Thoughts On Combining Compression and Encryption

One of the issues while talking about encryption and cryptography is how should we combine compression with encryption. Data compression is one of the tasks people often do. Combining compression and encryption needs some addressing since compression should always be done before the encryption and not the other way.

The results are generally not good if encryption is done before compressing the data. This is because of the nature of the encryption. Compression takes advantage of non-randomness of data but a good encryption generates the random stream of data which is unlikely to get good compression in cases of loss-less compressions. Of course, some image compression which are not loss-less will still get some compression.

Compression technology looks for the repeatability of data and performs compression by looking such patterns. Most encryption schemes transform the data such that it is random or very very close to being random. Output of good encryption scheme must be indistinguishable from truly random. And compressing the truly random data would not produce effective result. Hence, compress first and then do the encryption. :)