[bawolff]

Essentially if you zip something, repeated text will be deduplicated.

For example "FooFoo" will be smaller than "FooBar" since there is a repeated pattern in the first one.

The attacker can look at the file size and make guesses about how repetitive the text is if they know what the uncompressed or normal size is.

This gets more powerful if the attacker can insert some of their own plaintext.

For example if the plaintext is "Foo" and the attacker inserts "Fo" (giving "FooFo") the result will be smaller than if they inserted zq where there is no pattern. By making lots of guesses the attacker can figure out the secret part of the text a little bit at a time just by observing the size of the ciphertext after inserting different guesses.