In scenario when an attacker can see encrypted message (e.g. monitoring network traffic) and can affect part of the message that is being encrypted, he can use

In scenario when an attacker can see encrypted message (e.g. monitoring network traffic) and can affect part of the message that is being encrypted, he can use the compression to his advantage. He can for example try different inputs and observe the length of encrypted message, if with the certain input the length drops that means the given input contains string that's similar to another part of the decrypted message and the compressing algorithm did its job and used that to reduce the size. This was mentioned in 2012 when CRIME[1] (also included BEAST[2] exploit) and later BREACH[2] vulnerability (when it was considered cool to come up with cool sounding name, creating a logo and a website for specific vulnerabilities) [1] https://en.wikipedia.org/wiki/CRIME https://en.wikipedia.org/wiki/CRIME [2] https://en.wikipedia.org/wiki/Transport_Layer_Security#BEAST_attack https://en.wikipedia.org/wiki/Transport_Layer_Security#BEAST... [3] https://en.wikipedia.org/wiki/BREACH https://en.wikipedia.org/wiki/BREACH