On Compression of Data Encrypted with Block Ciphers

TL;DR

This paper explores methods to compress data encrypted with block ciphers like AES without knowing the secret key, demonstrating feasibility, security considerations, and fundamental limitations in certain modes.

Contribution

It introduces techniques for compressing encrypted data without key knowledge and analyzes the impact of chaining modes on compressibility and security.

Findings

Feasible compression of encrypted data without key knowledge.

Chaining modes enable compression without security compromise.

Fundamental limits exist for non-chained block cipher compressibility.

Abstract

This paper investigates compression of data encrypted with block ciphers, such as the Advanced Encryption Standard (AES). It is shown that such data can be feasibly compressed without knowledge of the secret key. Block ciphers operating in various chaining modes are considered and it is shown how compression can be achieved without compromising security of the encryption scheme. Further, it is shown that there exists a fundamental limitation to the practical compressibility of block ciphers when no chaining is used between blocks. Some performance results for practical code constructions used to compress binary sources are presented.