FsAsn1

FsAsn1 is an F# library for reading DER encoded ASN.1 data. ASN.1 is a binary format that is widespread in cryptography and public key infrastructure. For example X.509 certificates, which are used also in HTTPS, are defined by an ASN.1 schema.

ASN.1 encoded data cannot always be fully interpreted without corresponding schema. For this reason, FsAsn1 supports decoding ASN.1 data according to a schema. Since a lot of ASN.1 schemas are described in RFC files, in addition to a plain schema file, you can also provide text version of an RFC file (e.g. https://www.ietf.org/rfc/rfc5280.txt).

The main reason to build this library, apart from learning F#, was to create a web-based ASN.1 viewer. As far as I know, there is no free ASN.1 viewer with support for schemas and in my last job I would definitely make use of one. The viewer itself is built using the Fable compiler which compiles F# code to JavaScript.

Since the library was primarily intended to build the web-based viewer, its interface is not well thought out. For standalone usage it would need tidying up, adding documentation and creating a NuGet package.

You can try the web viewer here:

https://i-p.github.io/FsAsn1/

If you have a file which is not displayed correctly or if you'd like to have a support for another schema, feel free to open a Github issue.

• Schema.fs - type definitions used to represent an ASN.1 schema
• Types.fs - types used to represent an ASN.1 structure
• Reader.fs - reading and interpreting ASN.1 encoded data
• SchemaParser.fs - FParsec-based parser for reading ASN.1 schemas
• Examples.fsx - a few examples how to use the library

A proof of concept for an ASN.1 type provider.

NUnit-based tests that are also run in JavaScript after they are compiled by Fable.

• ParserTests.fs - tests for parsers defined in SchemaParser module
• FParsecTests.fs - tests to verify that FParsec functions work correctly after translation to JavaScript
• ReaderTests.fs - tests for reading ASN.1 encoded data
• TypeProviderTests.fsx - simple F# script used to test ASN.1 type provider

• Fable.Plugins.FParsec.fsx - Fable plugins for translating FParsec functions to JavaScript
• Fable.Plugins.Viewer.fsx - Fable plugins for translating various functions that are needed by FsAsn1.Viewer
• fparsec.es6.js - wrapper that provides FParsec-like interface to bennu library. Replacements defined by Fable plugin are then much simpler.
• Data.fs - example files and schemas
• Components.fs - functions for building user interface

• Fable - F# to JavaScript compiler
• FParsec - parser combinator library used to parse ASN.1 schemas
• bennu - JavaScript parser combinator library based on the same principles as FParsec, that allowed for an easy translation
• Font Awesome icons

In solution directory, run:

build.bat

To run ASN.1 Viewer locally, run:

build.bat WebServer

then open http://localhost:8080/asn1-viewer in your browser.

If you change something in FsAsn1.Viewer project, run the following command and reload the ASN.1 Viewer:

build.bat DevViewerJS

If you change something in FsAsn1 project and want to execute JavaScript tests, run:

build.bat DevRunTestsJS

ASN.1 type provider is currently just a proof of concept. For experimenting with it, see TypeProviderTests.fsx.

In order to prevent Visual Studio from locking the assemblies, it's important to refer to assemblies in the following way:

#load "load-tp-assemblies.fsx"

After you make some changes to FsAsn1 or FsAsn1.TypeProvider, run following command in NuGet Package Manager Console (menu Tools > Nuget Package Manager):

. .\Build-TypeProvider.ps1

This command will build uniquely named assemblies and update load-tp-assemblies.fsx. In this way, there are no issues with Visual Studio locking the assemblies.

• ASN.1 standard
  • ITU-T X.680 (08/2015) Information technology – Abstract Syntax Notation One (ASN.1): Specification of basic notation
  • ITU-T X.690 (08/2015) Information technology – ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)