twitst4tz

x509.js (19556B)

---

 // Copyright 2017 Joyent, Inc.
 
 module.exports = {
 	read: read,
 	verify: verify,
 	sign: sign,
 	signAsync: signAsync,
 	write: write
 };
 
 var assert = require('assert-plus');
 var asn1 = require('asn1');
 var Buffer = require('safer-buffer').Buffer;
 var algs = require('../algs');
 var utils = require('../utils');
 var Key = require('../key');
 var PrivateKey = require('../private-key');
 var pem = require('./pem');
 var Identity = require('../identity');
 var Signature = require('../signature');
 var Certificate = require('../certificate');
 var pkcs8 = require('./pkcs8');
 
 /*
  * This file is based on RFC5280 (X.509).
  */
 
 /* Helper to read in a single mpint */
 function readMPInt(der, nm) {
 	assert.strictEqual(der.peek(), asn1.Ber.Integer,
 	    nm + ' is not an Integer');
 	return (utils.mpNormalize(der.readString(asn1.Ber.Integer, true)));
 }
 
 function verify(cert, key) {
 	var sig = cert.signatures.x509;
 	assert.object(sig, 'x509 signature');
 
 	var algParts = sig.algo.split('-');
 	if (algParts[0] !== key.type)
 		return (false);
 
 	var blob = sig.cache;
 	if (blob === undefined) {
 		var der = new asn1.BerWriter();
 		writeTBSCert(cert, der);
 		blob = der.buffer;
 	}
 
 	var verifier = key.createVerify(algParts[1]);
 	verifier.write(blob);
 	return (verifier.verify(sig.signature));
 }
 
 function Local(i) {
 	return (asn1.Ber.Context | asn1.Ber.Constructor | i);
 }
 
 function Context(i) {
 	return (asn1.Ber.Context | i);
 }
 
 var SIGN_ALGS = {
 	'rsa-md5': '1.2.840.113549.1.1.4',
 	'rsa-sha1': '1.2.840.113549.1.1.5',
 	'rsa-sha256': '1.2.840.113549.1.1.11',
 	'rsa-sha384': '1.2.840.113549.1.1.12',
 	'rsa-sha512': '1.2.840.113549.1.1.13',
 	'dsa-sha1': '1.2.840.10040.4.3',
 	'dsa-sha256': '2.16.840.1.101.3.4.3.2',
 	'ecdsa-sha1': '1.2.840.10045.4.1',
 	'ecdsa-sha256': '1.2.840.10045.4.3.2',
 	'ecdsa-sha384': '1.2.840.10045.4.3.3',
 	'ecdsa-sha512': '1.2.840.10045.4.3.4',
 	'ed25519-sha512': '1.3.101.112'
 };
 Object.keys(SIGN_ALGS).forEach(function (k) {
 	SIGN_ALGS[SIGN_ALGS[k]] = k;
 });
 SIGN_ALGS['1.3.14.3.2.3'] = 'rsa-md5';
 SIGN_ALGS['1.3.14.3.2.29'] = 'rsa-sha1';
 
 var EXTS = {
 	'issuerKeyId': '2.5.29.35',
 	'altName': '2.5.29.17',
 	'basicConstraints': '2.5.29.19',
 	'keyUsage': '2.5.29.15',
 	'extKeyUsage': '2.5.29.37'
 };
 
 function read(buf, options) {
 	if (typeof (buf) === 'string') {
 		buf = Buffer.from(buf, 'binary');
 	}
 	assert.buffer(buf, 'buf');
 
 	var der = new asn1.BerReader(buf);
 
 	der.readSequence();
 	if (Math.abs(der.length - der.remain) > 1) {
 		throw (new Error('DER sequence does not contain whole byte ' +
 		    'stream'));
 	}
 
 	var tbsStart = der.offset;
 	der.readSequence();
 	var sigOffset = der.offset + der.length;
 	var tbsEnd = sigOffset;
 
 	if (der.peek() === Local(0)) {
 		der.readSequence(Local(0));
 		var version = der.readInt();
 		assert.ok(version <= 3,
 		    'only x.509 versions up to v3 supported');
 	}
 
 	var cert = {};
 	cert.signatures = {};
 	var sig = (cert.signatures.x509 = {});
 	sig.extras = {};
 
