Expanding web-of-trust with subkey

Daniel Kahn Gillmor dkg at fifthhorseman.net
Wed Feb 15 19:46:13 CET 2017

On Wed 2017-02-15 11:54:51 -0500, Teemu Likonen wrote:
> That makes things very simple, in a way. I use "trust-model direct" and
> do some checking in web pages or check consistent use of signatures. If
> the key seems ok I'll "--edit-key", type "trust" and assign marginal or
> full trust for that key. That's it. And because I have no use for other
> people's signatures I also have "keyserver-options import-clean" so my
> keyring remains small.

right, so your use of "trust-model direct" switches the meaning of the
"trust" flag from its usual "ownertrust" semantics to be what we'd
normally call "validity".

Note also that when you mark a key itself as "trusted" in this way,
you're asking GnuPG to treat *all* user IDs on it as valid.

So if the keyholder updates their key at some point in the future to add
a new User ID, your GnuPG installation is going to blindly accept that
User ID as legitimate.

Please see A405E58AB3725B396ED1B85C1318EFAC5FBBDBCE as an example of
this kind of thing.  The keyholder cheekily added a new User ID "Satoshi
Nakamoto (www.bitcoin.org) <satoshin at gmx.com>" after his OpenPGP
certificate was created.  I have met the keyholder, and i do not believe
he is actually Satoshi Nakamoto ;)

> When Debian 9 is released, with GnuPG 2.1, I'll try "trust-model
> tofu+pgp" (trust on first use plus web of trust). It seems useful too.

please be aware that if you switch from "trust-model direct" to
"trust-model tofu+pgp", then your previous assignments of "trust" will
transform into indications of "ownertrust".  So someone whose OpenPGP
certificate you previously meant to indicate was valid can now certify
*other* OpenPGP certificates, and the pgp trust model will accept those
certificates as correct :(

Transitioning between trust models without overhauling the ownertrust db
is not a workflow that seems particularly well-supported, unfortunately.

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