asn1lib

ASN.1 Parser for Dart

Encodes & decodes ASN1 using BER encoding.

Samples

Encoding

import 'package:asn1lib/asn1lib.dart';

var s = ASN1Sequence();
s.add(ASN1Integer(23));
s.add(ASN1OctetString('This is a test'));
s.add(ASN1Boolean(true));

// GET the BER Stream
var bytes = s
.
encodedBytes;

Decoding

import 'package:asn1lib/asn1lib.dart';

// e.g. bytes from the Encoding Example
var p = ASN1Parser(bytes);
var s2 = p.nextObject();
// s2 is a sequence...


// relaxed parsing, returning a generic ASN1Object from (yet) unsupported structures, instead of throwing
var p2 = ASN1Parser(bytes, relaxedParsing: true);
var s3 = p2.nextObject();
// s3 is a sequence...

ASN1 Object classes

The library has been most thoroughly tested against ASN1 primitive class types, such as strings, integers, sequences and sets.

There is minimal support for ASN1 Application, Context-specific and Private classes. If the parser encounters these tags, it will wrap them in an a corresponding object ( ASN1Application, ASN1ContextSpecific, ASN1Private). You can force decode these to another type, by doing something like this:

    var s = parser.nextObject();
    // We expect this is a context-specific class
    expect(s, isA<ASN1ContextSpecific>());
    // which is also an ASN1Object...
    expect(s, isA<ASN1Object>());
    // We expect this is a sequence, so "recast" to a sequence type
    var sequence = ASN1Sequence.fromBytes(s.encodedBytes);
    expect(sequence, isA<ASN1Sequence>());

Breaking Changes

As of 1.6, the default encoding of ASN1OctetString("some string") has changed. It now encodes octets using utf8 encoding. This is different than Dart's default utf16 encoding.

If you need to encode to utf16, you should convert to bytes first:

var x = ASN1OctetString(Uint8List.fromList('some string'.codeUnits));
// new getter
var y = x.utf16StringValue;

The getter ASN1OctetString.stringValue is deprecated. You should use ASN1OctetString.utf8StringValue or ASN1OctetString.utf16StringValue instead.

Issues

ASN1 parsing is complex, so bugs are expected. If you find a bug, please provide a test case that demonstrates the issue, along with the expected output. Bonus points if it is formatted as a dart test.

References

• https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/
• https://luca.ntop.org/Teaching/Appunti/asn1.html
• https://ldap.com/ldapv3-wire-protocol-reference-asn1-ber/