Dual-signed things should be easy to verify with one key

[Dimitri John Ledkov]

As part of Ubuntu key rotation strategy, we rely on dual-signing (inline, or detached) such that validation with at least one key available in a keyring should be trusted, without using web-of-trust.

However, it seems to be only correctly so far implemented by the apt's gpgv method.

Ideally, we should ship an easy enough to use the helper that is `like gpgv` to use, and possibly reusing apt's gpgv code and/or exposing it via apt-key's verify.

The problem seems to be that 1 good sig + 1 no public key available, results in gpgv exiting with 2, instead of 0 or 1.

Ideally it should be easy enough to use gpgv/gpg to verify that at least one signature is good, and decrypt/extract signed contents only.

More details and reproducers to follow.

[TJ (tj)]

I've created a shell wrapper than might be useful for this called "gpgpv-multisig" which is a multi-call executable. Given /usr/bin/gpgv-multisig

ln -s gpgv-multisig /usr/bin/gpgv-aptkeys

and called as 'gpgv-aptkeys' it will assume the keyring to be used is /etc/apt/trusted.gpg (set by APT_KEYRING).

Returns the same exit codes as detailed in man gpgv(1):

0 = all signatures good
1 = at least one signature good
2 = no signatures good

Many configuration variables can be over-ridden from the environment but adopt sensible defaults.

[Julian Andres Klode (juliank)]

That wrapper is conceptually wrong. It should be using --status-fd.

I'm in favour of not duplicating that stuff and see if we can reuse the existing apt code, by moving it into the library and providing a tool in apt-helper.

[TJ (tj)]

Julian - I hadn't realised there are two gpgv's!

Having found apt's own /usr/lib/methods/gpgv

I have been able to create a shell wrapper that can feed it the expected request headers and parse the response headers to ensure a GPGVOutput: GOODSIG ...

It's a proof of concept right now; if this approach is preferred then I'll tidy it up and publish.