Symmetric Encryption Requirement

Mark Jacobs mark.jacobs at
Fri Dec 19 11:08:22 CET 2003

On Friday 19 December 2003 11:00 am, David Shaw wrote:
> On Fri, Dec 19, 2003 at 10:53:22AM -0500, Mark Jacobs wrote:
> Content-Description: signed data
> > On Friday 19 December 2003 10:41 am, David Shaw wrote:
> >
> > <snip>
> >
> > > You can get the key with --show-session-key and write it to a file
> > > however you like, but it is unlikely you will be able to decrypt the
> > > data without doing some work, as OpenPGP programs use a special
> > > "OpenPGP cipher feedback mode" which may not be supported by your
> > > generic 3DES implementation.
> >
> > Is this special mode described anywhere other than the source code?
> RFC-2440:


> > Is there a way to request generic 3DES?
> You can modify the GnuPG code.  See cipher/cipher.c and cipher/des.c.
> The code is capable of doing regular CFB, but obviously that piece of
> the code doesn't get used too often since it isn't used in PGP.

Thanks again.

> > > Why not just run GnuPG on the mainframe?
> >
> > The mainframe process that we are looking for is by the nature of the
> > mainframe a batch process without human interaction, i.e. hard for the
> > mainframe to enter a passphrase when prompted by GNUPG.
> GnuPG can decrypt in batch processes - it can take the passphrase via
> stdin and never prompt the user.
> > Also GNUPG has not been ported to run in the mainframe UNIX or
> > native environment.
> The major difficulty in porting GnuPG to odd platforms is the need for
> a random number source.  Since you are only decrypting on the
> mainframe, no true randomness is needed for that.  This should make
> porting substantially easier.  What happens when you do ./configure on
> GnuPG on the mainframe?

I haven't tried yet. It is on my todo list this afternoon.

> David

Thanks a bunch for your assistance.
Mark Jacobs
Time Customer Service, Tampa FL
Time Warner


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