From: Brad Huntting <huntting@glarp.com>
Date: Sun, 29 Aug 93 09:26:15 PDT
To: Derek Atkins <warlord@athena.mit.edu>
Subject: Re: PGP-MIME
In-Reply-To: <9308290317.AA07547@podge.MIT.EDU>
Message-ID: <199308291623.AA03115@misc.glarp.com>
MIME-Version: 1.0
Content-Type: text/plain



>> For PGP to really make use of MIME, it could use "multipart" types
>> to separate the objects being encrypted and/or signed from the
>> signatures and encrypted session keys associated with them.

> No, this is WRONG.  Take a look at the PEM-MIME Internet Draft.  You
> *do not* want to separate the signature from the body of text being
> signed, since then you lose the delimiters of the signed message, and
> MIME can do anything with the data (like transfer tabs to spaces,
> etc.)  This is BAD.

If the signed object where of type text/*, then yes it might be
altered with impunity.  But MIME would not alter a body part type
such as application/pgp-object or message/external-body would it?

There's a definite advantage in using multipart/* to separate
signatures from the plain-text they apply to because then you can
easily encode the signatures with base64 while encoding the text
in "7bit" or "8bit".

> If you keep the message and signature together, it will work
better.  > MIME still does funky things, however, some times.

Then we need to get the MIME do's and dont's nailed down in the
next RFC (if they arn't already) specifying exactly which type can
be altered, and to what extent.

> Currently, you can easily use MIME as a transport mechanism for
PGP > messages.  However currently there is no way to use PGP
security for a > MIME message.  Hopefully we can take what the
PEM-MIME effort has > learned and apply that to PGP..

But I can put a MIME message headers in a PGP message.  And (though
I haven't tried it), I should be able to specify that application/pgp
messages will be piped through a command like "pgp -m|metamail" or
"pgp -m|mhn -file /dev/stdin".  Unfortunately, I've had problems
redirecting pgp's stdin and stdout.  I'm not sure that it opens
/dev/tty every time it wants to talk to the user.

Also, since you bring up PEM-MIME, what could be done to bring PGP
and PEM closer together?  I'm not suggesting changing the web of
trust model or using DES instead of IDEA.  Just make the formats
similar enough so that an application implementing one could be
painlessly modified to implement the other.

A common format wouldn't seem to be all that difficult to implement.
Since some folks want to use triple DES and others IDEA, add this
info to the RSA encrypted portion of the message.  This has the
advantage of hiding not only the key used to encrypt the body of
the message, but the algorithm as well.

As for the trust models, there not so different in theory, just in
practice.  PEM is planing on using X.509 type certificates, and
they are having problems figuring out how to map X.500 Distinguished
Names (DN's) onto e-mail addresses (I think they're making the
problem allot harder than it is).

Perhaps the model for future PEM/PGP systems is to keep both private
key rings, and public key rings with the latter being found in the
(Internet) X.500 directory.  This would nessesitate being able to
sign a private key with a public key, and vice versa.


brad