Motoko ASN.1

A Motoko implementation of ASN.1 (Abstract Syntax Notation One) encoding and decoding.

⚠️ Partial Implementation

This library implements the core ASN.1 DER encoding and decoding functionality, but is missing several ASN.1 types:

Missing Types (by usage frequency):

1. ENUMERATED - Used in protocols like SNMP, LDAP
2. BMPString - Common in certificates and internationalized applications
3. NumericString - Used in telephony and financial protocols
4. VisibleString/ISO646String - Legacy systems and restricted environments
5. TeletexString/T61String - Older X.509 certificates
6. REAL - Scientific and technical applications
7. DATE, DATE-TIME - Modern alternatives to UTCTime/GeneralizedTime
8. RELATIVE-OID - X.500 directory services
9. Other less common types (EXTERNAL, GraphicString, UniversalString, etc.)

Package

MOPS

mops add asn1

To set up MOPS package manager, follow the instructions from the MOPS Site

Quick Start

Example 1: Encoding ASN.1 to DER

import ASN1 "mo:asn1";
import Result "mo:base/Result";
import Debug "mo:base/Debug";
import Nat "mo:base/Nat";

// Create an ASN.1 SEQUENCE value
let sequence = #sequence([
    #integer(123),
    #utf8String("Hello ASN.1"),
    #boolean(true)
]);

// Encode to DER format
let bytes : [Nat8] = switch (ASN1.encodeDER(sequence)) {
    case (#err(msg)) return #err(msg);
    case (#ok(bytes)) bytes;
};
...

Example 2: Decoding DER to ASN.1

import ASN1 "mo:asn1";
import Result "mo:base/Result";
import Debug "mo:base/Debug";

// Assuming 'derBytes' contains DER-encoded data
let derBytes : [Nat8] = [...];

// Decode from DER
let value : ASN1.ASN1Value = switch (ASN1.decodeDER(derBytes.vals())) {
    case (#err(msg)) return #err(msg);
    case (#ok(value)) value;
};

Example 3: Pretty-Printing ASN.1 Values

import ASN1 "mo:asn1";
import Debug "mo:base/Debug";

// Create an ASN.1 structure
let certificate = #sequence([
    #objectIdentifier([1, 2, 840, 113549, 1, 1, 11]), // sha256WithRSAEncryption
    #utf8String("Example Certificate"),
    #contextSpecific({
        tagNumber = 0;
        constructed = true;
        value = ?#sequence([
            #utctime("220101000000Z"),
            #utctime("230101000000Z")
        ])
    })
]);

// Print a human-readable representation
let prettyText = ASN1.toText(certificate);
Debug.print(prettyText);
// Output:
// SEQUENCE {
//   OBJECT IDENTIFIER: 1.2.840.113549.1.1.11
//   UTF8String: Example Certificate
//   [0] CONSTRUCTED {
//     SEQUENCE {
//       UTCTime: 220101000000Z
//       UTCTime: 230101000000Z
//     }
//   }
// }

API Reference

Types

public type TagClass = {
    #universal;
    #application;
    #contextSpecific;
    #private_;
};

public type ASN1Value = {
    #boolean : Bool;
    #integer : Int;
    #bitString : [Bool];
    #octetString : [Nat8];
    #null_;
    #objectIdentifier : [Nat];
    #utf8String : Text;
    #printableString : Text;
    #ia5String : Text;
    #utctime : Text;
    #generalizedTime : Text;
    #sequence : [ASN1Value];
    #set : [ASN1Value];
    #contextSpecific : ContextSpecificASN1Value;
    #unknown : UnknownASN1Value;
};

public type ContextSpecificASN1Value = {
    tagNumber : Nat;
    constructed : Bool;
    value : ?ASN1Value;
};

public type UnknownASN1Value = {
    tagClass : TagClass;
    tagNumber : Nat;
    constructed : Bool;
    data : [Nat8];
};

Functions

// Main encoding/decoding functions
public func decodeDER(bytes : [Nat8]) : Result.Result<ASN1Value, Text>;

public func encodeDER(value : ASN1Value) : Result.Result<[Nat8], Text>;

// Utility function for pretty-printing
public func toText(value : ASN1Value) : Text;

License

This project is licensed under the MIT License - see the LICENSE file for details.