Disabling compression when encrypting

[David Shaw]

On May 24, 2008, at 2:12 PM, Robert J. Hansen wrote:

> David Shaw wrote:
>> Note that GPG also checks the input file to see if it is  
>> compressed.  If it sees a compressed input file, it automatically  
>> disables compression.
>
> How does it do this?  Does it look for known headers, or does it  
> check the entropy of the source text?  If the former, then what  
> headers does GnuPG know about?  zip, bz2 and gz, I'm assuming, but  
> what about more exotic formats like 7z and the like?

Known headers.  It's basically the same thing that file(1) does.   
You're correct in that it only catches zip, gzip, and bz2.

David