Art of the Problem | Fermat primality test (Prime Adventure Part 10) @ArtOfTheProblem | Uploaded 11 years ago | Updated 11 hours ago
Fermat Primality test, rough outline of how & why it works.

Note, this doesn't assume strong background in modular arithmetic, for those details please see full lesson here:
khanacademy.org/math/applied-math/cryptography/modarithmetic/a/what-is-modular-arithmetic

Link to program from video:
khanacademy.org/cs/level-10-fermats-primality-test/1641899107