I just implemented xtea for the AVR MCU using C, it might be hard to implement in Picbasic since 32 bit unsigned long integers are not supported (I think).

I just copied from http://www.answers.com/topic/xtea into my C compiler (IAR embedded workbench)and was able to use it without modification for AVR.

xtea is supposed to be one of the most secure ( I do not think it has been cracked yet) and fastest algorithims. I looked into DES and AES and they seemed to time consuming to implement. I think xtea or one of its variants is used for the xbox.

I am using xtea to encrypt firmware so people can flash upgrade stuff without me worrying that they will steal my work.