[tialaramex]

The chain "property" was never enforced anywhere of consequence and is gone in TLS 1.3

In practice other than the position of the end entity's certificate, the "chain" is just a set of documents which might aid your client in verifying that this end entity certificate is OK. If you receive, in addition to the end entity certificate, certs A, B, C and D it's completely fine if certificate D has expired, certificate B is malformed and certificate A doesn't relate to this end-entity certificate at all as far as you're concerned if you're able (perhaps with the aid of C) to conclude that yes, this is the right end entity and it's a trustworthy certificate.

Insisting on a chain imagines that the Web PKI's trust graph is a DAG and it is not. So since the trust graph we're excerpting has cycles and is generally a complete mess we need to accept that we can't necessarily turn a section of that graph (if it even was one graph which it isn't, each client possibly has a slightly different trust set) into a chain.

[stefan_]

You are overthinking it. Some sysadmin copying the same cert into the chain twice because AWS is confusing and doesn’t care and OpenSSL doesn’t care isn’t resolving the grand problem of the trust graph, it’s just a loss overall, for everyone. Nobody wins here.

(Of course the 1.3 approach of throwing a bunch of certificates and then asking to resolve over all of them breaks BearSSL comprehensively)

[toast0]

Yes, it's useless to include the CA cert, and to include extra copies, and all those other things.

But requiring the cert chain to be exactly correct is also useless if you need to address clients with different root cert packages. If some clients have only root A and some have only root B, but B did a cross-sign for A, you're ok if you send entity signed by intermediate, intermediate signed by A, A signed by B and clients with A only short circuit after they see an intermediate signed by A, and the clients with B only should be fine too. Of course it gets real weird when the B root has expired, and clients often have A and B, but some don't check if their roots expired, and some won't short circuit to validating with A, so they fail the cert because B is expired.

Oh, and TLS handshakes in the wild don't give you explicit information about what roots they have or what client / version they are. Sometimes you can get a little bit of information and return different cert chains to different clients, but there's also not a lot of support for that in most server stacks.

I don't necessarily like TLS 1.3's approach of end entity cert comes first and then just try all the permutations and accept any one that works, but at least it presents a way to get to success given the reality we live in. I'd also love to see some way to get your end entity cert signed by multiple intermediates, but that's a whole nother level of terrible.

#notbitter