Encryption library for an 8051 microcontroller
You can use simple XOR encryption, and I suppose if you had to use your 96 bit key you can rotate the register every next byte until you get to your last byte of the 96 bits, then you start again, and then on the receiver side just XOR it again to get the correct input. A changing XOR key every byte provides some small protection against brute-forcing the common key if an attacker tries to brute force multiple bytes, as they wont have the same XOR byte counterpart it will be much harder to crack. XOR encryption is very lightweight but its not the best, though with an 8051 it should be somewhat quick to compute and transmit.
There may be other libraries which rely on more complex encryption, although on an 8 bit MCU it's not going to be particularly fast and if you are just worried about having this info protected, then a rolling XOR key should be fine for use with the 8051 unless you have someone who's trying to target your system and break the encryption (which, if they build a program to do so, with a modern GPU it will be very quick to capture the transmitted bytes and brute force every single one (assuming a byte would be recognisable once decrypted) and recover your initial 96 bit key.