How to properly sign a GET request to Amazon's ItemLookup using client-side JavaScript only?

Question

Here's what I have so far:

function sha256(stringToSign, secretKey) {
  return CryptoJS.HmacSHA256(stringToSign, secretKey);
} 

function getAmazonItemInfo(barcode) {
  
  var parameters = 
    "Service=AWSECommerceService&"
    + "AWSAccessKeyId=" + appSettings.amazon.accessKey + "&"
    + "Operation=ItemLookup&"
    + "ItemId=" + barcode
    + "&Timestamp=" + Date.now().toString();

  var stringToSign =
    "GET\n"
    + "webservices.amazon.com\n"
    + "/onca/xml\n"
    + parameters;

  var signature = "&Signature=" + encodeURIComponent(sha256(stringToSign, appSettings.amazon.secretKey));

  var amazonUrl =  
    "http://webservices.amazon.com/onca/xml?"
    + parameters
    + signature;

  // perform a GET request with amazonUrl and do other stuff

}

When executed as an HTTP GET request, the value of amazonUrl in the above code results in the following response from Amazon:

<?xml version="1.0"?> 
  <ItemLookupErrorResponse xmlns="http://ecs.amazonaws.com/doc/2005-10-05/">
    <Error>
      <Code>SignatureDoesNotMatch</Code>
      <Message>
        The request signature we calculated does not match the signature you provided. 
        Check your AWS Secret Access Key and signing method. Consult the service 
        documentation for details.
      </Message>
   </Error>
   <RequestId>[REMOVED]</RequestId>
  </ItemLookupErrorResponse>

Useful links:

• ItemLookup - Product Advertising API Amazon Documentation

• Example REST Requests

• AWS Authentication Process

• CryptoJS

Answer 1

I hacked around with your code and I got it working.

function sha256(stringToSign, secretKey) {
  var hex = CryptoJS.HmacSHA256(stringToSign, secretKey);
  return hex.toString(CryptoJS.enc.Base64);
} 

function timestamp() {
    var date = new Date();
    var y = date.getUTCFullYear().toString();
    var m = (date.getUTCMonth() + 1).toString();
    var d = date.getUTCDate().toString();
    var h = date.getUTCHours().toString();
    var min = date.getUTCMinutes().toString();
    var s = date.getUTCSeconds().toString();

    if(m.length < 2) { m = "0" + m; }
    if(d.length < 2) { d = "0" + d; }
    if(h.length < 2) { h = "0" + h; }
    if(min.length < 2) { min = "0" + min; }
    if(s.length < 2) { s = "0" + s}

    var date = y + "-" + m + "-" + d;
    var time = h + ":" + min + ":" + s;
    return date + "T" + time + "Z";
}

function getAmazonItemInfo(barcode) {
    var PrivateKey = "";
    var PublicKey = "";
    var AssociateTag = "";

    var parameters = [];
    parameters.push("AWSAccessKeyId=" + PublicKey);
    parameters.push("ItemId=" + barcode);
    parameters.push("Operation=ItemLookup");
    parameters.push("Service=AWSECommerceService");
    parameters.push("Timestamp=" + encodeURIComponent(timestamp()));
    parameters.push("Version=2011-08-01");
parameters.push("AssociateTag=" + AssociateTag);

    parameters.sort();
    var paramString = parameters.join('&');

    var signingKey = "GET\n" + "webservices.amazon.com\n" + "/onca/xml\n" + paramString

    var signature = sha256(signingKey,PrivateKey);
        signature = encodeURIComponent(signature);

    var amazonUrl =  "http://webservices.amazon.com/onca/xml?" + paramString + "&Signature=" + signature;
    console.log(amazonUrl);
}

The Header of the Javascript I used for some reference.

<script src="hmac-sha256.js"></script>
<script src="http://crypto-js.googlecode.com/svn/tags/3.0.2/build/components/enc-base64-min.js"></script>
<script src="amazon.js"></script>

You will need to modify parts of it because I changed some parameters around and don't reference your "app" object.

For what I did to fix it (from what I can recall).

1. The parameters have to be alphabetical. I placed them in an array and then sort them. I follow this up by a join with the ampersand.

2. I modified the sha256 function to return the base64 of the RAW sha256. Before it was returning the hexbits in lowercase, which isn't correct.

3. I was going to add a base64 before encoding, but the sha256 now handles all of the signing.

4. The date format was incorrect. It was returning a epoch timestamp instead of a string timestamp. I hacked together a simple timestamp option.

   This code requires you to include the Base64 Library for CryptoJS also.

Answer 2

Use this Node.js library for AWS. It even includes an example specifically for the Product Advertising API.

Answer 3

Building on David's great answer, I made some tweaks. The solution below uses moment.js and crytpo-js, and can be used to search for items by keyword. I used the amazon scratch-pad to help build the target call. A couple of things I noticed:

• The scratch-pad needs to use the same location as your associates account, ".com" ".co.uk", etc.
• The end point you call to needs to be the same country as your associates account.
• The time-stamp you use needs to match the local time in the country your associates account is registered.

const getAmazonItemInfo = (keywords) => {

  let date = moment().startOf().add(-9, 'hours').format("YYYY-MM-DDThh:mm:ss.000") + 'Z'
  let SecretKey = "GENERATED_IN_AFFILATES_ACCOUNT";
  let AccessKey = "GENERATED_IN_AFFILATES_ACCOUNT";
  let AssociateTag = "FOUND_IN_AFFILATES_ACCOUNT";
  let parameters = [];
  let url = 'webservices.amazon.co.uk' // UK account
  //let url = 'webservices.amazon.com'// US account

  parameters.push("AWSAccessKeyId=" + AccessKey);
  parameters.push("Keywords=" + keywords);
  parameters.push("Operation=ItemSearch");
  parameters.push("SearchIndex=All");
  parameters.push("ResponseGroup=" + encodeURIComponent('Images,ItemAttributes,Offers'));
  parameters.push("Service=AWSECommerceService");
  parameters.push("Timestamp=" + encodeURIComponent(date));
  parameters.push("AssociateTag=" + AssociateTag);
  parameters.sort();

  let paramString = parameters.join('&');
  let string_to_sign = "GET\n" + url + "\n" + "/onca/xml\n" + paramString

  let signature = CryptoJS.HmacSHA256(string_to_sign, SecretKey);
  signature = CryptoJS.enc.Base64.stringify(signature);

  let amazonUrl = "http://" + url + "/onca/xml?" + paramString + "&Signature=" + signature;
  return amazonUrl;
}

let keywords = 'iphone'
console.log(getAmazonItemInfo(keywords))