How to use the algosdk.Seed.decode function in algosdk

signMessage(key: KeyPair, message: string | Buffer): Buffer {
    // key.prv actually holds the encoded seed, but we use the prv name to avoid breaking the keypair schema.
    // See jsdoc comment in isValidPrv
    let seed = key.prv;
    if (!this.isValidPrv(seed)) {
      throw new Error(`invalid seed: ${seed}`);
    }
    if (typeof seed === 'string') {
      try {
        seed = Seed.decode(seed).seed;
      } catch (e) {
        throw new Error(`could not decode seed: ${seed}`);
      }
    }
    const keyPair = generateAccountFromSeed(seed);

    if (!Buffer.isBuffer(message)) {
      message = Buffer.from(message);
    }

    return Buffer.from(NaclWrapper.sign(message, keyPair.sk));
  }

!_.has(params.txPrebuild, 'keys[0]') ||
      !_.has(params.txPrebuild, 'keys[1]') ||
      !_.has(params.txPrebuild, 'keys[2]')
    ) {
      throw new Error('missing public keys parameter to sign transaction');
    }

    if (!_.isNumber(addressVersion)) {
      throw new Error('missing addressVersion parameter to sign transaction');
    }

    // we need to re-encode our public keys using algosdk's format
    const keys = [params.txPrebuild.keys[0], params.txPrebuild.keys[1], params.txPrebuild.keys[2]];

    // re-encode sk from our prv (this acts as a seed out of the keychain)
    const seed = Seed.decode(prv).seed;
    const pair = generateAccountFromSeed(seed);
    const sk = pair.sk;

    return { txHex, addressVersion, keys, sk, isHalfSigned };
  }