@@ -1,4 +1,4 @@
/**
/*
* 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
@@ -15,14 +15,401 @@
* 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 MySigninggrp MySigning
* @{
*
*******************************
*
* DESCRIPTION
* Signing support created by Patrick "Anticimex" Fallberg <patrick@fallberg.net>
* The MySigning driver provides a generic API for various signing backends to offer
* @brief The signing driver provides a generic API for various signing backends to offer
* signing of MySensors messages.
*
* Signing support created by Patrick "Anticimex" Fallberg <patrick@fallberg.net>
*
* @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 swich, 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 need some way of determining that the message is authentic from Alice, when Bob receives it.
* This prevent an eavesdropper, Eve, to trick Bob into thinking it was Alice that sent a message Eve in fact transmitted. Bob also need
* 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
* @image latex MySigning/alicenfriends.eps "Alice with Eve and Bob" width=6cm
*
* 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
* @image latex MySigning/alicenfriends2.eps "Alice with Mallory and Bob" width=9cm
*
* 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
* @image latex MySigning/alicenbob.eps "Alice and Bob" width=8cm
* 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 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.
*
* @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 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 choise in your sketch. Currently, two compatible backends are
* supported; @ref MY_SIGNING_ATSHA204 (hardware backed) and @ref MY_SIGNING_SOFT (software backed). You can either enable these globally in
* @ref MyConfig.h or in your sketch for sketch specific/local use.
*
* <b>Firstly</b>, you need to make sure to pick a backend to use as described above.
* @code{.cpp}
* //#define MY_SIGNING_SOFT
* #define MY_SIGNING_ATSHA204
* #include <MySensor.h>
* ...
* @endcode
* Make sure to set the define before the inclusion of MySensor.h.
* It is legal to mix hardware- and software-based backends in a network. They work together.
*
* You also need to decide if the node (or gateway) in question require and verify signatures in addition to calculating them.
* This has to be set by at least one of the node 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. If this is set in a gateway, it will @b NOT force all nodes to sign messages to it. It will only require
* signatures from nodes that in turn require signatures.
*
* <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 and the default is to use pin A3.<br>
* 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 <MySensor.h>
* ...
* @endcode
* For the software backed signingbackend, an unconnected analog pin is required 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 and the default is to use pin A7. The same configuration
* possibilities exist as with the other configuration options.
*
* <b>Thirdly</b>, if you use the software backend, you need to create/set a HMAC key to use. This key is 32 bytes wide and should
* be an arbitrarily chosen value. A string is OK, but as this key never needs to be “parsed” a completely random number is recommended. The
* key is stored in software and can be set "globally" in @ref MyConfig.h or locally in your sketch. The define is @ref MY_SIGNING_SOFT_HMAC_KEY.
* @code{.cpp}
* #define MY_SIGNING_SOFT
* #define MY_SIGNING_SOFT_SERIAL 0x12,0x34,0x56,0x78,0x90,0x12,0x34,0x56,0x78
* #defune MY_SIGNING_SOFT_RANDOMSEED_PIN 7
* #define MY_SIGNING_SOFT_HMAC_KEY 0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x10,0x20,0x30,0x40,0x50,0x60,0x70,0x80,0x90,0x0a,0x0b,0x0c,0x0d,0x0e,0x0f,0xa0,0xb0,0xc0,0xd0,0xe0,0xf0,0x11,0x22
* #define MY_SIGNING_REQUEST_SIGNATURES
* #include <MySensor.h>
* ...
* @endcode
*
* An example of a node that require signatures is available in @ref SecureActuator.ino.
