This system is (almost) just a variant of the one-time pad. One-time pad: before Alice and Bob need to send secret messages to one another, they generate a lar

This system is (almost) just a variant of the one-time pad. One-time pad: before Alice and Bob need to send secret messages to one another, they generate a large blob of random bits and each take one copy of it. They keep these bits very secret. When one needs to send an N-bit message to the other, s/he takes the next N bits of the blob and XORs them with the message. This is trivial to do and provably impossible to break -- except that making and sharing and keeping all that key data (the big blob of bits) is really inconvenient and easy to get wrong. So, anyway, your system has the same weaknesses as any other one-time pad: you need new key data for every message you send, which means you have to make a lot of key data, share it with your partner in crime, and remember it for a long time, all without leaking any of it. It's not quite the same as a one-time pad. You can get by with fewer bits of key than bits of message, provided there's still "enough" key. On the other hand, "enough" is quite a lot, much more than the key length of a typical symmetric cryptosystem like AES. Anyway, I don't think this counts as "by far the simplest encryption to understand". A standard one-time pad is simpler and stronger, and the extra complexity of multiplying large prime numbers doesn't really buy you anything much to compensate.