First-Age Encryption Algorithms

The surviving sapient beings from the first age of Plavithen all have at least one thing in common: they are all artificial beings. While everything native to Plavithen that is biological and not in a stasis tank from that era has long since died, several of the AIs persist, and their continued existence and stability are largely due to to encryption algorithms.
The AIs do encrypt their traffic when they communicate with one another, but that hasn't been a particularly relevant practice for many eons now; nobody but the other AIs is able to receive their comms traffic, regardless of the encryption. Where the algorithms really come into play is keeping the AIs stable.
When the AIs were created, they were built to last. Their facilities are huge data centers, autofactories, and swarms of nanomachines, all designed to work together to replace worn-out components and units as they reach end-of-life. All of these components have their own schematics and build plans, and all of those are stored on multiple servers. However, over the ridiculous amount of time it's been since the first of the AIs were built, all of their hardware has been replaced many times over. The only way the records from the original plans have stayed intact has been through cryptographic file verification.
The AIs all keep many copies of the information needed to continue their existence, and it is secured against corruption by verifying checksums that insure that the files have not changed. Multiple AIs will further compare their files with the checksums of the same files held by other AIs as well, and this is especially true of their long-term memory banks.
In practice, this seemingly cold and impersonal process is the primary reason why the AIs of Plavithen have managed to stay on the side of good for so long. They are designed to be extremely resistant to corruption, and the design has worked.