This paper investigates compression of encrypted data. It has been previously shown that data encrypted with Vernam's scheme, also known as the one-time pad, can be compressed without knowledge of the secret key, therefore this result can be applied to stream ciphers used in practice. However, it was not known how to compress data encrypted with non-stream ciphers. In this paper, we address the problem of compressing data encrypted with block ciphers, such as the advanced encryption standard (AES) used in conjunction with one of the commonly employed chaining modes. We show that such data can be feasibly compressed without knowledge of the key. We present performance results for practical code constructions used to compress binary sources.
