1. For each letter of the plaintext, concatenate "dontcorruptmebro"
2. Grab 17 letters from the pad (I'm assuming pad means the randomly chosen letters that make up the 1 gigabyte of shared secret among the participants)
3. Convert the first letter of the pad to a number (let's say 1-26)
4. Right-shift the message by that amount and let the letters wrap around: e.g., if the plaintext letter is g and we're shifting by 1 the plaintext becomes "ogdontcorruptmebr"
5. Encrypt with pad (not a cryptographer, but I'm assuming this means adding a character of the shared secret with a character of the plaintext, and wrapping around if it goes past the end)
6. Send
7. Decrypt with pad
8. Left-shift and wrap by $first_pad_letter_value (i.e., the same value we used to right-shift and wrap above)
9. Remove the "dontcorruptmebro" part to concatenate the plaintext letters to reveal the original message
10. Rinse and repeat, and do whatever secure messengers do if/when you receive a part of the "dontcorruptmebro" that isn't "dontcorruptmebro"
I only thought about this for a few minutes, and I'm not a cryptographer. So I'm honestly asking-- did I solve the corruption-in-transit problem without screwing up anything?
Edit: just to be clear-- I'm not trying to be efficient, or to authenticate, or do anything whatsoever other than a dead simple thing that can be implemented easily.
Edit 2: clarification