[jeroenhd]

You can parse DER, but you have no idea what you've just parsed without the schema. In a software library, that's often not very useful, but at least you can verify that the message was loaded correctly, and if you're reverse engineering a proprietary protocol you can at least figure out the parts you need without having to understand the entire thing.

[woodruffw]

Yes, it's like JSON in that regard. But the key part is that the framing of DER doesn't require a schema; that isn't true for all encoding formats (notably protobuf, where types have overlapping encodings that need to be disambiguated through the schema).

[jeroenhd]

I'd argue that JSON is still easier as it allows you to reason about the structure and build up a (partial) schema at least. You have the keys of the objects you're trying to parse. Something like {"username":"abc","password":"def",userId:1,admin:false} would end up something like Utf8String(3){"abc"}+Utf8String(3){"def"}+Integer(1){1}+Integer(1){0} if encoded in DER style.

This has the fun side effect that DER essentially allows you to process data ("give me the 4th integer and the 2nd string of every third optional item within the fifth list") without knowing what you're interpreting.

[cryptonector]

It's really not an advantage that DER can be "parsed" without a schema. (As compared to: XDR, PER, OER, DCE RPC, etc., which really can't be.) It's only possible because of the use of tag-length value encoding, which is really wasteful and complicates life (by making it harder or impossible to do online encoding, since you have to compute the length before you can place the value because the length itself is variable length so you have to reserve the correct number of bytes for it and shoot-me-now).

[woodruffw]

I don't have a strong opinion about whether it's an advantage or not, that was more just about the claim that it can't be parsed without a schema.

(I don't think variable-length-lengths are that big of a deal in practice. That hasn't been a significant hurdle whenever I've needed to parse DER streams.)

[cryptonector]

Variable length lengths are not a big deal, but they prevent online encoding. The way you deal with that anyways is that you make your system use small messages and then stream those.