/*
* 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.
*/
/** @}*/