How to use the safer-buffer.Buffer.concat function in safer-buffer

DBCSDecoder.prototype.write = function(buf) {
    var newBuf = Buffer.alloc(buf.length*2),
        nodeIdx = this.nodeIdx, 
        prevBuf = this.prevBuf, prevBufOffset = this.prevBuf.length,
        seqStart = -this.prevBuf.length, // idx of the start of current parsed sequence.
        uCode;

    if (prevBufOffset > 0) // Make prev buf overlap a little to make it easier to slice later.
        prevBuf = Buffer.concat([prevBuf, buf.slice(0, 10)]);
    
    for (var i = 0, j = 0; i < buf.length; i++) {
        var curByte = (i >= 0) ? buf[i] : prevBuf[i + prevBufOffset];

        // Lookup in current trie node.
        var uCode = this.decodeTables[nodeIdx][curByte];

        if (uCode >= 0) { 
            // Normal character, just use it.
        }
        else if (uCode === UNASSIGNED) { // Unknown char.
            // TODO: Callback with seq.
            //var curSeq = (seqStart >= 0) ? buf.slice(seqStart, i+1) : prevBuf.slice(seqStart + prevBufOffset, i+1 + prevBufOffset);
            i = seqStart; // Try to parse again, after skipping first byte of the sequence ('i' will be incremented by 'for' cycle).
            uCode = this.defaultCharUnicode.charCodeAt(0);
        }

iconv.encode = function encode(str, encoding, options) {
    str = "" + (str || ""); // Ensure string.

    var encoder = iconv.getEncoder(encoding, options);

    var res = encoder.write(str);
    var trail = encoder.end();
    
    return (trail && trail.length > 0) ? Buffer.concat([res, trail]) : res;
}

if (cipher && key && iv) {
		var cipherStream = crypto.createDecipheriv(cipher, key, iv);
		var chunk, chunks = [];
		cipherStream.once('error', function (e) {
			if (e.toString().indexOf('bad decrypt') !== -1) {
				throw (new Error('Incorrect passphrase ' +
				    'supplied, could not decrypt key'));
			}
			throw (e);
		});
		cipherStream.write(buf);
		cipherStream.end();
		while ((chunk = cipherStream.read()) !== null)
			chunks.push(chunk);
		buf = Buffer.concat(chunks);
	}

	/* The new OpenSSH internal format abuses PEM headers */
	if (alg && alg.toLowerCase() === 'openssh')
		return (sshpriv.readSSHPrivate(type, buf, options));
	if (alg && alg.toLowerCase() === 'ssh2')
		return (rfc4253.readType(type, buf, options));

	var der = new asn1.BerReader(buf);
	der.originalInput = input;

	/*
	 * All of the PEM file types start with a sequence tag, so chop it
	 * off here
	 */
	der.readSequence();

assert.number(count, 'iteration count');

	var clen = CIPHER_LEN[cipher];
	assert.object(clen, 'supported cipher');

	salt = salt.slice(0, PKCS5_SALT_LEN);

	var D, D_prev, bufs;
	var material = Buffer.alloc(0);
	while (material.length < clen.key + clen.iv) {
		bufs = [];
		if (D_prev)
			bufs.push(D_prev);
		bufs.push(passphrase);
		bufs.push(salt);
		D = Buffer.concat(bufs);
		for (var j = 0; j < count; ++j)
			D = crypto.createHash('md5').update(D).digest();
		material = Buffer.concat([material, D]);
		D_prev = D;
	}

	return ({
	    key: material.slice(0, clen.key),
	    iv: material.slice(clen.key, clen.key + clen.iv)
	});
}

function pbkdf2(hashAlg, salt, iterations, size, passphrase) {
	var hkey = Buffer.alloc(salt.length + 4);
	salt.copy(hkey);

	var gen = 0, ts = [];
	var i = 1;
	while (gen < size) {
		var t = T(i++);
		gen += t.length;
		ts.push(t);
	}
	return (Buffer.concat(ts).slice(0, size));

	function T(I) {
		hkey.writeUInt32BE(I, hkey.length - 4);

