DER Support

[masihyeganeh]

Hi.
My project heavily depends on parsing DER encoded data.
Your crate is by far the most interesting ASN1 parsers.
Is it hard to implement DER for it too?
I can contribute to that but I'm not sure where to start.

[kellerkindt]

Hi there,
I dont know how hard it is to implement DER-serialization. Maybe you can gather useful information from this pdf or directly from ITU / ITU-DER.

I'd suggest the following steps in implementing DER support:

• create a DER module with Der-, Read- and Write-traits that provide all primitive (de-)serialization types (like for UPER in src/io/uper.rs)
• add a new GeneratorSupplement impl for DerSerializer, that generates the Rust impl code for the DER-(de-)serialization-trait (see asn1rs-model/src/gen/rust/uper.rs)

Feel free to contribute via PRs whenever you like to :)

[kellerkindt]

@masihyeganeh I suggest to await the current refactoring (see #11) which should make it easy-ish to extend this crate with another encoding (DER in your case) and without the need to codegen for it.

[kellerkindt]

Hi there, refactoring is almost complete. I'll get rid of (the) old (uper) code soon/next.

If you want to, you can look into implementing a DerReader and DerWriter, analogue to println and uPER. The showcase demonstrates basic inline-usage. The idea is to only have to swap the UperWriter/-Reader with a DerWriter/-Reader in the uper_proof function to create a der_proof function without further code generation.

I guess some ASN syntax and types you'll need are not parsed or implemented yet. If you show some samples I'll might focus on them next. Otherwise I'll probably keep on trying to get LDAP working next.

I'll gladly accept a PR for a working implementation 😃

[masihyeganeh]

Fantastic.
Would you please release a version on crates.io so I can play with it?
I couldn't make it work with master branch.

[kellerkindt]

@masihyeganeh I just did, but I would also like to know why it did not work for you. Maybe its and issue with my repository layout? If so, I might be able to fix it, if you are give me a more concrete error description.

[masihyeganeh]

Thanks.
It was because of my other dependencies.
My old version of reqwest was conflicting with yours.
Can you please implement SET and SET OF before I start to figure out how to implement DER? It is really similar to SEQUENCE I think.
I'm not that familiar with detail of ASN, I should try to implement it somehow based on rfc

[masihyeganeh]

I just started to implement DER, but I think I'm lost.
I copied src/syn/io/uper.rs to src/syn/io/der.rs and started changing it, but I found out that in other parts of code, like this one, there is direct dependency on Uper*:

asn1rs/src/io/buffer.rs

Line 27 in e9ac059

impl UperReader for LegacyBitBuffer<'_> {

It is not clear to me how to implement it, what files I need to change and ...
Considering that DER is the simplest implementation of this family, I hope we can do it soon.

And by the way, I tested your experimental-set-setof branch with my asn file and it worked well (almost, there was a minor catch). I hope you merge it soon.

[kellerkindt]

I just added the der-support branch where I copied the UPER related modules for DER. Let me explain real quick:

• io::buf::OctetBuffer is the analogy to io::per::unaligned::buffer::BitBuffer: store and access octets (so in this case it is just a fancy wrapper around a Vec<u8>)
• io::der::DistinguishedRead and -Write traits shall define read/write functions for the "primitive" types of DER according to the mentions in X.690 (I don't know whether they are the same as for (u)PER, which is why I didn't copy over the function definitions from PackedRead and -Write).
• io::der::octet_aligned-module implements theses DistinguishedRead/-Write traits for the OctetBuffer (OctetBuffer is probably? re-used with additional impls for other encoding rules like BER? So not putting the impls directly in the io::buff module is just an attempt to keep things separate and organized)
• syn::io::der::DerReader and -Writer adapt the calls from the generated data structs to the OctetBuffer/to the DistinguishedRead/-Write implementation of the OctetBuffer. Maybe you need to add a similar Scope enum like the one used for the UperRead/-Write implementation. But again, I haven't looked into DER.

Depending on how similar the DER/(u)PER traits are, I will/we can try to unify them later. But I suggest to 'just get it working' for now - even if this means duplicat-ish code/declarations atm. Refactoring working code with proper tests is much easier than trying to get the structure, abstraction and impl right at once.

I hope this helps you to get you going 😄

EDIT: regarding the experimental-set-setof branch, #13 describes the current blocker for merging.
EDIT2: Poke me, if you need access to the tags which aren't yet represented in the type constraints and thus not available in the read/write adapter calls. But I can prioritize this if you need it.

[masihyeganeh]

Thanks man.
This should make working on it a lot easier.
I looked at the implementation of your uper for hour and I think I know how it works now but not actually sure about it.
I need to confirm it with you.

Look, der (and ber and cer) is way simpler than uper;
There is no difference between types.
Always there is a 2 bit class, 1 bit to tell if it is primitive or constructed and 5 bits tag in first octet.
Second octed is like : 1 bit to tell it's length type and usually next 7 bits tell it's actual length or how many octets should we read as number to know the length.
Next n octets is the value

That's all. Nothing more. No especial case or anything. I'm sure you can implement it in minutes, but I had to read your code for hours to know what is going on.
For example I'm not sure it it is ok to directly call read_bit or not. I'm not sure how ro read 2 bits as a number (or enum). I saw some ranges in there and I think I need to set start and end of buffer, but it's confusing for me.

If you can give some of your time, here is a simple explaiantion of ber implementation (der is just ber, but simpler, always there is length):
https://en.m.wikipedia.org/wiki/X.690#BER_encoding

Or if you don't have time, please guide me through it.
Thanks

[mouse07410]

@kellerkindt @masihyeganeh Any update on this? It would be great to have Rust code for DER encoding/decoding generated directly from the ASN.1 files.

[kellerkindt]

So, I've set up a basic workflow for the (Distinguished) Encoding Rules support, see the very very early boolean example

• example / test
• (De-)Serialization impl
• Reader/Writer mappings for proc-macro of rust types

I noticed the https://asn1.io/asn1playground is now rate limited pretty early, which makes it hard for me to verify my implementations. Does anybody know an alternative for DER?

EDIT: Started to use asn1js as reference

[mouse07410]

Please consider https://GitHub.com/mouse07410/asn1c, as a cross-testing platform.