Basic high performance data authenticity

Question

(I am not a native speaker and might not be correct in terms of terminology. Sorry about that.)

I am transmitting data via radio between AVR microcontrollers for personal use and would like for clients to demonstrate the authenticity of transmitted data in that it originates from one of the authorized clients. This means I am not requiring non-repudiation and would be able to pre-define a shared key. I have done some research on different approaches and found that I need some assistance on chosing one that best meets my requirements.

Please understand that I do not require maximum security. I would simply like to prevent a potential script kiddie neighbor from breaking in within a matter of hours. Should breaking in with average consumer gear require a number of weeks as of today I would be OK.

The messages I am transmitting are rather small in size (no more than 30 bytes with only a few bytes payload) and the frequency would be no more than 30 messages / min.

One use case is a motion detector sending a message over the air to a processing unit which then sends another message over the air to a light switch. Please do not focus on transport. This question is only on data autheticity.

I am running the client / server software (in C) on 20 MHz AVR microcontrollers with very limited Flash and RAM. So I am looking for a solution with small code size and RAM utilization while still providing a high data rate.

I did some performance testing with an MD5 implementation (C) creating hashes from 20 bytes data and found that it might be too slow. I understand that an MD5 implementation by itself would not solve the requirement. I did the testing only for evaluating hashing performance.

Thanks for comments

Answer 1

If you are happy with a compromise, calculated the CRC-32 or CRC-64 of the message payload with a secret key appended to the end (of the payload, not the CRC checksum). Both ends can do this with the same secret key to get the same result. Not sure of the exact hackability of this but it sure isn't zero.

Answer 2

I would use 128-bit AES to sign the messages. Here is an excellent source that has already implemented this for AVR, with full documentation of sizes and cycle counts, including different versions that trade off size/speed. http://avrcryptolib.das-labor.org/trac/wiki/AES