How to sign Amazon Simple Pay button in node.js / Javascript - node.js

Question

I've spent the past 6 hours trying to follow Amazon's instructions for signing a Simple Pay button form in node.js. I can't get it to the accept the sig and I've tried every permutation of the confusing instructions. Can anyone help me out of my misery?
The error I get is
Input parameter signature is invalid
Here is my procedure
var params={
"returnUrl": "[confidential]",
"ipnUrl": "[confidential]",
"processImmediate": "1",
"accessKey" :"[AWS key]",
"collectShippingAddress" :"0",
"isDonationWidget" :"0",
"amazonPaymentsAccountId" :"[the button generator creates this but there is no mention in the docs]",
"referenceId" :ref,
"cobrandingStyle" :"logo",
"immediateReturn" :"1",
"amount" :"USD "+amount,
"description" : desc,
"abandonUrl" :"[confidential]",
"signatureMethod": "HmacSHA256", //docs not clear if signatureMethod and signatureVersion should be included, but I've tried all permutations and can't get it to work
"signatureVersion" :"2"
}
//Docs say it should confirm to
/*
StringToSign = HTTPVerb + "\n" +
ValueOfHostHeaderInLowercase + "\n" +
HTTPRequestURI + "\n" +
CanonicalizedQueryString <from the preceding step>
*/
//sort parameters (natural byte order)
var p=_.pairs(params);
var psorted=_.sortBy(p, function(p) { return p[0];});
//start to construct the form
var input='';
for(var i=0; i<psorted.length;i++) {
input+="<input type='hidden' name='"+psorted[i][0]+"' value='"+psorted[i][1]+"'>";
}
//prepare the string to be signed
var qstring='POST'+'\n';
qstring+='https://authorize.payments.amazon.com'+'\n';
qstring+='/pba/paypipeline'+'\n';
for(var i=0; i<psorted.length;i++) {
psorted[i][0]=encodeURI(psorted[i][0]);
psorted[i][1]=encodeURI(psorted[i][1]);
qstring+=psorted[i][0]+'='+psorted[i][1];
if (i<psorted.length-1) {qstring+='&';}
};
console.log(qstring+'\n\n');
var sig=crypto.createHmac("SHA256", "[AWS Secret Key") 6
.update(qstring)
.digest('base64');
input+="<input type='hidden' name='signature' value='"+sig+"'>"; //doesn't matter whether or not i url encode this
console.log(input);
That converts the parameters into
POST
authorize.payments.amazon.com
/pba/paypipeline
abandonUrl=XXXX&accessKey=XXXXX&amazonPaymentsAccountId=XXXXXX&amount=USD%203&cobrandingStyle=logo&collectShippingAddress=0&description=sadasdd&immediateReturn=1&ipnUrl=XXXXXx&isDonationWidget=0&processImmediate=1&referenceId=324324&returnUrl=XXXXXXXX&signatureMethod=HmacSHA256&signatureVersion=2
I concatenate and paste the the output to this form for testing
<form action="https://authorize.payments.amazon.com/pba/paypipeline" method="POST">
<input type='hidden' name='abandonUrl' value='[confidential]'>
<input type='hidden' name='accessKey' value='[confidential]'>
<input type='hidden' name='amazonPaymentsAccountId' value='[confidential]'>
<input type='hidden' name='amount' value='USD 3'>
<input type='hidden' name='cobrandingStyle' value='logo'>
<input type='hidden' name='collectShippingAddress' value='0'>
<input type='hidden' name='description' value='sadasdd'>
<input type='hidden' name='immediateReturn' value='1'>
<input type='hidden' name='ipnUrl' value='[confidential]'>
<input type='hidden' name='isDonationWidget' value='0'>
<input type='hidden' name='processImmediate' value='1'>
<input type='hidden' name='referenceId' value='324324'>
<input type='hidden' name='returnUrl' value='[confidential]'>
<input type='hidden' name='signatureMethod' value='HmacSHA256'>
<input type='hidden' name='signatureVersion' value='2'>
<input type='hidden' name='signature' value='fHSA+p37r5ooOJOUnjYBdhNFe/pAEg/KunAEOudUvGs='>
<input type="submit">
</form>
Here are the Amazon docs
http://docs.aws.amazon.com/AmazonSimplePay/latest/ASPAdvancedUserGuide/Sig2CreateSignature.html
How to Generate a Signature
To create the signature
Create the canonicalized query string that you need later in this
procedure:
Sort the UTF-8 query string components by parameter name with natural
byte ordering.
The parameters can come from the GET URI or from the POST body (when
Content-Type is application/x-www-form-urlencoded).
URL encode the parameter name and values according to the following
rules:
Do not URL encode any of the unreserved characters that RFC 3986
defines.
These unreserved characters are A-Z, a-z, 0-9, hyphen ( - ),
underscore ( _ ), period ( . ), and tilde ( ~ ).
Percent encode all other characters with %XY, where X and Y are hex
characters 0-9 and uppercase A-F.
Percent encode extended UTF-8 characters in the form %XY%ZA....
Percent encode the space character as %20 (and not +, as common
encoding schemes do).
Note Currently all AWS service parameter names use unreserved
characters, so you don't need to encode them. However, you might want
to include code to handle parameter names that use reserved
characters, for possible future use. Separate the encoded parameter
names from their encoded values with the equals sign ( = ) (ASCII
character 61), even if the parameter value is empty.
Separate the name-value pairs with an ampersand ( & ) (ASCII code 38).
Create the string to sign according to the following pseudo-grammar
(the "\n" represents an ASCII newline).
StringToSign = HTTPVerb + "\n" + ValueOfHostHeaderInLowercase + "\n" +
HTTPRequestURI + "\n" + CanonicalizedQueryString The HTTPRequestURI component is the HTTP absolute path
component of the URI up to, but not including, the query string. If
the HTTPRequestURI is empty, use a forward slash ( / ).
Calculate an RFC 2104-compliant HMAC with the string you just created,
your Secret Access Key as the key, and SHA256 or SHA1 as the hash
algorithm.
For more information, go to http://www.ietf.org/rfc/rfc2104.txt.
Convert the resulting value to base64.
Use the resulting value as the value of the Signature request
parameter.
Important The final signature you send in the request must be URL
encoded as specified in RFC 3986 (for more information, go to
http://www.ietf.org/rfc/rfc3986.txt). If your toolkit URL encodes your
final request, then it handles the required URL encoding of the
signature. If your toolkit doesn't URL encode the final request, then
make sure to URL encode the signature before you include it in the
request. Most importantly, make sure the signature is URL encoded only
once. A common mistake is to URL encode it manually during signature
formation, and then again when the toolkit URL encodes the entire
request. For information on the high-level process for creating a
button, see Creating Button Forms Dynamically .
In the following examples, new lines have been inserted to make the
examples easier to read. Explicit '\n' is used wherever new line is
required.
The following is an example Amazon Simple Pay request using POST.
The following is an example of a string to use for
StringToSign in the preceding example.
POST\n authorize.payments-sandbox.amazon.com\n /pba/paypipeline\n
SignatureMethod=HmacSHA256 &SignatureVersion=2
&accessKey=YourCallerKey &amount=USD%201.1 &cobrandingStyle=logo
&description=Test%20Widget &immediateReturn=0
&ipnUrl=http%3A%2F%2Fyourwebsite.com%2Fipn &processImmediate=1
&referenceId=YourReferenceId
&returnUrl=http%3A%2F%2Fyourwebsite.com%2Freturn.html For more
examples of generating a signature, see Appendix: Sample Code.
For information on signing your button form correctly, see How to Sign
your Button Form Correctly.