 	cert.serial = readMPInt(der, 'serial');
 
 	der.readSequence();
 	var after = der.offset + der.length;
 	var certAlgOid = der.readOID();
 	var certAlg = SIGN_ALGS[certAlgOid];
 	if (certAlg === undefined)
 		throw (new Error('unknown signature algorithm ' + certAlgOid));
 
 	der._offset = after;
 	cert.issuer = Identity.parseAsn1(der);
 
 	der.readSequence();
 	cert.validFrom = readDate(der);
 	cert.validUntil = readDate(der);
 
 	cert.subjects = [Identity.parseAsn1(der)];
 
 	der.readSequence();
 	after = der.offset + der.length;
 	cert.subjectKey = pkcs8.readPkcs8(undefined, 'public', der);
 	der._offset = after;
 
 	/* issuerUniqueID */
 	if (der.peek() === Local(1)) {
 		der.readSequence(Local(1));
 		sig.extras.issuerUniqueID =
 		    buf.slice(der.offset, der.offset + der.length);
 		der._offset += der.length;
 	}
 
 	/* subjectUniqueID */
 	if (der.peek() === Local(2)) {
 		der.readSequence(Local(2));
 		sig.extras.subjectUniqueID =
 		    buf.slice(der.offset, der.offset + der.length);
 		der._offset += der.length;
 	}
 
 	/* extensions */
 	if (der.peek() === Local(3)) {
 		der.readSequence(Local(3));
 		var extEnd = der.offset + der.length;
 		der.readSequence();
 
 		while (der.offset < extEnd)
 			readExtension(cert, buf, der);
 
 		assert.strictEqual(der.offset, extEnd);
 	}
 
 	assert.strictEqual(der.offset, sigOffset);
 
 	der.readSequence();
 	after = der.offset + der.length;
 	var sigAlgOid = der.readOID();
 	var sigAlg = SIGN_ALGS[sigAlgOid];
 	if (sigAlg === undefined)
 		throw (new Error('unknown signature algorithm ' + sigAlgOid));
 	der._offset = after;
 
 	var sigData = der.readString(asn1.Ber.BitString, true);
 	if (sigData[0] === 0)
 		sigData = sigData.slice(1);
 	var algParts = sigAlg.split('-');
 
 	sig.signature = Signature.parse(sigData, algParts[0], 'asn1');
 	sig.signature.hashAlgorithm = algParts[1];
 	sig.algo = sigAlg;
 	sig.cache = buf.slice(tbsStart, tbsEnd);
 
 	return (new Certificate(cert));
 }
 
 function readDate(der) {
 	if (der.peek() === asn1.Ber.UTCTime) {
 		return (utcTimeToDate(der.readString(asn1.Ber.UTCTime)));
 	} else if (der.peek() === asn1.Ber.GeneralizedTime) {
 		return (gTimeToDate(der.readString(asn1.Ber.GeneralizedTime)));
 	} else {
 		throw (new Error('Unsupported date format'));
 	}
 }
 
 function writeDate(der, date) {
 	if (date.getUTCFullYear() >= 2050 || date.getUTCFullYear() < 1950) {
 		der.writeString(dateToGTime(date), asn1.Ber.GeneralizedTime);
 	} else {
 		der.writeString(dateToUTCTime(date), asn1.Ber.UTCTime);
 	}
 }
 
 /* RFC5280, section 4.2.1.6 (GeneralName type) */
 var ALTNAME = {
 	OtherName: Local(0),
 	RFC822Name: Context(1),
 	DNSName: Context(2),
 	X400Address: Local(3),
 	DirectoryName: Local(4),
 	EDIPartyName: Local(5),
 	URI: Context(6),
 	IPAddress: Context(7),
 	OID: Context(8)
 };
 
 /* RFC5280, section 4.2.1.12 (KeyPurposeId) */
 var EXTPURPOSE = {
 	'serverAuth': '1.3.6.1.5.5.7.3.1',
 	'clientAuth': '1.3.6.1.5.5.7.3.2',
 	'codeSigning': '1.3.6.1.5.5.7.3.3',
 
