Can you clarify when data compression is used?

[David Shaw]

On Sun, Feb 03, 2008 at 11:48:13PM -0600, Kevin Hilton wrote:
> Is the data compression algorithm applied to the text prior to being
> converted to ciphertext, or is the ciphertext compressed, or is it the
> combination of the ciphertext and encrypted session key that is compressed?

Prior.  Ciphertext doesn't compress particularly well.  Plus, there is
a minor (as in "don't rely on it, but it's nice to have") security
improvement in encrypting already-compressed data.

> I can't seem to find any documentation discussing this.

RFC-4880 documents this.

David