Most people--even programmers--”are confused about the basic concepts underlying digital signatures. Therefore, most people should read this section, even if it looks trivial at first sight.
Digital signatures can prove both **authenticity** and **integrity** to a reasonable degree of certainty. **Authenticity** ensures that a given file was indeed created by the person who signed it (i.e., that it was not forged by a third party). **Integrity** ensures that the contents of the file have not been tampered with (i.e., that a third party has not undetectably altered its contents *en route*).
Digital signatures **cannot** prove any other property, e.g., that the signed file is not malicious. In fact, there is nothing that could stop someone from signing a malicious program (and it happens from time to time in reality).
The point is, of course, that people must choose who they will trust (e.g., Linus Torvalds, Microsoft, the Qubes Project, etc.) and assume that if a given file was signed by a trusted party, then it should not be malicious or buggy in some horrible way. But the decision of whether to trust any given party is beyond the scope of digital signatures. It's more of a sociological and political decision.
Once we make the decision to trust certain parties, digital signatures are useful, because they make it possible for us to limit our trust only to those few parties we choose and not to worry about all the "Bad Things That Can Happen In The Middle" between us and the them, e.g., server compromises (qubes-os.org will surely be compromised one day), dishonest IT stuff at the hosting company, dishonest stuff at the ISPs, Wi-Fi attacks, etc.
By verifying all the files we download which purport to be authored by a party we've chosen to trust, we eliminate concerns about the bad things discussed above, since we can easily detect whether any files have been tampered with (and subsequently choose to refrain from executing, installing, or opening them).
However, for digital signatures to make any sense, we must ensure that the public keys we use for signature verification are indeed the original ones. Anybody can generate a GPG key pair that purports to belong to "The Qubes Project," but of course only the key pair that we (i.e., the Qubes developers) generated is the legitimate one. The next section explains how to verify the validity of the Qubes signing keys.
Every file published by the Qubes Project (rpm, tgz, git repositories) is digitally signed by one of the developer or release keys. Each such key is signed by the Qubes Master Signing Key (`0x36879494`).
There should also be a copy of this key at the project's main website, as well as in the archives of the project's [developer](https://groups.google.com/forum/#!msg/qubes-devel/RqR9WPxICwg/kaQwknZPDHkJ) and [user](https://groups.google.com/d/msg/qubes-users/CLnB5uFu_YQ/ZjObBpz0S9UJ) mailing lists.
Once you have downloaded and verified the fingerprint of the Master Signing Key, you should import this key and set its trust level to "ultimate" (oh, well), so that it can be used to automatically verify all the developers' keys:
The developer keys are set to be valid for 1 year only, while the Qubes Master Signing Key has no expiration date. This latter key was generated and is kept only within a dedicated, air-gapped "vault" machine, and the private portion will (hopefully) never leave this isolated machine.
Developers who fetch code from our Git server should always verify tags on the latest commit. Any commits that are not followed by a signed tag should not be trusted!
