sporatic decryption issue

[David Shaw]

On Thu, Feb 16, 2006 at 02:27:00PM -0600, Elliot Nathanson wrote:
> All:
> 
> I'm having a really frustrating problem, whereby I'm encrypting
> files for a client and they are *occasionally* unable to decrypt
> a file. The failure rate could be about 1/100; one e-mail said
> they processed 400 transactions without a problem.
> 
> The process goes like this. We sign and encrypt the files via
> gpg and send them to an FTPS server. The client fetches the
> data from the FTPS server and decrypts them. I am using GPG
> v1.0.7 on Solaris 8.  The client is using WSFTPPRO on Windows;
> I'm not sure what version. I tried the both default encryption
> as well as the OpenPGP flag; both produce sporatic decryption
> issues.
> 
> Some of the errors the client has reported include: Unknown OpenPGP
> packet (xx) and Invalid Signature.
> 
> Of course we are having difficulty reproducing the problem and
> the client is having problems trying to decrypt the files via
> GPG.

The first thing to do is establish whether this is an encryption
problem or transport problem.  You say you encrypt the files, and
transfer them to a FTPS server, where the client fetches them via
FTPS.  Space allowing, try keeping the originals on the encrypt box
for a while.  If and when the client has a problem with a file,
compare a hash of the local copy with the one the client has.  That
is, both you and the client run "gpg --print-md sha1 (thefile)" and
verify it matches.

If it doesn't match, you have a transport problem.  If it does match,
you may have an encryption software problem.  This can chop the
possible problem space way down.  Right now, there are too many
variables.

David