*
* @anchor perzonalization If you use the “real” ATSHA204A, 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 Sha204Personalizer.ino
*
* The process of personalizing the ATSHA204A involves
* - Writing and locking chip configuration
* - (optionally) Generating and (mandatory) writing HMAC key
* - (optionally) Locking data sections
*
* First execute the sketch without modifications to make sure communications with the device can be established. It is highly recommended that
* the first time this is done, a device with serial debug possibilities is used.<br>
* When this has been confirmed, it is time to decide what type of personalization is desired.<br>
* There are a few options here.<br>
* Firstly, enable @ref LOCK_CONFIGURATION to allow the sketch to lock the chip configuration. The sketch will write the default settings to the
* chip because these are fine for our purposes. This also enables the RNG which is required to allow the sketch to automatically generate a PSK
* if this is desired.<br>
* Now it is possible to execute the sketch to lock the configuration and enable the RNG.<br>
* Next step is to decide if a new key should be generated or an existing key should be stored to the device. This is determined using
* @ref USER_KEY_DATA, which, if defined, will use the data in the variable user_key_data.<br>
* If @ref USER_KEY_DATA is disabled, the RNG will be used to generate a key. This key obviously need to be made available to you so you can use
* it in other devices in the network, and this key is therefore also printed on the serial console when it has been generated.<br>
* The key (generated or provided) will be written to the device unless @ref SKIP_KEY_STORAGE is set. As long as the data zone is kept unlocked
* the key can be replaced at any time. However, Atmel suggests the data region to be locked for maximum security. On the other hand, they also
* claim that the key is not readable from the device even if the data zone remains unlocked so the need for locking the data region is optional
* for MySensors usage.<br>
* For devices that does not have serial debug possibilities, it is possible to set @ref SKIP_UART_CONFIRMATION, but it is required to set
* @ref USER_KEY_DATA if this option is enabled since a generated and potentially unknown key could be written to the device and thus rendering
* it useless (if the data zone is also locked).<br>
* For devices with serial debug possibilities it is recommended to not use @ref SKIP_UART_CONFIRMATION as the sketch without that setting will
* ask user to send a ‘ ’
* this irreversible operation is done. However, if a number of nodes are to undergo personalization, this option can be enabled to streamline
* the personalization.<br>
* This is a condensed description of settings to fully personalize and lock down a set of sensors (and gateways):
* - Pick a “master” device with serial debug port.
* - Set the following sketch configuration of the personalizer:
* -# Enable @ref LOCK_CONFIGURATION
* -# Disable @ref LOCK_DATA
* -# Enable @ref SKIP_KEY_STORAGE
* -# Disable @ref SKIP_UART_CONFIRMATION
* -# Disable @ref USER_KEY_DATA
* - Execute the sketch on the “master” device to obtain a randomized key. Save this key to a secure location and keep it confidential so that\n
you can retrieve it if you need to personalize more devices later on.
* - Now reconfigure the sketch with these settings:
* -# Enable @ref LOCK_CONFIGURATION
* -# Enable @ref LOCK_DATA (if you are sure you do not need to replace/revoke the key, this is the most secure option to protect from key readout\n
* according to Atmel, but they also claim that key is not readable even if data region remains unlocked from the slot we are using)
* -# Disable @ref SKIP_KEY_STORAGE
* -# Enable @ref SKIP_UART_CONFIRMATION
* -# Enable @ref USER_KEY_DATA
* - Put the saved key in the user_key_data variable.
* - Now execute the sketch on all devices you want to personalize with this secret key.
*
* That’
*
* If a node does require signing, any unsigned message sent to the node will be rejected.<br>
* This also applies to the gateway. However, the difference is that the gateway will only require signed messages from nodes it knows in turn
* require signed messages.<br>
* A node can also inform a different node that it expects to receive signed messages from it. This is done by transmitting an internal message
* of type @ref I_REQUEST_SIGNING and provide a boolean for payload, set to @c true.<br>
* 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 the gateway reboots, the nodes does not have to retransmit a signing request to the gateway for the gateway to realize that the node
* expect signed messages.<br>
* Also, the nodes that do not require signed messages will also inform gateway of this, so if you reprogram a node to stop require signing,
* the gateway will adhere to this as soon as the new node has presented itself to the gateway.