Answer 1

For posterity, here is the solution:
1) Don't mess with Simple Pay Buttons - use FPS instead
2) Of the myriad overlapping documents, I found this to be the simplest and clearest: http://docs.aws.amazon.com/AmazonFPS/latest/FPSBasicGuide/SendingaCBUIRequest.html
3) Use encodeURIComponent not encodeURI - this was my biggest most frustrating mistake
This code will correctly sign an Amazon FPS request (assumes crypto for hmac and nconf for configuration)
var crypto = require('crypto');
var _ = require('underscore');
var nconf = require('nconf').argv().env().file({
file: "./config.json"
});
exports.azPayRequest=function (amount, desc,ref) {
var params={
"returnUrl": nconf.get("awsPayments:returnURL"), //callback
"callerKey" : nconf.get("awsPayments:callerKey"), //aws id
"callerReference": ref,
"pipelineName":"SingleUse",
"cobrandingStyle" :"logo",
"currencyCode" :"USD",
"transactionAmount" : amount,
"paymentReason" : desc,
"signatureMethod": "HmacSHA256",
"signatureVersion" :"2"
}
/*
StringToSign = HTTPVerb + "\n" +
ValueOfHostHeaderInLowercase + "\n" +
HTTPRequestURI + "\n" +
CanonicalizedQueryString <from the preceding step>
*/
//sort parameters
var p=_.pairs(params);
var psorted=_.sortBy(p, function(p) { return p[0];});
//method, host, path
var method='GET';
var host=nconf.get('awsPayments:host'); // e.g., authorize.payments.amazon.com;
var path=nconf.get('awsPayments:path'); //e.g. /cobranded-ui/actions/start;
//url encode parameters
var qstring='';
for(var i=0; i<psorted.length;i++) {
psorted[i][0]=encodeURIComponent(psorted[i][0]);
psorted[i][1]=encodeURIComponent(psorted[i][1]);
qstring+=psorted[i][0]+'='+psorted[i][1];
if (i<psorted.length-1) {qstring+='&';}
};
//calculate hmac
var nl=String.fromCharCode(10);
var encode_request=method+nl+host+nl+path+nl+qstring;
console.log("STRING TO ENCODE\n"+encode_request+'\n\n');
var sig=crypto.createHmac("SHA256", nconf.get("awsPayments:awsSecretAccessKey"))
.update(encode_request)
.digest('base64');
var url="https://"+host+path+"?"+qstring+'&signature='+encodeURIComponent(sig);
return url;
}