A tool for the investigation of data encoded in ASN.1 (DER). It takes an example of encoded data and produces an interactive HTML document showing the structure of the data along with values contained within.
Q: what is ASN.1?
A: abstract syntax notation one, a flexible way of defining an encoding structure for data that, at least in theory, is very amenable to machine processing.
Q: what is DER?
A: ASN.1 allows data with the same structure to be encoded with different rule sets. DER are the distinguised encoding rules, which are really just a subset of BER (basic encoding rules). DER defines policies which mean that there is only ever a single way of encoding a given construct, whereas BER might allow that construct to be encoded in different ways. DER is thus appropriate for use where data might be encoded multiple times, and the resultant bytes need to be equal in each instance. DER encoding is quite often used in cryptographic protocols, due to its reproduceability. It is thus the target of this tool.
Q: should I use ASN.1 for a new application?
A: no; use Protocol Buffers instead. ASN.1 was very powerful and flexible for its time, but three decades of experience have produced something much more simple and interoperable while retaining all of the same power.
A lot of existing standards use ASN.1 in practice, and so it is important to be able to understand and process such data if required. This tool is an aid to investigating instances of such data.