256 Encrpted serial communication question

I am looking to encrypt my communication between processors. I will be useing either i2c or serial but want to encrypt blocks of data with AES encryption.

I have found a few AES librarys but when i attempt to decrypt the block on another processor it gives me garbage data. If i decrypt on the same processor is seems to work fine. I also noticed there is some sort of data that is stored locally in the library's that can actually decrypt the blocks.

Can someone point me to a library that can accomplish what I am looking for? Or quote me on how much one would charge to accomplish this? I have no background in crpytography and don't think i could learn the trade in the timeframe I would like to finish this project in.

I don't plan on passing more than 16 bytes of data. Each "set" of arduino prosessors will be married with a dedicated key built into the source code. each processor will encrpyt is block and transmit only the encrypted block to the other. Then useing the key must decrypt the block back into a readable string so that the receiving processor can perform a task.

toonamo: I have found a few AES librarys

I don't know which ones you've found or which one you're using, but I suggest looking for one with an example that shows encrypting and decrypting independently.

Yes I was thinking the same thing I have tried

https://github.com/DavyLandman/AESLib

http://www.literatecode.com/aes256

http://forum.arduino.cc/index.php/topic,88890.0.html (needs origal string to decrypt?)

All of them appear to encrypt fine, and also decrypt (although the last one passes the original string in so I am not sure it actually works). However if you save the encrpyted char array and attempt to directly jump to decryption with the same key they don't seem to work.

Because I will be decrypted on a different processor I am looking for a way to make this work.