresentation
 *  - SGN:<b>SGN</b>	from @ref signerSignMsg
 *  - SGN:<b>VER</b>	from @ref signerVerifyMsg
 *  - SGN:<b>SKP</b>	from @ref signerSignMsg or @ref signerVerifyMsg (skipSign)
 *  - SGN:<b>NCE</b>	from @ref signerProcessInternal (signerInternalProcessNonceRequest)
 *  - SGN:<b>BND</b>	from the signing backends
 *
 * MySigning debug log messages:
 *
 * |E| SYS | SUB | Message									| Comment
 * |-|-----|-----|--------------------------|----------------------------------------------------------------------------
 * | | SGN | INI | BND OK										| Backend has initialized ok
 * |!| SGN | INI | BND FAIL									| Backend has not initialized ok
 * | | SGN | PER | OK												| Personalization data is ok
 * |!| SGN | PER | TAMPERED									| Personalization data has been tampered
 * | | SGN | PRE | SGN REQ									| Signing required
 * | | SGN | PRE | SGN REQ,TO='node'				| Tell 'node' that we require signing
 * | | SGN | PRE | SGN REQ,FROM='node'			| Node 'node' require signing
 * | | SGN | PRE | SGN NREQ									| Signing not required
 * | | SGN | PRE | SGN REQ,TO='node'				| Tell 'node' that we do not require signing
 * | | SGN | PRE | SGN NREQ,FROM='node'			| Node 'node' does not require signing
 * |!| SGN | PRE | SGN NREQ,FROM='node' REJ	| Node 'node' does not require signing but used to (requirement remain unchanged)
 * | | SGN | PRE | WHI REQ									| Whitelisting required
 * | | SGN | PRE | WHI REQ,TO='node'				| Tell 'node' that we require whitelisting
 * | | SGN | PRE | WHI REQ,FROM='node'			| Node 'node' require whitelisting
 * | | SGN | PRE | WHI NREQ									| Whitelisting not required
 * | | SGN | PRE | WHI NREQ,TO='node'				| Tell 'node' that we do not require whitelisting
 * | | SGN | PRE | WHI NREQ,FROM='node'			| Node 'node' does not require whitelisting
 * |!| SGN | PRE | WHI NREQ,FROM='node' REJ	| Node 'node' does not require whitelisting but used to (requirement remain unchanged)
 * | | SGN | PRE | XMT,TO='node'						| Presentation data transmitted to 'node'
 * |!| SGN | PRE | XMT,TO='node' FAIL				| Presentation data not properly transmitted to 'node'
 * | | SGN | PRE | WAIT GW									| Waiting for gateway presentation data
 * |!| SGN | PRE | VER='version'						| Presentation version 'version' is not supported
 * | | SGN | PRE | NSUP											| Received signing presentation but signing is not supported
 * | | SGN | PRE | NSUP,TO='node'						| Informing 'node' that we do not support signing
 * | | SGN | SGN | NCE REQ,TO='node'				| Nonce request transmitted to 'node'
 * |!| SGN | SGN | NCE REQ,TO='node' FAIL		| Nonce request not properly transmitted to 'node'
 * |!| SGN | SGN | NCE TMO									| Timeout waiting for nonce
 * | | SGN | SGN | SGN											| Message signed
 * |!| SGN | SGN | SGN FAIL									| Message failed to be signed
 * | | SGN | SGN | NREQ='node'							| 'node' does not require signed messages
 * | | SGN | SGN | 'sender'!='us' NUS				| Will not sign because 'sender' is not 'us' (repeater)
 * |!| SGN | SGN | STATE  									| Security system in a invalid state (personalization data tampered)
 * |!| SGN | VER | NSG											| Message was not signed, but it should have been
 * |!| SGN | VER | FAIL											| Verification failed
 * | | SGN | VER | OK												| Verification succeeded
 * | | SGN | VER | LEFT='number'						| 'number' of failed verifications left in a row before node is locked
 * |!| SGN | VER | STATE  									| Security system in a invalid state (personalization data tampered)
