@f-o-t/asn1

ASN.1 DER encoding and decoding with builder helpers.

Installation

bun add @f-o-t/asn1

API

encodeDer(node: Asn1Node): Uint8Array

Encode an ASN.1 node tree to DER format.

decodeDer(data: Uint8Array): Asn1Node

Decode a DER-encoded byte array into an ASN.1 node tree.

oidToBytes(dotNotation: string): Uint8Array

Convert an OID dot-notation string (e.g. "1.2.840.113549") to DER value bytes.

bytesToOid(data: Uint8Array): string

Convert DER OID value bytes back to dot-notation string.

Builder Helpers

Convenience functions for constructing ASN.1 nodes:

| Function | Tag | Description | |---|---|---| | sequence(...children) | 0x10 | Constructed SEQUENCE | | set(...children) | 0x11 | Constructed SET | | integer(value) | 0x02 | INTEGER (number or bigint) | | oid(dotNotation) | 0x06 | OBJECT IDENTIFIER | | octetString(data) | 0x04 | OCTET STRING | | bitString(data, unusedBits?) | 0x03 | BIT STRING | | utf8String(value) | 0x0C | UTF8String | | ia5String(value) | 0x16 | IA5String | | printableString(value) | 0x13 | PrintableString | | boolean(value) | 0x01 | BOOLEAN | | nullValue() | 0x05 | NULL | | utcTime(date) | 0x17 | UTCTime | | generalizedTime(date) | 0x18 | GeneralizedTime | | contextTag(tag, children, explicit?) | custom | Context-specific tag |

Usage

import {
  encodeDer,
  decodeDer,
  sequence,
  integer,
  oid,
  octetString,
  utf8String,
  nullValue,
} from "@f-o-t/asn1";

// Build an ASN.1 structure
const node = sequence(
  oid("1.2.840.113549.1.1.11"),  // sha256WithRSAEncryption
  nullValue(),
);

// Encode to DER bytes
const der = encodeDer(node);

// Decode back to a node tree
const decoded = decodeDer(der);

console.log(decoded.tag);         // 0x10 (SEQUENCE)
console.log(decoded.constructed); // true

Context Tags

import { contextTag, sequence, integer } from "@f-o-t/asn1";

// Explicit context tag [0] wrapping an integer
const explicit = contextTag(0, [integer(3)]);

// Implicit context tag [0] replacing the child's tag
const implicit = contextTag(0, [integer(3)], false);

Types

type Asn1Class = "universal" | "context" | "application" | "private";

type Asn1Node = {
  tag: number;
  constructed: boolean;
  class: Asn1Class;
  value: Uint8Array | Asn1Node[];
};