		var hmac = crypto.createHmac(hashAlg, passphrase);
		hmac.update(hkey);

		var Ti = hmac.digest();
		var Uc = Ti;
		var c = 1;
		while (c++ < iterations) {
			hmac = crypto.createHmac(hashAlg, passphrase);
			hmac.update(Uc);
			Uc = hmac.digest();
			for (var x = 0; x < Ti.length; ++x)
				Ti[x] ^= Uc[x];

assert.object(clen, 'supported cipher');

	salt = salt.slice(0, PKCS5_SALT_LEN);

	var D, D_prev, bufs;
	var material = Buffer.alloc(0);
	while (material.length < clen.key + clen.iv) {
		bufs = [];
		if (D_prev)
			bufs.push(D_prev);
		bufs.push(passphrase);
		bufs.push(salt);
		D = Buffer.concat(bufs);
		for (var j = 0; j < count; ++j)
			D = crypto.createHash('md5').update(D).digest();
		material = Buffer.concat([material, D]);
		D_prev = D;
	}

	return ({
	    key: material.slice(0, clen.key),
	    iv: material.slice(clen.key, clen.key + clen.iv)
	});
}

var parts = algInfo.parts;

	var buf = new SSHBuffer({});

	buf.writeString(alg);

	for (i = 0; i < parts.length; ++i) {
		var data = key.part[parts[i]].data;
		if (algInfo.normalize !== false) {
			if (key.type === 'ed25519')
				data = utils.zeroPadToLength(data, 32);
			else
				data = utils.mpNormalize(data);
		}
		if (key.type === 'ed25519' && parts[i] === 'k')
			data = Buffer.concat([data, key.part.A.data]);
		buf.writeBuffer(data);
	}

	return (buf.toBuffer());
}

assert.object(clen, 'supported cipher');

	salt = salt.slice(0, PKCS5_SALT_LEN);

	var D, D_prev, bufs;
	var material = Buffer.alloc(0);
	while (material.length < clen.key + clen.iv) {
		bufs = [];
		if (D_prev)
			bufs.push(D_prev);
		bufs.push(passphrase);
		bufs.push(salt);
		D = Buffer.concat(bufs);
		for (var j = 0; j < count; ++j)
			D = crypto.createHash('md5').update(D).digest();
		material = Buffer.concat([material, D]);
		D_prev = D;
	}

	return ({
	    key: material.slice(0, clen.key),
	    iv: material.slice(clen.key, clen.key + clen.iv)
	});
}

var cipherStream = crypto.createDecipheriv(cinf.opensslName,
		    ckey, iv);
		cipherStream.setAutoPadding(false);
		var chunk, chunks = [];
		cipherStream.once('error', function (e) {
			if (e.toString().indexOf('bad decrypt') !== -1) {
				throw (new Error('Incorrect passphrase ' +
				    'supplied, could not decrypt key'));
			}
			throw (e);
		});
		cipherStream.write(privKeyBlob);
		cipherStream.end();
		while ((chunk = cipherStream.read()) !== null)
			chunks.push(chunk);
		privKeyBlob = Buffer.concat(chunks);
		break;
	default:
		throw (new Error(
		    'OpenSSH-format key uses unknown KDF "' + kdf + '"'));
	}

	buf = new SSHBuffer({buffer: privKeyBlob});

	var checkInt1 = buf.readInt();
	var checkInt2 = buf.readInt();
	if (checkInt1 !== checkInt2) {
		throw (new Error('Incorrect passphrase supplied, could not ' +
		    'decrypt key'));
	}

	var ret = {};

Signer.prototype.sign = function () {
	var sig = nacl.sign.detached(
	    new Uint8Array(Buffer.concat(this.chunks)),
	    new Uint8Array(Buffer.concat([
		this.key.part.k.data, this.key.part.A.data])));
	var sigBuf = Buffer.from(sig);
	var sigObj = Signature.parse(sigBuf, 'ed25519', 'raw');
	sigObj.hashAlgorithm = 'sha512';
	return (sigObj);
};