 * | | SGN | SKP | MSG CMD='cmd',TYPE='type'| Message with command 'cmd' and type 'type' does not need to be signed
 * | | SGN | SKP | ECHO CMD='cmd',TYPE='type'| ECHO messages does not need to be signed
 * | | SGN | NCE | LEFT='number'						| 'number' of nonce requests between successful verifications left before node is locked
 * | | SGN | NCE | XMT,TO='node'						| Nonce data transmitted to 'node'
 * |!| SGN | NCE | XMT,TO='node' FAIL				| Nonce data not properly transmitted to 'node'
 * |!| SGN | NCE | GEN											| Failed to generate nonce
 * | | SGN | NCE | NSUP (DROPPED)						| Ignored nonce/request for nonce (signing not supported)
 * | | SGN | NCE | FROM='node'							| Received nonce from 'node'
 * | | SGN | NCE | 'sender'!='dst' (DROPPED)| Ignoring nonce as it did not come from the designation of the message to sign
 * |!| SGN | BND | INIT FAIL								| Failed to initialize signing backend
 * |!| SGN | BND | PWD<8										| Signing password too short
 * |!| SGN | BND | PER											| Backend not personalized
 * |!| SGN | BND | SER											| Could not get device unique serial from backend
 * |!| SGN | BND | TMR											| Backend timed out
 * |!| SGN | BND | SIG,SIZE,'message'>'max'	| Refusing to sign 'message' because it is bigger than 'max' allowed size
 * | | SGN | BND | SIG WHI,ID='id'					| Salting message with our 'id'
 * | | SGN | BND | SIG WHI,SERIAL='serial'	| Salting message with our 'serial'
 * |!| SGN | BND | VER ONGOING							| Verification failed, no ongoing session
 * |!| SGN | BND | VER,IDENT='identifier'		| Verification failed, 'identifier' is unknown
 * | | SGN | BND | VER WHI,ID='sender'			| 'sender' found in whitelist
 * | | SGN | BND | VER WHI,SERIAL='serial'	| Expecting 'serial' for this sender
 * |!| SGN | BND | VER WHI,ID='id' MISSING	| 'id' not found in whitelist
 * | | SGN | BND | NONCE='nonce'						| Calculating signature using 'nonce'
 * | | SGN | BND | HMAC='hmac'							| Calculated signature is 'hmac'
 */
/** @}*/

/**
 * @defgroup MySigningTroubleshootinggrp Signing troubleshooting
 * @ingroup MySigninggrpPub
 * @{
 *
 * @brief Typical signing related failure cases and how to solve them
 *
 * @section MySigningTroubleshootingSymptoms Symptoms and solutions
 *
 * The first thing to do if you suspect signing is causing problems, is to enable the verbose debug
 * flag for the signing backend. @see MY_DEBUG_VERBOSE_SIGNING
 *
 * If you are having trouble getting signing to work, please see the following troubleshooting tips.
 *
 * @subsection MySigningTroubleshootingSymptomStFail Signing fails and logs show st=fail on transmissions
 *
 * This is actually not a signing problem, although often st=fail becomes st=ok when signing is disabled.
 * This is by far the most commonly reported problem with signing, but the problems is not with signing,
 * it is with radio performance.<br>
 * This is a typical log which might look like a signing related issue but isn't:
 * @code{.unparsed}
 * 0;255;3;0;9;Skipping security for command 3 type 16
 * 0;255;3;0;9;read: 3-3-0 s=255,c=3,t=16,pt=0,l=0,sg=0:
 * 0;255;3;0;9;Signing backend: ATSHA204Soft
 * 0;255;3;0;9;SHA256: 86DEAE1DAF50D577A4E2262B33ABF9DEE05DD8FAF84F94F50900000000000000
 * 0;255;3;0;9;Skipping security for command 3 type 17
 * 0;255;3;0;9;send: 0-0-3-3 s=255,c=3,t=17,pt=6,l=25,sg=0,st=fail:86DEAE1DAF50D577A4E2262B33ABF9DEE05DD8FAF84F94F509
 * 0;255;3;0;9;Failed to transmit nonce!
