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up vote 4 down vote favorite

I am current working with Google Apps script and am attempting to write & sign an HTTP request to AWS CloudWatch.

On the Amazon API documentation here regarding how to create a signing key, they use pseudo to explain that the HMAC algorithm is to return binary format.

HMAC(key, data) represents an HMAC-SHA256 function 
that returns output in binary format.

Google apps script offers a method to do such a hash,

Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
                                            data,
                                            key);

but the return type is always a byte array.

Byte[]

How do I convert the Byte[] to the binary data AWS wants? Or is there a vanilla javascript function I can use in Google Apps Script to compute the hash?

Thanks

share|improve this question
1  
Have you tried converting to String first (from Byte[]) and then, converting it to Binary would be easier? –  Rpranata Jun 18 '13 at 5:18
    
Yes I, have tried that by converting the Byte[] to a blob and then reading the data as string to be passed into the next iteration of hashing, but to no avail –  user2495753 Jun 18 '13 at 6:01

1 Answer 1

up vote 1 down vote accepted

The conversion from byte array to the binary data required should be simple:

kDate = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
             '20130618', 'AWS4' + kSecret);
kDate = Utilities.newBlob(kDate).getDataAsString();
kRegion = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, 
             'eu-west-1', kDate);

BUT you have to look onto this open issue in the bugtracker - there could be some issues in conversion.

maybe you could try to make a String.fromCharCode() loop and avoid negative numers:

kDateB = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
             '20130618', 'AWS4' + kSecret);
kDate = '';
for (var i=0; i<kDateB.length; i++)
  kDate += String.fromCharCode(kDateB[i]<0?256+kDateB[i]:0+kDateB[i]);
kRegion = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, 
             'eu-west-1', kDate);
share|improve this answer
    
Thank you for the reply. This is definitely frustrating. I made a comment on the bugtracker, so hopefully this will get the google team to at least respond and give an update to this known issue. –  user2495753 Jun 18 '13 at 8:30
    
did you try the ´String.fromCharCode` option? The problem seems to appear with negative Byte values. Adding 256 to these should solve the problem. –  Taras Jun 18 '13 at 8:54
    
I did try to do this and get rid of negative values by adding 256, but unfortunately I am still getting the incorrect result. Google has said that this is a known issue and is working on it, but they have also stated that there is no update to this bug at the current time. –  user2495753 Jun 20 '13 at 2:20
    
I managed to get the signature to work, but only by using CryptoJS which I imported into Google Script by copy and pasting the raw javascript code, but I still haven't managed to get pure Google code to work. –  user2495753 Jun 20 '13 at 5:04

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