Verifing the first link in the apt-get verification chain

Question

I'm asking myself the following question lately: How do you really know, that you've installed the package you ordered under debian/ubuntu with the use of APT?

I think I understood the basics of the verification chain -- except the very first link.

So when I tell apt to install a package, it downloads the Release file from the repository-mirror specified in sources.list. That file is signed by the repository maintainers secret key. Apt verifies the signature with the local trusted keys (apt-key list). If everything is okay, apt uses the checksums in the "Release" file to verify further Packages files and the checksums in them to verify that the .deb files I'm installing have not been tampered with.

What I don't get at the moment is how do I verify that I've got a good set of trusted keys to verify the signature of the "release" file (ie. the first link in the chain).

In the common case I receive the keys via the debian-archive-keyring packages or its ubuntu equivalent. But since my keyring is possibly empty at the beginning before installing it, i can't verify this package effectively. There has to be another option.

1. For debian, I have found this nice webpage https://ftp-master.debian.org/keys.html which I can access via HTTPS that lists all valid repository-maintainer keys. So I can use apt-key to verify the fingerprints of my local keys with those of the website. Seems good to me. But I can't find something similar for ubuntu. And this seems not to be the 'popular' method for verifying the keyring.

   They even have this sentence on the page:

   Please note that the details here are for information only, you should not rely on them and use other ways to verify them.

2. Via the Web of Trust. I've read some things about it, but I don't now if I'm convinced. eg.: When I search for the current debian-stable key on pgp.mit.edu I see that the key is signed by four different keys. But I know none of them. To be fair, three of those four are heavily signed by a lot of people. But I don't know any of them either.

   https://pgp.mit.edu/pks/lookup?op=vindex&search=0xCBF8D6FD518E17E1

Wouldn't it be better if every 'repository creator' (like debian, ubuntu) had an easily and secure accessible https page where you can easily compare your local key to, instead of digging through the WOT in order to hopefully find a convincing link to the debian project?

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Bonus question: Is there a possibility to 'verify', that a package is the result of a build process from a specific sourcecode you can look at?

So far I think, that the repository maintainer who signs the release file 'promises' (don't know a better word) me this with his signature. Is that correct?

Answer 1

The Debian keyring is shipped with the distribution installer, so you get bootstrapped your repository trust by the installation medium. This also applies to (probably) all other Linux distributions relying on OpenPGP/GnuPG for verification in package managers.

If you don't trust your installation medium, you should verify it by any of the means provided (fetching it from a trusted source, comparing with others, checking the OpenPGP signatures issued, ...).

If you want to construct a trust path into the Debian keyring manually, find some Debian developers and verify their identity/sign their keys. Usually, they will gladly meet with you if asked nicely.

Wouldn't it be better if every 'repository creator' (like debian, ubuntu) had an easily and secure accessible https page where you can easily compare your local key to, instead of digging through the WOT in order to hopefully find a convincing link to the debian project?

There are people around that do not trust HTTPs and the hierarchical trust system for some (valid) reasons (but this is far beyond this answer, and already discussed lots of times) and prefer to rely on the decentralized OpenPGP web of trust, especially for bootstrapping trust.

In the end verification boils down to finding a trusted CD image source, anyway.

But I know none of them. To be fair, three of those four are heavily signed by a lot of people. But I don't know any of them either.

Basically, you'd have to start your trust network somewhere (by meeting with others already connected to the web of trust, verifying their ID and signing their key), and go on extending your personal view on the web of trust by recursively selecting people you want to trust until reaching the Debian signing keys. For sure, this is not a trivial task.

Deciding whether to trust an HTTPs page suffers the same issues: again and again, there are reports of hacked and broken certification authorities. Do you know all of those hundreds of CAs, and do you trust each of them? They're all trusted and can issue valid Debian certificates!

Bonus question: Is there a possibility to 'verify', that a package is the result of a build process from a specific sourcecode you can look at?

Debian promises you by signing the packages that it was built using the release file from respective Debian sources. The packages are build on build servers and signed by special keys, this is nothing the individual Debian developers do manually.