*
* The following sequence diagram illustrate how messages are passed in a MySensors network with respect to signing:
* @image html MySigning/signingsequence.png
* @image latex MySigning/signingsequence.eps "Signing sequence diagram" width=9cm
*
* None of this activity is “visible” to you (as the sensor sketch implementor). All you need to do is to set your preferences in MyConfig.h or
* in your sketch. Depending on chosen backend, do personalization or key configurations and set @ref MY_SIGNING_REQUEST_SIGNATURES on the node that
* you want protected. 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).
*
* @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 (therefore secure and out of reach from prying eyes) that controls your garage door motor.\n
* This node require 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.
*
* One day your keyfob gets stolen or you lost it or it simply broke down.
*
* You know end up with a problem; you need some way of telling your door node that the keyfob in question cannot be trusted any more. Furthermore,
* you maybe locked the data region in your door nodes ATSHA device and is not able to revoke/change your PSK, or you have some other reason for
* not wanting to replace the PSK. 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, 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. This is unique for each device. The serial number is printed in a copy+paste friendly format by the
* personalizer for this purpose. For the software backend, it is defined by @ref MY_SIGNING_SOFT_SERIAL. You should keep this value unique for each
* node in the network.<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>
* For whitelisting to work, the sending node also need to have whitelisting enabled.<br>
*
* Whitelisting is achieved by 'salting' the signature with some node-unique information known to the receiver. In the case of ATSHA204A 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 which might not be desirable for every user. So if you want the ability to
* provide and use whitelists, as well as transmitting to a node with a whitelist, you need to enable @ref MY_SIGNING_NODE_WHITELISTING.<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 <MySensor.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).
*
* The "soft" backend of course also support whitelisting. However, since it does not contain a unique identifier, you have to provide an additional
* setting (@ref MY_SIGNING_SOFT_SERIAL) when you enable whitelisting as illustrated in this example:
* @code{.cpp}
* #define MY_SIGNING_SOFT
* #define MY_SIGNING_SOFT_SERIAL 0x12,0x34,0x56,0x78,0x90,0x12,0x34,0x56,0x78
* #defune MY_SIGNING_SOFT_RANDOMSEED_PIN 7
* #define MY_SIGNING_SOFT_HMAC_KEY 0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x10,0x20,0x30,0x40,0x50,0x60,0x70,0x80,0x90,0x0a,0x0b,0x0c,0x0d,0x0e,0x0f,0xa0,0xb0,0xc0,0xd0,0xe0,0xf0,0x11,0x22
* #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 <MySensor.h>
* ...
* @endcode
*
* For a node that should transmit whitelisted messages but not receive whitelisted messages, you can set the whitelist flag as follows:
* @code{.cpp}
* #define MY_SIGNING_NODE_WHITELISTING {}
* @endcode
* Remember that for the software backend, you still need to set @ref MY_SIGNING_SOFT_SERIAL (a default is set in MyConfig.h if you do not provide one
* in your sketch.
*
* It is important to emphasize that you do not have to provide a whitelist that has entries for all nodes that transmit signed messages to the node
* in question. You only need to have entries for the nodes that in turn have enabled @ref MY_SIGNING_NODE_WHITELISTING. Nodes that does not have
* this option enabled can still transmit "regular" signed messages as long as they do not match a NodeId in the receivers whitelist.
*
* @section MySigningtechnical The technical stuff
*
* 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
* @image latex MySigning/signingillustrated1.eps "Message structure" width=11cm
*
* 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. 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
* @image latex MySigning/signingillustrated2.eps "HMAC processing" width=10cm
*
* Exactly how this is done can be reviewd in the source for the ATSHA204SOFT backend 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_REQUEST_SIGNING <br>
* @ref I_GET_NONCE <br>
* @ref I_GET_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 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 usecases" chapter below.<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 usecases
*
* "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 should then keep the data section of your gateway and your lock node unlocked. Locking the data (and therefore the PSK) will require you
* to replace at least the signing circuit in case you need to revoke the PSK because some other node in your network gets compromised.