 	/* See https://github.com/joyent/oid-docs/blob/master/root.md */
 	'joyentDocker': '1.3.6.1.4.1.38678.1.4.1',
 	'joyentCmon': '1.3.6.1.4.1.38678.1.4.2'
 };
 var EXTPURPOSE_REV = {};
 Object.keys(EXTPURPOSE).forEach(function (k) {
 	EXTPURPOSE_REV[EXTPURPOSE[k]] = k;
 });
 
 var KEYUSEBITS = [
 	'signature', 'identity', 'keyEncryption',
 	'encryption', 'keyAgreement', 'ca', 'crl'
 ];
 
 function readExtension(cert, buf, der) {
 	der.readSequence();
 	var after = der.offset + der.length;
 	var extId = der.readOID();
 	var id;
 	var sig = cert.signatures.x509;
 	if (!sig.extras.exts)
 		sig.extras.exts = [];
 
 	var critical;
 	if (der.peek() === asn1.Ber.Boolean)
 		critical = der.readBoolean();
 
 	switch (extId) {
 	case (EXTS.basicConstraints):
 		der.readSequence(asn1.Ber.OctetString);
 		der.readSequence();
 		var bcEnd = der.offset + der.length;
 		var ca = false;
 		if (der.peek() === asn1.Ber.Boolean)
 			ca = der.readBoolean();
 		if (cert.purposes === undefined)
 			cert.purposes = [];
 		if (ca === true)
 			cert.purposes.push('ca');
 		var bc = { oid: extId, critical: critical };
 		if (der.offset < bcEnd && der.peek() === asn1.Ber.Integer)
 			bc.pathLen = der.readInt();
 		sig.extras.exts.push(bc);
 		break;
 	case (EXTS.extKeyUsage):
 		der.readSequence(asn1.Ber.OctetString);
 		der.readSequence();
 		if (cert.purposes === undefined)
 			cert.purposes = [];
 		var ekEnd = der.offset + der.length;
 		while (der.offset < ekEnd) {
 			var oid = der.readOID();
 			cert.purposes.push(EXTPURPOSE_REV[oid] || oid);
 		}
 		/*
 		 * This is a bit of a hack: in the case where we have a cert
 		 * that's only allowed to do serverAuth or clientAuth (and not
 		 * the other), we want to make sure all our Subjects are of
 		 * the right type. But we already parsed our Subjects and
 		 * decided if they were hosts or users earlier (since it appears
 		 * first in the cert).
 		 *
 		 * So we go through and mutate them into the right kind here if
 		 * it doesn't match. This might not be hugely beneficial, as it
 		 * seems that single-purpose certs are not often seen in the
 		 * wild.
 		 */
 		if (cert.purposes.indexOf('serverAuth') !== -1 &&
 		    cert.purposes.indexOf('clientAuth') === -1) {
 			cert.subjects.forEach(function (ide) {
 				if (ide.type !== 'host') {
 					ide.type = 'host';
 					ide.hostname = ide.uid ||
 					    ide.email ||
 					    ide.components[0].value;
 				}
 			});
 		} else if (cert.purposes.indexOf('clientAuth') !== -1 &&
 		    cert.purposes.indexOf('serverAuth') === -1) {
 			cert.subjects.forEach(function (ide) {
 				if (ide.type !== 'user') {
 					ide.type = 'user';
 					ide.uid = ide.hostname ||
 					    ide.email ||
 					    ide.components[0].value;
 				}
 			});
 		}
 		sig.extras.exts.push({ oid: extId, critical: critical });
 		break;
 	case (EXTS.keyUsage):
 		der.readSequence(asn1.Ber.OctetString);
 		var bits = der.readString(asn1.Ber.BitString, true);
 		var setBits = readBitField(bits, KEYUSEBITS);
 		setBits.forEach(function (bit) {
 			if (cert.purposes === undefined)
 				cert.purposes = [];
 			if (cert.purposes.indexOf(bit) === -1)
 				cert.purposes.push(bit);
 		});
 		sig.extras.exts.push({ oid: extId, critical: critical,
 		    bits: bits });
 		break;
 	case (EXTS.altName):
 		der.readSequence(asn1.Ber.OctetString);
 		der.readSequence();
 		var aeEnd = der.offset + der.length;
 		while (der.offset < aeEnd) {
 			switch (der.peek()) {
 			case ALTNAME.OtherName:
 			case ALTNAME.EDIPartyName:
 				der.readSequence();
 				der._offset += der.length;
 				break;
 			case ALTNAME.OID:
 				der.readOID(ALTNAME.OID);
 				break;
 			case ALTNAME.RFC822Name:
 				/* RFC822 specifies email addresses */
 				var email = der.readString(ALTNAME.RFC822Name);
 				id = Identity.forEmail(email);
 				if (!cert.subjects[0].equals(id))
 					cert.subjects.push(id);
 				break;
 			case ALTNAME.DirectoryName:
 				der.readSequence(ALTNAME.DirectoryName);
 				id = Identity.parseAsn1(der);
 				if (!cert.subjects[0].equals(id))
 					cert.subjects.push(id);
 				break;
 			case ALTNAME.DNSName:
 				var host = der.readString(
 				    ALTNAME.DNSName);
 				id = Identity.forHost(host);
 				if (!cert.subjects[0].equals(id))
 					cert.subjects.push(id);
 				break;
 			default:
 				der.readString(der.peek());
 				break;
 			}
 		}
 		sig.extras.exts.push({ oid: extId, critical: critical });
 		break;
 	default:
 		sig.extras.exts.push({
 			oid: extId,
 			critical: critical,
 			data: der.readString(asn1.Ber.OctetString, true)
 		});
 		break;
 	}
 
