[briansmith]

> he situation described above just generated an "Invalid certificate" message. More use of anyhow::Context would be helpful. I don't disagree with Rustls disallowing decade-obsolete crypto. It's the "silently ignores" part that's a problem.

Because of how X.509 certificate validation works, in general it's not possible to tell you why an issuer couldn't be found, because there are many possible reasons.

Regardless https://github.com/briansmith/webpki/issues/206 tracks improving the situation.

[tialaramex]

Brian surely knows this, but for other people's context: Certificates in the Web PKI [and likely in any non-trivial PKI] form a Directed Graph with cycles in it. So "is this end entity certificate trustworthy?" is already a pretty tricky graph problem that you're going to have to solve unless you either out-source the problem entirely to the application (lots of older or toy TLS implementations) or just rely on your peer showing you exactly the right set of certificates matching your requirements (the "chain" you may have seen in tools like Certbot) without you ever saying what those actually are.

In a sense it's going to have a big undigestible list of reasons the certificate wasn't found trustworthy, like if you asked grep to tell you why all the non-matching lines in a file don't match a regular expression. "The first letter on this line wasn't a match, and then the second letter wasn't a match, and then the third..."

However, as that ticket says, one relatively easy thing the code could do is notice if there was only one consistent reason and if so tell you what that was.

Also I agree with several commenters that webpki's current behaviour, in which it says "Unknown issuer" even when that's not the problem at all is undesirable and an even vaguer error might actually be better for these cases. See also, languages in which the parser too easily gets confused and reports "Syntax error" when your syntax was correct but something else is wrong, "Parse failed" is vaguer but at least doesn't gaslight me.

[briansmith]

Are you assuming that the set of certificates that is given as input is the complete search space? It isn't; the server might have failed to send some certificate that, if present, would have fixed the "unknown issuer" problem. Also, we fail fast on errors; as soon as we find a problem, we stop the search on that path. Because of these reasons, identifying a single error would be, potentially, misleading.

In some cases, e.g. the end-entity certificate is signed with an algorithm that isn't supported, or an RSA key that is too small, we could add special logic to diagnose that problem. However, all this special diagnostic logic would probably approach the size of the rest of the path building logic itself. It doesn't seem appropriate for something in the core of the TLB of the system. Perhaps at some point in the not-too-distant future we can provide some mode with more diagnostic logic, and find a way to clearly separate this diagnostic logic from the actual validation logic to ensure that the diagnostic logic doesn't influence the result.