ASN.1 doesn't have a schema language. It has a schema spec, how you encode the schema is up to you. This is a huge boon.
ASN.1 has many encoding standards, but you don't need to implement them all, only the specific one for your needs.
ASN.1 has a standard and an easy to follow spec, which Protobuf doesn't.
In sum: I could cobble together a working ASN.1 implementation over a weekend.
In contrast, getting to a clean-room working Protobuf library is a month's work.
Caveat: I have not had to deal with PKI stuff. My experience with ASN.1 is from LDAP, one of the easiest protocols to implement ever, IMO.
my experience is from helping a coworker writing a ASN.1 serialization/deserialization library limited to a subset of encodings but (close to) the full spec each encoding ;)
> to follow spec, which Protobuf doesn't.
I can't say so personally, but from what I heard from the coworker I helped the spec isn't always easy to follow as there are many edge cases where you can "guess" what they probably mean but they aren't precise enough. Through they had a surprising amount of success to get clarification from authoritative sources (I think some author or maintainer of the standard but I'm not fully sure, it was a few years ago).
In general there seem to be a huge gap between writing something which works for some specific usage(s) of ASN.1 and something which works "in general/all the standard" (for the relevant encodings (mainly DER, but also at least part of non-DER BER as far as I remember)).
> Protobuf doesn't.
yes but it's wire format is relatively simple and documented (not as a specification but documented anyway) so getting something going which can (de-)serialize the wire format really isn't that hard and the mapping from that to actual data types is also simpler (through also won't work for arbitrary types due to dump design limitations). I would be surprised if you need a month of work to get something going there. Through if you want to reproduce all the tooling eco-system or special ways some libraries can interact with it (e.g. in place edits etc.) it's a different project altogether. What I mean is just a (de-)serializer for the wire format with appropriate mapping to data types (objects,structs, whatever the language you use prefers).
ASN.1 has many encoding standards, but you don't need to implement them all, only the specific one for your needs.
ASN.1 has a standard and an easy to follow spec, which Protobuf doesn't.
In sum: I could cobble together a working ASN.1 implementation over a weekend. In contrast, getting to a clean-room working Protobuf library is a month's work.
Caveat: I have not had to deal with PKI stuff. My experience with ASN.1 is from LDAP, one of the easiest protocols to implement ever, IMO.