How to use the asn1.BerWriter function in asn1

q = tmp.data;
    offset = tmp.offset;

    tmp = readNext(buffer, offset);
    g = tmp.data;
    offset = tmp.offset;

    tmp = readNext(buffer, offset);
    y = tmp.data;
  } catch (e) {
    console.log(e.stack);
    throw new Error('Invalid ssh key: ' + key);
  }

  // DER is a subset of BER
  der = new asn1.BerWriter();

  der.startSequence();

  der.startSequence();
  der.writeOID('1.2.840.10040.4.1');

  der.startSequence();
  writeInt(der, p);
  writeInt(der, q);
  writeInt(der, g);
  der.endSequence();

  der.endSequence();

  der.startSequence(0x03); // bit string
  der.writeByte(0x00);

function signAsync(cert, signer, done) {
	if (cert.signatures.x509 === undefined)
		cert.signatures.x509 = {};
	var sig = cert.signatures.x509;

	var der = new asn1.BerWriter();
	writeTBSCert(cert, der);
	var blob = der.buffer;
	sig.cache = blob;

	signer(blob, function (err, signature) {
		if (err) {
			done(err);
			return;
		}
		sig.algo = signature.type + '-' + signature.hashAlgorithm;
		if (SIGN_ALGS[sig.algo] === undefined) {
			done(new Error('Invalid signing algorithm "' +
			    sig.algo + '"'));
			return;
		}
		sig.signature = signature;

function write(key, options, type) {
	assert.object(key);

	var alg = {
	    'ecdsa': 'EC',
	    'rsa': 'RSA',
	    'dsa': 'DSA',
	    'ed25519': 'EdDSA'
	}[key.type];
	var header;

	var der = new asn1.BerWriter();

	if (PrivateKey.isPrivateKey(key)) {
		if (type && type === 'pkcs8') {
			header = 'PRIVATE KEY';
			pkcs8.writePkcs8(der, key);
		} else {
			if (type)
				assert.strictEqual(type, 'pkcs1');
			header = alg + ' PRIVATE KEY';
			pkcs1.writePkcs1(der, key);
		}

	} else if (Key.isKey(key)) {
		if (type && type === 'pkcs1') {
			header = alg + ' PUBLIC KEY';
			pkcs1.writePkcs1(der, key);

case 'ed25519':
		if (format === 'ssh') {
			buf = new SSHBuffer({});
			buf.writeString(stype);
			buf.writePart(this.part.sig);
			return (buf.toBuffer());
		} else {
			return (this.part.sig.data);
		}
		break;

	case 'dsa':
	case 'ecdsa':
		var r, s;
		if (format === 'asn1') {
			var der = new asn1.BerWriter();
			der.startSequence();
			r = utils.mpNormalize(this.part.r.data);
			s = utils.mpNormalize(this.part.s.data);
			der.writeBuffer(r, asn1.Ber.Integer);
			der.writeBuffer(s, asn1.Ber.Integer);
			der.endSequence();
			return (der.buffer);
		} else if (format === 'ssh' && this.type === 'dsa') {
			buf = new SSHBuffer({});
			buf.writeString('ssh-dss');
			r = this.part.r.data;
			if (r.length > 20 && r[0] === 0x00)
				r = r.slice(1);
			s = this.part.s.data;
			if (s.length > 20 && s[0] === 0x00)
				s = s.slice(1);

function sign(cert, key) {
	if (cert.signatures.x509 === undefined)
		cert.signatures.x509 = {};
	var sig = cert.signatures.x509;

	sig.algo = key.type + '-' + key.defaultHashAlgorithm();
	if (SIGN_ALGS[sig.algo] === undefined)
		return (false);

	var der = new asn1.BerWriter();
	writeTBSCert(cert, der);
	var blob = der.buffer;
	sig.cache = blob;

	var signer = key.createSign();
	signer.write(blob);
	cert.signatures.x509.signature = signer.sign();

