gnupg 1.0.4 <-> 1.0.0 interoperability problem

[Werner Koch]

On Wed, 13 Dec 2000, Peter Bloecher (EED) wrote:

> After upgrading to gpg 1.0.4 I discovered a problem when exchanging messages
> with people using gpg 1.0.0. What happens is that during decryption they are asked

Is it the case that the keys of the people using 1.0.0 have neen
generated with 1.0.4?  Than it is pretty ovious what happens:

> Calling gpg 1.0.4 with --list-packets gives

> :encrypted data packet:
>         length: 4294967295
>         mdc_method: 2

Since 1.0.3, keys generated with gpg are created with preferences to
TWOFISH (and AES since 1.0.4) and that also means that they have the
capability to use the new MDC encryption method.  This will go into
OpenPGP soon and is also suppoted by PGP 7.  This new method avoids
a (not so new) attack on all email encryption systems.

The NEWS for 1.0.3 tell you that there is an incompatibility.

> The length field of the encrypted packet looks suspicious, but that does not
> have to be connected to the problem.

Indeed.  It only effects the lising and I will fix it in the next
release.

> Just upgrading everybody to 1.0.4 does not seem reasonable. I also fear that
> there might be interoperability problems with, e.g., PGP.

Not if you have the latest PGP (7) - I checked with the PGP
developers that MDC works for both of us.

As workaround I can suggest to add 

disable-cipher-algo RIJNDAEL
disable-cipher-algo TWOFISH

to the options file of gpg 1.0.4.  Or foce the use of one cipher
without caring about any preferences by using:

cipher-algo cast5


IIRC, there is something about it in the FAQ.  For security reasons,
I'd suggest to upgrade to 1.0.4 anyway.


Hth,

  Werner

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