Amazonの製品カタログデータにアクセスできる公式API「Product Advertising API」(以下、PA-API)のバージョン5.0が昨年公開され、来月にも完全移行(旧バージョンでの利用が不可になる)が予定されています。

ブログ等を書く際の使い勝手向上のため自分本位に開発・公開しているChrome拡張「Associates Link Generator」も今のままでは使用出来なくなってしまうので新版リリースの準備を始めたのですが、Amazonが用意してくれているScratchpad(≠実働サンプル)にはJAVA、PHP、cURLあたりの実装例しか載っていなかったのでPHP版を参考にChrome拡張で使えるJavaScriptのClassを作成。

お約束ですが…参考にされる場合、自己責任でお願いします。(フォローやサポートは致しかねます)

'use strict';

export class AwsV4 {
  constructor(accessKey, secretKey) {
    this.accessKey       = accessKey;
    this.secretKey       = secretKey;
    this.path            = null;
    this.regionName      = null;
    this.serviceName     = null;
    this.httpMethodName  = null;
    this.queryParametes  = [];
    this.awsHeaders      = [];
    this.payload         = "";

    this.HMACAlgorithm   = 'AWS4-HMAC-SHA256';
    this.aws4Request     = 'aws4_request';
    this.strSignedHeader = null;
    this.xAmzDate        = this._getTimeStamp();
    this.currentDate     = this._getDate();
  }

  setPath(path) {
    this.path = path;
  }

  setServiceName(serviceName) {
    this.serviceName = serviceName;
  }

  setRegionName(regionName) {
    this.regionName = regionName;
  }

  setPayload(payload) {
    this.payload = payload;
  }

  setRequestMethod(method) {
    this.httpMethodName = method;
  }

  addHeader(headerName, headerValue) {
    this.awsHeaders[headerName] = headerValue;
  }

  _prepareCanonicalRequest() {
    let canonicalURL = '';
    canonicalURL += this.httpMethodName + "\n";
    canonicalURL += this.path + "\n" + "\n";
    let signedHeaders = '';
    for (let key in this.awsHeaders) {
      signedHeaders += key + ";";
      canonicalURL  += key + ":" + this.awsHeaders[key] + "\n";
    }
    canonicalURL += "\n";
    this.strSignedHeader = signedHeaders.slice(0, - 1);
    canonicalURL += this.strSignedHeader + "\n";
    canonicalURL += this._generateHex(this.payload);
    return canonicalURL;
  }

  _prepareStringToSign(canonicalURL) {
    let stringToSign = '';
    stringToSign += this.HMACAlgorithm + "\n";
    stringToSign += this.xAmzDate + "\n";
    stringToSign += this.currentDate + "/" + this.regionName + "/" + this.serviceName + "/" + this.aws4Request + "\n";
    stringToSign += this._generateHex(canonicalURL);
    return stringToSign;
  }

  _calculateSignature(stringToSign) {
    const signatureKey    = this._getSignatureKey (this.secretKey, this.currentDate, this.regionName, this.serviceName);
    const signature       = this._hash_hmac(stringToSign, signatureKey);
    const strHexSignature = CryptoJS.enc.Hex.stringify(signature).toLowerCase();
    return strHexSignature;
  }

  _ksort(obj) {
    const keys = Object.keys(obj).sort();
    let sortedObj = {};
    for (let i in keys) {
      sortedObj[keys[i]] = obj[keys[i]];
    }
    return sortedObj;
  }

  getHeaders() {
    this.awsHeaders['x-amz-date'] = this.xAmzDate;
    this.awsHeaders = this._ksort(this.awsHeaders);

    const canonicalURL = this._prepareCanonicalRequest();
    const stringToSign = this._prepareStringToSign(canonicalURL);
    const signature    = this._calculateSignature(stringToSign);

    if (signature) {
      this.awsHeaders ['Authorization'] = this._buildAuthorizationString(signature);
      return this.awsHeaders;
    }
  }

  _buildAuthorizationString($strSignature) {
    return this.HMACAlgorithm + " " + "Credential=" + this.accessKey + "/" + this._getDate () + "/" + this.regionName + "/" + this.serviceName + "/" + this.aws4Request + "," + "SignedHeaders=" + this.strSignedHeader + "," + "Signature=" + $strSignature;
  }

  _generateHex(data) {
    return CryptoJS.enc.Hex.stringify(CryptoJS.SHA256(data)).toLowerCase();
  }

  _hash_hmac(s, k) {
    return CryptoJS.HmacSHA256(s, k);
  }

  _getSignatureKey(key, date, regionName, serviceName) {
    const kSecret  = "AWS4" + key;
    const kDate    = this._hash_hmac(date, kSecret);
    const kRegion  = this._hash_hmac(regionName, kDate);
    const kService = this._hash_hmac(serviceName, kRegion);
    const kSigning = this._hash_hmac(this.aws4Request, kService);
    return kSigning;
  }

  _getTimeStamp() {
    return new Date().toISOString().replace(/-|:|\.[0-9]{3}/g, "");
  }

  _getDate() {
    return new Date().toISOString().replace(/-/g, "").substr(0,8);
  }
}

動作には別途CryptoJSが必要です。