Compression used in an encrypted message

[Peter Pentchev]

On Fri, Mar 11, 2011 at 12:50:26PM -0500, Avi wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA512
> 
> Forgive my ignorance, but is there a way to take a given
> encrypted message/file and determine which compression algorithm
> was used (and which level)? I know how to set compression
> algorithm and level prefs, but I'm curious to see what others
> use, if possible.

If the file has been encrypted to you (or, more specifically, to
one of the secret keys currently accessible to you), then, yes, you
most probably can - "gpg --list-packets filename" should tell you
what compression algorithm has been used, then it's just a matter of
looking it up in RFC 4880 :)

If the message has been encrypted to someone else's key, then you
most probably won't be able to examine it - at least GnuPG does
the compression before the encryption, so that the information about
the compression algorithm used is contained within the encrypted data.
You may still give it a shot with --list-packets, but don't expect
too much :)

Hope that helps.

G'luck,
Peter

-- 
Peter Pentchev	roam at ringlet.net roam at FreeBSD.org peter at packetscale.com
PGP key:	http://people.FreeBSD.org/~roam/roam.key.asc
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This sentence contains exactly threee erors.
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