How does validating the PGP signature of a downloaded executable against the publisher's public key show that the binary has not been tampered with?

Question

Websites that host downloadable executables often provide measures to confirm the integrity of the data that is available to download. Such measures include:

• Hosting the website under HTTPS;
• Providing the SHA-256 sum of the downloaded binary (the end user is advised to compute the checksum on their own and see if both sums match);
• Providing the publisher's PGP public key and the PGP signature of the binary (the end user is advised to check the signature against the provided public key).

To my understanding these are the three major threats that may compromise the integrity of downloaded data:

• Due to a MitM attack the binary is downloaded from a different source than intended - hosting the website under HTTPS is supposed to prevent this;
• Due to network errors the downloaded data is accidentally modified (SHA or even MD5 sums are supposed to not match if this occurs);
• The website hosting the downloads is compromised and a third party has injected their own malicious binaries to the website: PGP signatures are supposed to fail to verify if this is the case.

This is the part I fail to understand. Let me provide an example.

Example: VeraCrypt

VeraCrypt installers & binaries, as well as their respective PGP signatures are available to be downloaded from https://www.veracrypt.fr/en/Downloads.html . This same webpage provides the PGP public key:

PGP Public Key: https://www.idrix.fr/VeraCrypt/VeraCrypt_PGP_public_key.asc (ID=0x680D16DE, Fingerprint=5069A233D55A0EEB174A5FC3821ACD02680D16DE)

Now assume that the idrix.fr website is compromised and serving malicious binaries. In that case what prevents the malicious 3rd party from computing their own public/private key pair, replace the public key provided by the website with their own (they own the website now) and replace the signatures of the binaries with ones signed by their own key?

Since the same website serves the public key, signatures and actual downloads I cannot see how is this supposed to increase security? If the website is not compromised then the measure is unnecessary; if it is compromised then the attacker can modify all three and the measure fails to provide security.

If the public key was hosted somewhere else (like Ubuntu keyserver) then it would make more sense to me: the Ubuntu keyserver would now verify the legitimacy of the public key (with the assumption that it is unlikely that both idrix.fr and keyserver.ubuntu.com domains are compromised simultaneously). However, it doesn't seem to me to be the case.

What am I missing here? What am I failing to understand?

Answer 1

The main thing that prevents it is that it would be an extremely visible action. Lots of people already have the VeraCrypt PGP public key in their keychain, and if the site suddenly had a new key that wasn't even signed by the old one's private key, there would be a massive and immediate uproar. PGP public keys are expected to be signed, usually by multiple parties, especially if they come from a well-known and security-conscious source.

With that said, there's nothing theoretically wrong with your attack. If you had some obscure product where nobody or almost nobody had the key already, or even any keys related to the project owner, somebody would have to take it on faith that the key on your website is legitimately yours. You could also try carrying out the attack narrowly (only against specific IPs, or only very briefly such that by the time anybody tried to verify a claim of the site serving the wrong keys/binaries, they'd be back). For that matter, most people probably don't check signatures even if they have the tools to do so, so maybe you could get away with it for a while. But people would definitely eventually notice, when the binaries don't verify using the known key and the new key is different and not signed by the old one or the old one's own signers.