[gnupg] trustdb problems, con't

[Thomas Roessler]

I have already been talking about the fact that "validity" is a
concept which is defined on (pubkey, user-id) pairs.  Gnupg
calculates "validity" as a function of the pubkey alone.  This is
worse than just a user interface problem.

In collect_paths(), this is done by looping over all user ID records
on a key and counting the number of fully or marginally trusted
certificates. Note that this counting is done on a per-key basis,
not per user ID. [1]

To see why this is wrong, assume we have four marginally trusted
introducers named ca_1, ..., ca_4 and a user.  Assume that this user
has e-mail addresses a_1 to a_4 and a public key p, and that ca_k
has certified the associatioin (p, a_k) for k = 1, ..., 4.  Let
marginals_needed be set to the default value of 3.

With PGP, we get four IDs with marginal validity, and the key won't
be used as an "introducer" - which is safe [2].  With gnupg, we get
a key with four marginally trusted certificates which lead to full
validity of the key.  It will be used as an introducer, just like a
key which has a fully certified user ID.  This is obviously wrong.

tlr


[1] There is another issue here: I'm not sure whether fully trusted
    signatures should be counted as marginals, too. Counting the
    various signature types separately and checking if

    full_count/full_needed + marginal_count/marginals_needed >= 1

    may be better.  Equivalent: full_count * marginals_needed +
    marginal_count * full_needed >= marginals_needed * full_needed.

[2] Though debatable.  One may wish to use the validity of a key
    (which will be something like the maximum user ID validity) as
    some kind of weight for the owner's trust.  For a clean approach
    to the web of trust, including recommendations, see Maurer's
    paper "Modelling a public key infrastructure".  (Thanks to gec
    for telling me about it.) Note that OpenPGP [RFC2440] actually
    defines recommendation packets.  They are called "trust
    signatures" there.

-- 
Thomas Roessler · 74a353cc0b19 · dg1ktr · http://home.pages.de/~roessler/
     2048/CE6AC6C1 · 4E 04 F0 BC 72 FF 14 23 44 85 D1 A1 3B B0 73 C1
> Hi!  I'm Signature Virus 99!  Copy me into your signature and join the fun!