Re: Stealth PGP work

[aba@atlas.ex.ac.uk]

Derek Atkins <warlord@mit.edu> writes on cpunks:
> > It seems that there a market demand for a stealth-capable product.
> > Many peoples here seems to discuss it.  And for the time being, AFAIK,
> > this type of products are used by a specific class of peoples, most of
> > which knows what 'stealth' means.
> > 
> > So why is it that they design a program that would not permit the use
> > of a feature considered desirable by it's customer base?
> 
> The big question I have for you is, what do you mean by "stealth" PGP?

I presume he means stealth, and the functionality that it provides, as
a filter for pgp versions 2.x.  The question in the minds of users of
this utility (for steganography applications) I think will be will
stealth still work with pgp3.0, and would it useful to include as
built-in functionality for pgp3.0.  I raised this question myself to
the pgp3 development team some time ago, and the reply I got was
essentially that it would still be possible to have as an add-on, so
there was no need to clutter the pgp3 functionality.

Henry Hastur's stealth 1.3 is included with pgp2.6.3i in the
contributions directory...

> Do you want a PGP message which doesn't say to whom it is encrypted?
> Or do you want a PGP message which does not even acknowledge that it
> is a PGP message?  If what you want is the former, then that can fit
> under the PGP API fairly well.  If you want the latter, it will not.

For steganography applications both are required (actually a
transformation formulated by Hal Finney, or an equivalent by Bodo
Moeller is necessary to add more than casual strength to the degree of
plausible deniability due to the fact that the rsa encrypted header
will always be less than the rsa modulus).

I haven't looked at the PGP3 API, but why will a stealth option not
fit?  If stealth can perform the operation as a separate program, why
is it infeasible or difficult for pgp3 to support a stealth option,
and another to support unstealth to a particular recipient.  (With
stealth on decrypt the user provides the user id they think the
message will be addressed to).

On encrypt, the stealth operation should be real easy to implement
inside PGP, it is just another post-filter operation like ascii armor,
stealth instead.

On decrypt, it is more tricky because firstly you loose information
about what kind of data it was, and secondly because you loose the
recipient user id if it was encrypted data.

However, if you provide an API call to unarmor with out decrypting,
and a call to decrypt with out uncompressing, etc then a call to test
for a particular user id on the assumption that it is addressed to
that user id and is an encrypted message would fit in a similar way?

Also note that the rest of the information, once this operation has
suceeded is retained inside the encryption envelope - the length tag,
signature, whether it is further compressed, and so forth.

The stealth operation as implemented in Henry's 1.3 is not all that
secure, and I had an attempt at implementing Hal Finney's algorithm to
produce a stealth 2.0.  My attempts are at:

	http://www.dcs.ex.ac.uk/~aba/stealth/

This is beta code, and I presume the reason that Stale included
stealth 1.3 rather than stealth2.01b with pgp263i was either that 2.01
is beta, or because he did not know it existed.

The reason that the code is beta, is that I had a problem in
implementing stealth as a standalone.  The problem is that Hal's
algorithm has a requirement for cryptographic quality random numbers.
It seems silly to duplicate all this code inside stealth when pgp
provides it internally.  And the (+makerandom=n file) option is not
convenient, because stealth is invoked as a filter with pipes, and
invoking pgp twice doesn't seem like a good idea.  It might even be
dangerous (interferring with the normal sequence of randseed
stirring)?  Besides I had gone to some pains in stealth2.0 to ensure
that nothing hit the disk at all if sufficient memory was available to
do without.

> The reason is that PGP, by definition, is a self-describing packet
> format.  Without that description there is no general way for the PGP
> library to discover what kind of message it is parsing order to
> perform the proper operation to open the message.

stealth allows you to specify encrypt or decrypt, if decrypt the user
id to insert.  It also supports conventional encryption.

> OTOH, if just the keyID is missing, the library will happily try all
> the keys on your secret keyring until one succeeds or they all fail
> (I'm not sure if this is implemented, but it fits quite nicely under
> the API).

That would be a useful functionality from stealths point of view.

> The other question I have is: who do you think the "customers" of PGP
> are?  

He expressed his request in terms of customer demand...

> If you think the majority of PGP's customers are the crypto-privacy
> activitst types, you are highly mistaken.  PGP has hit the main
> stream, and is being used by many non-crypto-aware people.  Probably
> more of them than there are of us.

PGP was started on idealogical grounds, my argument for including
stealth, or as much stealth compatible options as fit within the API
model, is that there are countries where crypto is illegal.  It is not
100% certain that the US won't one day join them (what do you rate the
odds of a mandatory GAK appearing in the US?).  I'd view it as part of
PGP's guerrilla privacy features, and a precaution against such worst
case scenarios.  Besides stego applications have their own set of
uses, albeit they are not in mainstream use in the same way that pgp
is.

> If you want to discuss this more, let's take it to private email,
> please.

I'd prefer to see the discussion on the list, or at least cc me in
such discussion.  Any reason for private email?  The topic is
cryptography related,

Adam