*
* @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 usecase because it is really no reason to sign sensor values. If you for some reason want to obfuscate sensor data,
* encryption is a better alternative.
*
* @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.
*
* @subsection MySigningsketches Relevant sketches
*
* - @ref SecureActuator.ino
* - @ref Sha204Personalizer.ino
*/
/**
* @file MySigning.h
*
* @brief API declaration for MySigning signing backend
*/
# ifndef MySigning_h
# define MySigning_h
@@ -35,53 +422,94 @@
# ifdef MY_SIGNING_NODE_WHITELISTING
typedef struct {
uint8_t nodeId ;
uint8_t serial [ SHA204_SERIAL_SZ ] ;
uint8_t nodeId ; /**< @brief The ID of the node */
uint8_t serial [ SHA204_SERIAL_SZ ] ; /**< @brief Node specific serial number */
} whitelist_entry_t ;
# endif
// Macros for manipulating signing requirement table
/** @brief Return 'true' if provided node ID is requiering signed messages */
# define DO_SIGN(node) (node == 0 ? (~_doSign[0]&1) : (~_doSign[node>>4]&(node%16)))
/** @brief Mark provided node ID to require signed messages in table */
# define SET_SIGN(node) (node == 0 ? (_doSign[0]&=~1) : (_doSign[node>>4]&=~(node%16)))
/** @brief Mark provided node ID to not require signed messages in table */
# define CLEAR_SIGN(node) (node == 0 ? (_doSign[0]|=1) : (_doSign[node>>4]|=(node%16)))
/** @brief Helper macro to determine the number of elements in a array */
# define NUM_OF(x) (sizeof(x) / sizeof(x[0]))
// Stores signing identifier and a new nonce in provided message for signing operations.
// All space in message payload buffer is used for signing identifier and nonce .
// Returns false if subsystem is busy processing an ongoing signing operation.
// If successful, this marks the start of a signing operation at the receiving side so
// implementation is expected to do any necessary initializations within this call.
/**
* @brief Stores signing identifier and a new nonce in provided message for signing operations .
*
* All space in message payload buffer is used for signing identifier and nonce.<br>
* Returns @c false if subsystem is busy processing an ongoing signing operation.<br>
* If successful, this marks the start of a signing operation at the receiving 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_GET_NONCE message.
*
* @param msg The message to put the nonce in.
* @returns @c true if nonce is successfully put in provided message.
*/
bool signerGetNonce ( MyMessage & msg ) ;
// Check timeout of verification session.
// Nonce will be purged if it takes too long for a signed message to be sent to the receiver .
// This function should be called on regular intervals.
/**
* @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 ) ;
// Get nonce from provided message and store for signing operations.
// Returns false if subsystem is busy processing an ongoing signing operation.
// Returns false if signing identifier found in message is not supported by the used backend.
// 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.
/**
* @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_GET_NONCE_RESPONSE
* message.
*
* @param msg The message to get the nonce from.
* @returns @c true if successful, else @c false.
*/
bool signerPutNonce ( MyMessage & msg ) ;
// 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 using getNonce() .
// Nonce will be cleared when this function is called to prevent re-use of nonce.
// Returns false if subsystem is busy processing an ongoing signing operation.
// Returns false if not two bytes or more of free payload space is left in provided message.
// 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.
/**
* @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 using @ref signerGetNonce().<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_GET_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 ) ;
// Verifies signature in provided message.
// Nonce used for verification is the one pre viously set using putNonce() .
// Nonce will be cleared when this function is called to prevent re-use of nonce.
// Returns false if subsystem is b usy processing an ongoing signing operation.
// Returns false if signing identifier fo und in message is not supported by the used backend.
// 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.
/**
* @brief Verifies signature in pro vided message .
*
* Nonce used for verification is the one previo usl y 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 ) ;
# endif
/** @}*/
Patrick Fallberg