	return (true);
}

function sign(cert, key) {
	if (cert.signatures.x509 === undefined)
		cert.signatures.x509 = {};
	var sig = cert.signatures.x509;

	sig.algo = key.type + '-' + key.defaultHashAlgorithm();
	if (SIGN_ALGS[sig.algo] === undefined)
		return (false);

	var der = new asn1.BerWriter();
	writeTBSCert(cert, der);
	var blob = der.buffer;
	sig.cache = blob;

	var signer = key.createSign();
	signer.write(blob);
	cert.signatures.x509.signature = signer.sign();

	return (true);
}

function sign(cert, key) {
	if (cert.signatures.x509 === undefined)
		cert.signatures.x509 = {};
	var sig = cert.signatures.x509;

	sig.algo = key.type + '-' + key.defaultHashAlgorithm();
	if (SIGN_ALGS[sig.algo] === undefined)
		return (false);

	var der = new asn1.BerWriter();
	writeTBSCert(cert, der);
	var blob = der.buffer;
	sig.cache = blob;

	var signer = key.createSign();
	signer.write(blob);
	cert.signatures.x509.signature = signer.sign();

	return (true);
}

function write(key, options, type) {
	assert.object(key);

	var alg = {
	    'ecdsa': 'EC',
	    'rsa': 'RSA',
	    'dsa': 'DSA',
	    'ed25519': 'EdDSA'
	}[key.type];
	var header;

	var der = new asn1.BerWriter();

	if (PrivateKey.isPrivateKey(key)) {
		if (type && type === 'pkcs8') {
			header = 'PRIVATE KEY';
			pkcs8.writePkcs8(der, key);
		} else {
			if (type)
				assert.strictEqual(type, 'pkcs1');
			header = alg + ' PRIVATE KEY';
			pkcs1.writePkcs1(der, key);
		}

	} else if (Key.isKey(key)) {
		if (type && type === 'pkcs1') {
			header = alg + ' PUBLIC KEY';
			pkcs1.writePkcs1(der, key);

case 'ed25519':
		if (format === 'ssh') {
			buf = new SSHBuffer({});
			buf.writeString(stype);
			buf.writePart(this.part.sig);
			return (buf.toBuffer());
		} else {
			return (this.part.sig.data);
		}
		break;

	case 'dsa':
	case 'ecdsa':
		var r, s;
		if (format === 'asn1') {
			var der = new asn1.BerWriter();
			der.startSequence();
			r = utils.mpNormalize(this.part.r.data);
			s = utils.mpNormalize(this.part.s.data);
			der.writeBuffer(r, asn1.Ber.Integer);
			der.writeBuffer(s, asn1.Ber.Integer);
			der.endSequence();
			return (der.buffer);
		} else if (format === 'ssh' && this.type === 'dsa') {
			buf = new SSHBuffer({});
			buf.writeString('ssh-dss');
			r = this.part.r.data;
			if (r.length > 20 && r[0] === 0x00)
				r = r.slice(1);
			s = this.part.s.data;
			if (s.length > 20 && s[0] === 0x00)
				s = s.slice(1);

function write(key, options, type) {
	assert.object(key);

	var alg = {'ecdsa': 'EC', 'rsa': 'RSA', 'dsa': 'DSA'}[key.type];
	var header;

	var der = new asn1.BerWriter();

	if (PrivateKey.isPrivateKey(key)) {
		if (type && type === 'pkcs8') {
			header = 'PRIVATE KEY';
			pkcs8.writePkcs8(der, key);
		} else {
			if (type)
				assert.strictEqual(type, 'pkcs1');
			header = alg + ' PRIVATE KEY';
			pkcs1.writePkcs1(der, key);
		}

	} else if (Key.isKey(key)) {
		if (type && type === 'pkcs1') {
			header = alg + ' PUBLIC KEY';
			pkcs1.writePkcs1(der, key);