Decoding base64 input with a Buffer object

Question

I have a client interface that sends binary data encoded in base64. The data is a 29 byte custom formatted payload of bytes that describes an event. The 29 bytes is made up of a number of fields; each with unique lengths.

I need the server to decode that so I can extract the fields. I've tried using the Buffer object as I did with another use case where the data was passed in hex format, without success.

buff = new Buffer('AR0AAAEKCgsLDAwAAATSAAAADsgAAAAAAAAAzMQ=', 'base64');

// track the current position
// ... get out to the data portion of the message
var position = 3;

// event type
var event_type = buff.slice(position,(position+3)).toString('utf8');
position += 3;
console.log('... event type: ' + event_type + ' /');

// address
var addr = buff.slice(position,(position+3)).toString('utf8');
position += 3;
console.log('... addr: ' + addr + ' /');

The .toString('utf8') is likely the root cause. How can I get to a String that represents the desired bytes in these slice() calls?

I'm not storying binary data on the server as one might normally do with base64 encoding. So should I be going from base64 to some other encoding to access the individual bytes?

I've used the same code on a known text string that is encoded with base64 to verify the basic logic. But when the source data is binary before being encoded, the console statements don't print anything. Can I go from binary to base64 to strings?

What doesn't work? Also, is the data in the fields really utf8? — thejh
– thejh, Commented Nov 23, 2011 at 17:49
Nothing is printed in those console statements... I just updated the question to reflect the fact that I've verified the code path with a known text string that is encoded with base64. — Greg
– Greg, Commented Nov 23, 2011 at 17:51
Works for me (although there are weird bytes in your data, it's just whitespace and a onebyte) — thejh
– thejh, Commented Nov 23, 2011 at 18:06
One thing I've discovered along the way is that the parameters used in slice() are indexes - as opposed to byte offsets. So my current approach simply won't work. I need to learn how to transform the base64 data into a byte stream or array of hex values and chope those up to get down to the individual bytes within the data. — Greg
– Greg, Commented Nov 23, 2011 at 18:17

Teemu Ikonen · Accepted Answer · 2011-11-23 19:27:02Z

0

The data does not seem to have valid ASCII or UTF-8 strings, the event_type and addr is some binary data. Use hexadecimal presentation and print that.

var event_type = buff.slice(3,6).toString('hex');  => '00010a'

answered Nov 23, 2011 at 19:27

Teemu Ikonen

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Greg · Accepted Answer · 2011-11-23 21:46:10Z

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The problem with this implementation is the use of slice() to dissect the byte stream. The parameters to slice() are indexes - not byte offsets. This seems like a pretty big flaw. I'm not sure why anyone would do that when interacting with a buffer.

Regardless, the solution is to access the buff as an array.

// event type
var event_type = buff[position].toString('utf8');
position += 1;
console.log('... event type: ' + event_type + ' /');

// address
var addr = buff[position].toString('utf8');
position += 1;
console.log('... addr: ' + addr + ' /');

If you want to concatenate multiple bytes (the original example has 3 byte lengths), you have to loop over the array indexes.

answered Nov 23, 2011 at 21:46

Greg

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