 	der._offset = after;
 }
 
 var UTCTIME_RE =
     /^([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})?Z$/;
 function utcTimeToDate(t) {
 	var m = t.match(UTCTIME_RE);
 	assert.ok(m, 'timestamps must be in UTC');
 	var d = new Date();
 
 	var thisYear = d.getUTCFullYear();
 	var century = Math.floor(thisYear / 100) * 100;
 
 	var year = parseInt(m[1], 10);
 	if (thisYear % 100 < 50 && year >= 60)
 		year += (century - 1);
 	else
 		year += century;
 	d.setUTCFullYear(year, parseInt(m[2], 10) - 1, parseInt(m[3], 10));
 	d.setUTCHours(parseInt(m[4], 10), parseInt(m[5], 10));
 	if (m[6] && m[6].length > 0)
 		d.setUTCSeconds(parseInt(m[6], 10));
 	return (d);
 }
 
 var GTIME_RE =
     /^([0-9]{4})([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})([0-9]{2})?Z$/;
 function gTimeToDate(t) {
 	var m = t.match(GTIME_RE);
 	assert.ok(m);
 	var d = new Date();
 
 	d.setUTCFullYear(parseInt(m[1], 10), parseInt(m[2], 10) - 1,
 	    parseInt(m[3], 10));
 	d.setUTCHours(parseInt(m[4], 10), parseInt(m[5], 10));
 	if (m[6] && m[6].length > 0)
 		d.setUTCSeconds(parseInt(m[6], 10));
 	return (d);
 }
 
 function zeroPad(n, m) {
 	if (m === undefined)
 		m = 2;
 	var s = '' + n;
 	while (s.length < m)
 		s = '0' + s;
 	return (s);
 }
 
 function dateToUTCTime(d) {
 	var s = '';
 	s += zeroPad(d.getUTCFullYear() % 100);
 	s += zeroPad(d.getUTCMonth() + 1);
 	s += zeroPad(d.getUTCDate());
 	s += zeroPad(d.getUTCHours());
 	s += zeroPad(d.getUTCMinutes());
 	s += zeroPad(d.getUTCSeconds());
 	s += 'Z';
 	return (s);
 }
 
 function dateToGTime(d) {
 	var s = '';
 	s += zeroPad(d.getUTCFullYear(), 4);
 	s += zeroPad(d.getUTCMonth() + 1);
 	s += zeroPad(d.getUTCDate());
 	s += zeroPad(d.getUTCHours());
 	s += zeroPad(d.getUTCMinutes());
 	s += zeroPad(d.getUTCSeconds());
 	s += 'Z';
 	return (s);
 }
 
