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Problems encoding Amazon Flexible Payments secret string in PHP

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By User


I am trying to use Amazon Payment Services, and they require me to do something like this:



Here is the complete signature so you can see I added the signature method: 





$string_to_sign = "GET\n

authorize.payments-sandbox.amazon.com\n

cobranded-ui/actions/start?

SignatureMethod=HmacSHA256&SignatureVersion=2&callerKey=my_key&callerReference=YourCallerReference&paymentReason=donation&pipelineName=SingleUse&returnUrl=http%3A%2F%2Fyourwebsite.com%2Freturn.html&transactionAmount=4.0";


and then I encrypt it like below. 





$encoded_string_to_sign = URLEncode(Base64_Encode(hash_hmac("sha256", $string_to_sign, 'my_secret_key')));


I do that, but then I get an error from them saying: 





Caller Input Exception: The following input(s) are either invalid or absent:[signatureMethod]


Any idea what might be going wrong here?



Here is the entire code for this: (the variables are assigned values above) 





<?php

$string_to_sign = 'GET

authorize.payments-sandbox.amazon.com/cobranded-ui/actions/startSignatureMethod=HmacSHA256&SignatureVersion=2&callerKey=AKIAJENBYSJCJX2IDWDQ&callerReference=YourCallerReference&paymentReason=donation&pipelineName=SingleUse&returnUrl=http%3A%2F%2Fproblemio.com&transactionAmount=4.0';



    $encoded_string_to_sign = URLEncode(Base64_Encode(hash_hmac("sha256", $string_to_sign, 'my_secret_key')));



$amazon_request_sandbox = 'https://authorize.payments-sandbox.amazon.com/cobranded-ui/actions/start?SignatureVersion=2&returnUrl='.$return_url.'&paymentReason='.$payment_reason.'&callerReference=YourCallerReference&callerKey='.$my_access_key_id.'&transactionAmount=4.0&pipelineName=SingleUse&SignatureMethod=HmacSHA256&Signature='.$encoded_string_to_sign;



//echo $amazon_request_sandbox; - use this if you want to see the resulting request and paste it into the browser



header('Location: '.$amazon_request_sandbox);

?>


Thanks!!


Source: Tips4all
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Comments

  1. UserMay 8, 2012 at 2:00 AM

    Check if you included &SignatureMethod=HmacSHA256 on the request

    This kind of errors has 3 basic natures:


    Missing Keys/Values
    Typos on Keys/Values
    Incorrect encoding or spaces on Keys/Values


    Hope that helps!

    Regards

    ReplyDelete
  2. UserMay 8, 2012 at 2:00 AM

    Have you set your signature method? from the AWS documentation:


    You must set the SignatureMethod request parameter to either
    HmacSHA256 or HmacSHA1 to indicate which signing method you're using

    ReplyDelete
  3. UserMay 8, 2012 at 2:00 AM

    I don't believe you need to base64 encode the hash (after all, it's already being urlencoded) -- try removing Base64_Encode.

    ReplyDelete
  4. UserMay 8, 2012 at 2:00 AM

    Your $string_to_sign variable is missing a '?' between start and SignatureMethod for your encoded Signature.


    Signature version 2 is an enhanced signing method for both Amazon
    Simple Pay and Amazon Flexible Payments Service.

    For inbound requests (from your application to Amazon Payments), it
    uses the entire request URI as the basis for the signature, with
    encryption based on the unique security credentials for your account.

    For outbound requests (from Amazon Payments to your application),
    Amazon signs the response which you can verify using the
    VerifySignature API


    EDIT:

    As @Jonathan Spooner mentioned already and what I use is the function varifySignature() located in


    /amazon-fps-2010-08-28-php5-library/src/Amazon/FPS/Samples/Client.php


    which can be downloaded here. It also has an example as to how to use it in


    /amazon-fps-2010-08-28-php5-library/src/Amazon/FPS/Samples/VerifySignatureSample.php


    It makes the whole process much easier. It may be worth a shot...

    ReplyDelete
  5. UserMay 8, 2012 at 2:00 AM

    Have you tried this

    base64_encode(hash_hmac('sha256', $Request, $AmazonSecretKey, true));

    Pass a boolean to pass it as a raw output.

    ReplyDelete
  6. UserMay 8, 2012 at 2:00 AM

    You're most definitely missing the last parameter for hash_hmac which has to be set true to get RFC 2104-compliant HMAC signature:

    base64_encode(
    hash_hmac($hash, $data, $key, true)
    );


    And in the complete example you're missing new lines in $string_to_sign.

    ReplyDelete

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