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Problems encoding Amazon Flexible Payments secret string in PHP

$string_to_sign = "GETn

authorize.payments-sandbox.amazon.comn

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";

       

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

       

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

       

<?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);

?>

       

$return_url = rawurlencode('http://problemio.com');

$payment_reason = 'donation';

$transaction_amount = rawurlencode('4.0');

 

$secret_key = '<Your_Secret_Key>';

$my_access_key_id = '<Your_Access_Key>';

 

$string_to_sign = 'GET

authorize.payments-sandbox.amazon.com

/cobranded-ui/actions/start

SignatureMethod=HmacSHA256&SignatureVersion=2&callerKey=' . $my_access_key_id . '&callerReference=YourCallerReference&paymentReason=' . $payment_reason . '&pipelineName=SingleUse&returnUrl=' . $return_url . '&transactionAmount=' . $transaction_amount;

 

$encoded_string_to_sign = URLEncode(Base64_Encode(hash_hmac("sha256", $string_to_sign, $secret_key, true)));

 

$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;

       

<?php

 

require_once 'CBUISingleUsePipeline.php';

require_once 'CBUIPipeline.php';

 

$secret_key = '<Your_Secret_Key>';

$my_access_key_id = '<Your_Access_Key>';

 

$return_url = 'http://problemio.com';

$transaction_amount = '4.0';

$caller_reference = '<Your_Caller_Reference>';

$payment_reason = 'donation';

 

$base = 'https://authorize.payments-sandbox.amazon.com/cobranded-ui/actions/start';

 

$pipeline = new Amazon_FPS_CBUISingleUsePipeline($my_access_key_id, $secret_key);

$pipeline->setMandatoryParameters($caller_reference, $return_url, $transaction_amount);

$pipeline->addParameter('paymentReason', $payment_reason);

$uRL = $pipeline->getURL($base);

 

?>

       

base64_encode(

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

);