 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 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;
 		done();
 	});
 }
 
 function write(cert, options) {
 	var sig = cert.signatures.x509;
 	assert.object(sig, 'x509 signature');
 
 	var der = new asn1.BerWriter();
 	der.startSequence();
 	if (sig.cache) {
 		der._ensure(sig.cache.length);
 		sig.cache.copy(der._buf, der._offset);
 		der._offset += sig.cache.length;
 	} else {
 		writeTBSCert(cert, der);
 	}
 
 	der.startSequence();
 	der.writeOID(SIGN_ALGS[sig.algo]);
 	if (sig.algo.match(/^rsa-/))
 		der.writeNull();
 	der.endSequence();
 
 	var sigData = sig.signature.toBuffer('asn1');
 	var data = Buffer.alloc(sigData.length + 1);
 	data[0] = 0;
 	sigData.copy(data, 1);
 	der.writeBuffer(data, asn1.Ber.BitString);
 	der.endSequence();
 
 	return (der.buffer);
 }
 
 function writeTBSCert(cert, der) {
 	var sig = cert.signatures.x509;
 	assert.object(sig, 'x509 signature');
 
 	der.startSequence();
 
 	der.startSequence(Local(0));
 	der.writeInt(2);
 	der.endSequence();
 
 	der.writeBuffer(utils.mpNormalize(cert.serial), asn1.Ber.Integer);
 
 	der.startSequence();
 	der.writeOID(SIGN_ALGS[sig.algo]);
 	if (sig.algo.match(/^rsa-/))
 		der.writeNull();
 	der.endSequence();
 
 	cert.issuer.toAsn1(der);
 
 	der.startSequence();
 	writeDate(der, cert.validFrom);
 	writeDate(der, cert.validUntil);
 	der.endSequence();
 
 	var subject = cert.subjects[0];
 	var altNames = cert.subjects.slice(1);
 	subject.toAsn1(der);
 
 	pkcs8.writePkcs8(der, cert.subjectKey);
 
 	if (sig.extras && sig.extras.issuerUniqueID) {
 		der.writeBuffer(sig.extras.issuerUniqueID, Local(1));
 	}
 
 	if (sig.extras && sig.extras.subjectUniqueID) {
 		der.writeBuffer(sig.extras.subjectUniqueID, Local(2));
 	}
 
 	if (altNames.length > 0 || subject.type === 'host' ||
 	    (cert.purposes !== undefined && cert.purposes.length > 0) ||
 	    (sig.extras && sig.extras.exts)) {
 		der.startSequence(Local(3));
 		der.startSequence();
 
 		var exts = [];
 		if (cert.purposes !== undefined && cert.purposes.length > 0) {
 			exts.push({
 				oid: EXTS.basicConstraints,
 				critical: true
 			});
 			exts.push({
 				oid: EXTS.keyUsage,
 				critical: true
 			});
 			exts.push({
 				oid: EXTS.extKeyUsage,
 				critical: true
 			});
 		}
 		exts.push({ oid: EXTS.altName });
 		if (sig.extras && sig.extras.exts)
 			exts = sig.extras.exts;
 
 		for (var i = 0; i < exts.length; ++i) {
 			der.startSequence();
 			der.writeOID(exts[i].oid);
 
 			if (exts[i].critical !== undefined)
 				der.writeBoolean(exts[i].critical);
 
