Support for reading and writing zero-length OctetStrings

[justinstenning]

We will be working on a migration from OpenDNP3 to stepfunc/dnp3 C# bindings soon. One of the customisations we had to make within OpenDNP3 was support for reading and writing OctetStrings, including zero-length ones.

We understand that zero-length OctetStrings is probably a non-standard implementation, so perhaps this could live behind some kind of enableNonStandardFeatures flag?

As discussed previously, our team are happy to help with implementation and can assist with testing C# bindings if required.

[jadamcrain]

I think that writing them could always be allowed in the master. There's a security issue with reading them. The encoding for a zero-length string is nothing but a header, e.g. you can say "this packets contains group 110 variation 0 start: 0 stop: 65535" and it only takes ~5 bytes to encode 65K values.... repeat this to fill an ASDU and it's like a ZIP bomb if the master makes a callback for each value:

https://www.cisa.gov/news-events/ics-advisories/icsa-14-149-01

It was kinda vague in the spec before I pointed out how problematic this is, then the committee "clarified" that 0-length strings weren't allowed b/c variation had always been reserved for requests to mean "send whatever your default variation is".

All of that said, I just think we need a configuration option to enable parsing 0-length strings on the master, and we can always allow writing them.