Arduino secure two way communication over radio 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?
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.