 			if (exts[i].oid === EXTS.altName) {
 				der.startSequence(asn1.Ber.OctetString);
 				der.startSequence();
 				if (subject.type === 'host') {
 					der.writeString(subject.hostname,
 					    Context(2));
 				}
 				for (var j = 0; j < altNames.length; ++j) {
 					if (altNames[j].type === 'host') {
 						der.writeString(
 						    altNames[j].hostname,
 						    ALTNAME.DNSName);
 					} else if (altNames[j].type ===
 					    'email') {
 						der.writeString(
 						    altNames[j].email,
 						    ALTNAME.RFC822Name);
 					} else {
 						/*
 						 * Encode anything else as a
 						 * DN style name for now.
 						 */
 						der.startSequence(
 						    ALTNAME.DirectoryName);
 						altNames[j].toAsn1(der);
 						der.endSequence();
 					}
 				}
 				der.endSequence();
 				der.endSequence();
 			} else if (exts[i].oid === EXTS.basicConstraints) {
 				der.startSequence(asn1.Ber.OctetString);
 				der.startSequence();
 				var ca = (cert.purposes.indexOf('ca') !== -1);
 				var pathLen = exts[i].pathLen;
 				der.writeBoolean(ca);
 				if (pathLen !== undefined)
 					der.writeInt(pathLen);
 				der.endSequence();
 				der.endSequence();
 			} else if (exts[i].oid === EXTS.extKeyUsage) {
 				der.startSequence(asn1.Ber.OctetString);
 				der.startSequence();
 				cert.purposes.forEach(function (purpose) {
 					if (purpose === 'ca')
 						return;
 					if (KEYUSEBITS.indexOf(purpose) !== -1)
 						return;
 					var oid = purpose;
 					if (EXTPURPOSE[purpose] !== undefined)
 						oid = EXTPURPOSE[purpose];
 					der.writeOID(oid);
 				});
 				der.endSequence();
 				der.endSequence();
 			} else if (exts[i].oid === EXTS.keyUsage) {
 				der.startSequence(asn1.Ber.OctetString);
 				/*
 				 * If we parsed this certificate from a byte
 				 * stream (i.e. we didn't generate it in sshpk)
 				 * then we'll have a ".bits" property on the
 				 * ext with the original raw byte contents.
 				 *
 				 * If we have this, use it here instead of
 				 * regenerating it. This guarantees we output
 				 * the same data we parsed, so signatures still
 				 * validate.
 				 */
 				if (exts[i].bits !== undefined) {
 					der.writeBuffer(exts[i].bits,
 					    asn1.Ber.BitString);
 				} else {
 					var bits = writeBitField(cert.purposes,
 					    KEYUSEBITS);
 					der.writeBuffer(bits,
 					    asn1.Ber.BitString);
 				}
 				der.endSequence();
 			} else {
 				der.writeBuffer(exts[i].data,
 				    asn1.Ber.OctetString);
 			}
 
 			der.endSequence();
 		}
 
 		der.endSequence();
 		der.endSequence();
 	}
 
 	der.endSequence();
 }
 
 /*
  * Reads an ASN.1 BER bitfield out of the Buffer produced by doing
  * `BerReader#readString(asn1.Ber.BitString)`. That function gives us the raw
  * contents of the BitString tag, which is a count of unused bits followed by
  * the bits as a right-padded byte string.
  *
  * `bits` is the Buffer, `bitIndex` should contain an array of string names
  * for the bits in the string, ordered starting with bit #0 in the ASN.1 spec.
  *
  * Returns an array of Strings, the names of the bits that were set to 1.
  */
 function readBitField(bits, bitIndex) {
 	var bitLen = 8 * (bits.length - 1) - bits[0];
 	var setBits = {};
 	for (var i = 0; i < bitLen; ++i) {
 		var byteN = 1 + Math.floor(i / 8);
 		var bit = 7 - (i % 8);
 		var mask = 1 << bit;
 		var bitVal = ((bits[byteN] & mask) !== 0);
 		var name = bitIndex[i];
 		if (bitVal && typeof (name) === 'string') {
 			setBits[name] = true;
 		}
 	}
 	return (Object.keys(setBits));
 }
 
 /*
  * `setBits` is an array of strings, containing the names for each bit that
  * sould be set to 1. `bitIndex` is same as in `readBitField()`.
  *
  * Returns a Buffer, ready to be written out with `BerWriter#writeString()`.
  */
 function writeBitField(setBits, bitIndex) {
 	var bitLen = bitIndex.length;
 	var blen = Math.ceil(bitLen / 8);
 	var unused = blen * 8 - bitLen;
 	var bits = Buffer.alloc(1 + blen); // zero-filled
 	bits[0] = unused;
 	for (var i = 0; i < bitLen; ++i) {
 		var byteN = 1 + Math.floor(i / 8);
 		var bit = 7 - (i % 8);
 		var mask = 1 << bit;
 		var name = bitIndex[i];
 		if (name === undefined)
 			continue;
 		var bitVal = (setBits.indexOf(name) !== -1);
 		if (bitVal) {
 			bits[byteN] |= mask;
 		}
 	}
 	return (bits);
 }