Use application/pem-certificate-chain for PEMs

[cipherboy]

As mentioned in #10948, it appears we're incorrectly using the
application/pkix-cert media type for PEM blobs, when
application/x-pem-file is more appropriate. Per RFC 5280 Section
4.2.1.13, application/pkix-crl is only appropriate when the CRL is in
DER form. Likewise, Section 4.2.2.1 states that application/pkix-cert
is only applicable when a single DER certificate is used.

Per recommendation in RFC 8555 ("ACME"), Section 7.4.2 and 9.1, we use
the newer application/pem-certificate-chain media type for
certificates. However, this is not applicable for CRLs, so we use fall
back to application/x-pem-file for these. Notably, no official IETF
source is present for the latter. On the OpenSSL PKI tutorial
(https://pki-tutorial.readthedocs.io/en/latest/mime.html), this type is
cited as coming from S/MIME's predecessor, PEM, but neither of the main
PEM RFCs (RFC 934, 1421, 1422, 1423, or 1424) mention this type.

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

---

While ietf-wg-acme/acme#120 (comment) seems to suggest application/x-pem-file is appropriate for PEM blobs in general (regardless of type), they ended up removing this in the final draft and introducing a new type.

[stevendpclark]

Looks good to me, but another pair of eyes would be a good thing.

[cipherboy]

@abriening what's your thoughts on using application/pem-certificate-chain instead of application/x-pem-file?

For what its worth, Dogtag produces application/x-pem-file: https://github.com/dogtagpki/pki/blob/master/base/common/src/main/java/com/netscape/certsrv/authority/AuthorityResource.java#L42-L45 but respects the ACME standard application/pem-certificate-chain: https://github.com/dogtagpki/pki/blob/master/base/acme/src/main/java/org/dogtagpki/acme/server/ACMECertificateService.java#L35-L39

[kitography]

This makes sense to me :)

(I'm following the slack chat, but I think it's very unlikely that someone is relying on an incorrect contentType)

[abriening]

I've used the application/pkix-* content-types in PKI testing. I only added the raw/pem endpoint to have parity with the existing raw(pem) endpoint.

Looks like ACME draft was application/x-pem-file then created application/pem-certificate-chain specifically:
ietf-wg-acme/acme@709a654

Seems like application/pem-certificate-chain is appropriate, just hope no one sends Accept: application/pem-certificate-chain expecting the full chain.

[cipherboy]

@abriening Thanks! For what it is worth, it appears that Vault doesn't respect (or check) the Accept header either way, so in that regard we'll at least be consistent with other APIs.

That said, we have approvals and consensus internally that we're fine with this change (but not backporting it), so I think it is OK to merge.

Thanks all for the reviews!