My intuition says that if your ciphertext after using a one-time pad is trivially compressible, it means there's a strong correlation between your plaintext and

My intuition says that if your ciphertext after using a one-time pad is trivially compressible, it means there's a strong correlation between your plaintext and your key. It's an interesting thought. Consider using basic modular addition of letters, with A=1, B=2 ... Z = 26. If your plaintext is HELLO, and you get AAAAA as ciphertext, then your key has to be SVOOL. If you look, that's just an inversion of each letter. H is the 8th letter of the alphabet, whereas S is the 8th from the end, etc. So, your one-time pad was incidentally a simple translation of your message. I'm not sure if there's any interesting information being leaked this way. Perhaps by seeing that the encrypted message can be heavily compressed, or knowing that it has been heavily compressed, you can assume that the encryption key has some very improbable patterns, and use that to bound guesses about the encryption key. Not an expert. Not even an intermediate.