 * 0;255;3;0;9;Message is not signed, but it should have been!
 * 0;255;3;0;9;verify fail
 * @endcode
 *
 * The reason for this is that when signing is used, the messages transmitted become relatively large.<br>
 * Because of this, the message is more sensitive to noise, and the chance for a message to get scrambled
 * increase with the message size. Please refer to the troubleshooting section at the MySensors forum for
 * information on how to improve radio performance.<br>
 * This is a good place to start: https://forum.mysensors.org/topic/666/debug-faq-and-how-ask-for-help
 *
 * @subsection MySigningTroubleshootingSymptomNonce Failed to generate nonce
 *
 * The signing backend failed to generate the nonce needed to sign a message. This indicate a hardware
 * problem. Please post the debug info on the forum together with a description of your hardware setup.
 *
 * @subsection MySigningTroubleshootingSymptomSign Failed to sign message
 *
 * The signing backend failed to sign the message. Typically this happens if your message is so large,
 * that there is no room left in the buffer to store a signature.
 *
 * @subsection MySigningTroubleshootingSymptomWrongSource Nonce did not come from the destination (XX) of the message to be signed! It came from YY
 *
 * This should not really happen. The reason for this message is that the signing backend is only capable
 * of handling one signed message session at any time. If for some reason multiple nodes send a nonce message to
 * the same node, only the nonce from a node that is the destination of the current message signing session will be
 * accepted. Any other nonces will be dropped. This should not happen as no node should send a nonce unless asked to,
 * and a node will only ask for a nonce to one destination for every signing session.<br>
 * If you see this message, please post the debugging details on the MySensors forum so it can be investigated further
 * together with a description of your setup.
 *
 * @subsection MySigningTroubleshootingSymptomNotSigned Message is not signed, but it should have been
 *
 * A node has failed to comply with the signing preferences of this node. Check that the node has received a
 * signing presentation message from this node. This is automatically transmitted to gateways. For other nodes,
 * you need to transmit the presentation from the sketch. @see signerPresentation
 *
 * @subsection MySigningTroubleshootingSymptomNotSignedGeneral "Messages do not appear to be signed but I think they should be..."
 *
 * Make sure you have enabled the flag to require signatures to require signatures and have enabled one of the signing
 * backends. @see MY_SIGNING_REQUEST_SIGNATURES @see MY_SIGNING_ATSHA204 @see MY_SIGNING_SOFT
 *
 * @subsection MySigningTroubleshootingSymptomNotWorkingWhitelisting Signature verification failed!
 *
 * Make sure both source and destination of the signed message has undergone @ref personalization with the same HMAC key.<br>
 * Also, if whitelisting is used, make sure the proper serial is paired with the proper node ID at the destination.
 * Whitelisting preferences are communicated with the signing presentation (done automatically from nodes to gateway but
 * has to be explicitly done by sketch for node to node communication). @see signerPresentation
 *
 * @subsection MySigningTroubleshootingSymptomStTampered Signing backend reports tampered even after personalization
 *
 * The signing backend validates that the secure elements in EEPROM remain unmodified after personalization using a checksum. If the check fails,
 * the backend reports
 * @code
 * !SGN:PER:Tampered
 * @endcode
 * This usually indicate that the sketch has modified the secure elements in EEPROM, but if you experience this even after a node is freshly
 * personalized on a atmega device, it could be that the EESAVE fuse bit is not set which mean that the EEPROM is erased when a new firmware is flashed.
 * You will need to enable the EESAVE bit in order to have the security personalization persist in the node.
 */
/** @}*/