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I would like to implement a secure communication channels between 2 arduinos communicating over 433 mhz radio modules (to be more precise using the barebone Atmega 326 chips).

I know that the lack of resources are a problem with the Atmega chips. The best starting point I could find was this library, which never got written:

https://forum.arduino.cc/index.php?topic=314845.0

The way I see it it does not matter what kind of encoding we use if an attacker can capture and replay that OTA, there has to be a 2nd component between the devices which is not known for an attacker and changes the same way like a DS based timer. If a client could encrypt its messages using key=timeXpassword key->encrypt(datatobesent) then the server could decrypt it the same way if it arrived in a proper time period like +-/5 seconds where the time does not have to be the current accurate time just a clock which is in sync between the devices. When the message is received by the server it would calculate 5 different data possibilities for the 5 seconds and look into a decrypted data for certain string to determine if that is a VALID data packet.

For this again there would be the need of an external timer circuit like the DSXXXX series because the internal timer in the chip is not reliable/overflows after a while.

Any other ideas for secure communication?

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You may want to take a look at my RC4-with-diffusion algorithm RC4D which is very resource friendly as it takes less than 500 bytes of flash space (when encrypting inplace) and is much faster than AES128 on an Arduino.

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  • RC4 should no longer be used for cryptographic applications; it's outmoded.
    – dandavis
    Sep 4 '18 at 15:51
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Since you have two-way communication you can be far more cunning that just including a timestamp. You can use real authentication.

The idea here is the "sender" first sends a request to the "receiver" saying "Let me in".

The "receiver" then sends back a message saying "Ok, here's your session key".

The "sender" then encrypts the connection credentials using that session key in some way (or encrypts it in some way including that key with the credentials) - ideally with a trapdoor hash algorithm, and sends it to the "receiver".

The "receiver" then does the same operation with the data it expects the "sender" should send, and compares it with what it received.

If they match, then the "receiver" can allow communication from that "client". The "receiver" would then send a new session key to the "sender" which would then be used to encrypt all communication thereafter. That key must be different to the first, and would need to use symmetric encryption.

This all means that:

  • The authentication key changes every time - there is no chance of "replaying" it
  • The communication key changes with each session, so again there is no chance of replaying messages at a later data, since the key will be wrong.

Yes, there are a lot of overheads with this method, but it's secure.

The authentication methodology is basically the Challenge Handshake Authentication Protocol (server sends challenge, client responds with credentials along with challenge, trapdoor encrypted, server compares with its own encryption and allows or denies). The rest is basic encryption of your choosing using the unique key the remote end of the connection provides.

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