How to use the fast-srp-hap.Client function in fast-srp-hap

private completePairing(pin: string): Promise {
    this.srp = srp.Client(
      srp.params['3072'],
      this.deviceSalt,
      Buffer.from('Pair-Setup'),
      Buffer.from(pin),
      this.key
    );
    this.srp.setB(this.devicePublicKey);
    this.publicKey = this.srp.computeA();
    this.proof = this.srp.computeM1();

    // console.log("DEBUG: Client Public Key=" + this.publicKey.toString('hex') + "\nProof=" + this.proof.toString('hex'));

    let that = this;
    let tlvData = tlv.encode(
      tlv.Tag.Sequence, 0x03,
      tlv.Tag.PublicKey, that.publicKey,

getM3Request() {
    const identity = Buffer.from('Pair-Setup');
    const password = Buffer.from(this._pin);  // Accessory pin

    this._srp = new srp.Client(this._params, this._salt, identity, password, this._key);
    this._srp.setB(this._accessoryPublicKey);

    this._rangerPublicKey = this._srp.computeA();
    this._rangerProof = this._srp.computeM1();

    const tlv = {};
    tlv[TLVType.State] = Buffer.from([this._state]);
    tlv[TLVType.PublicKey] = this._rangerPublicKey;
    tlv[TLVType.Proof] = this._rangerProof;

    const payload = {};
    payload[TlvKeys.Value] = TLV8Encoder.encode(tlv);
    payload[TlvKeys.ReturnResponse] = new Buffer([1]);

    return {
      address: this._address,

public async finishPairing (pin: string) {
        // Stage 1 response
        const { pk: serverPk
              , salt: serverSalt } = await this.pairSetupPin1();

        // SRP params must 2048-bit SHA1
        const srpParams = srp6a.params[2048];
        srpParams.hash = "sha1";

        // Create SRP client
        const srpClient = new srp6a.Client(
                srpParams                                // Params
              , serverSalt                               // Receiver salt
              , Buffer.from(this.credentials.clientId)   // Username
              , Buffer.from(pin)                         // Password (receiver pin)
              , Buffer.from(this.credentials.clientSk)); // Client secret key

        // Add receiver's public key
        srpClient.setB(serverPk);

        // Stage 2 response
        await this.pairSetupPin2(
                srpClient.computeA()    // SRP public key
              , srpClient.computeM1()); // SRP proof

        // Stage 3 response
        await this.pairSetupPin3(srpClient.computeK());