Generate DER encoding from known ECDSA private key in JS

der.js

"use strict";

var ECKey = require('eckey')
var fs = require('fs')

var seed = process.argv[2] 

var bytes = new Uint8Array(32);

for (var i = 0; i < seed.length; i += 2) {
    var b = parseInt(seed.substr(i, 2), 16)
    bytes[i/2] = b
}

var ck = new ECKey(bytes, false)

var pubkeyhex = ck.publicKey.toString('hex')
console.log("privkey export: " + ck.privateExportKey.toString('hex'))
console.log("pubkey: " + ck.publicKey.toString('hex'))
console.log("pubpoint: " + ck.publicPoint.toString('hex'))

var sequence = [];

// id-ecPublicKey OBJECT IDENTIFIER
// 1, 2, 840, 10045, 2, 1 (06072a8648ce3d0201)
sequence.push(0x06)
sequence.push(0x07)
sequence.push(0x2a)
sequence.push(0x86)
sequence.push(0x48)
sequence.push(0xce)
sequence.push(0x3d)
sequence.push(0x02)
sequence.push(0x01)

// secp256r1 OBJECT IDENTIFIER
// 1, 2, 840, 10045, 3, 1, 7 (06082a8648ce3d030107)
sequence.push(0x06)
sequence.push(0x08)
sequence.push(0x2a)
sequence.push(0x86)
sequence.push(0x48)
sequence.push(0xce)
sequence.push(0x3d)
sequence.push(0x03)
sequence.push(0x01)
sequence.push(0x07)

sequence.unshift(sequence.length);
sequence.unshift(0x30);

sequence.push(0x03);
sequence.push(0x42); // size for rest of content
sequence.push(0x00); // no padding necessary
for (var i = 0; i < pubkeyhex.length; i += 2) {
  var b = "0x" + pubkeyhex.substr(i, 2)
    sequence.push(b)
}

sequence.unshift(sequence.length);
sequence.unshift(0x30);

var pubkeyder = new Buffer(sequence);

fs.writeFile("node.der", pubkeyder, function(err) {
  if(err) {
    return console.log(err);
  }
});