How to get a BigInteger from an array of bytes in Javascript

Question

Here is my problem:

I have a base64 encoded string, a very long one (344 characters).
When I decode this string I can obtain an array of bytes.

Let's say my base64 encoded string is ALVuSLbT.
Decoded it gives me the following array: [0, 181, 110, 72, 182, 211].

And I need to obtain this number: 779239339731 (the original encoded value).
I know how to do this manually.
000000000000000000000000000000000000000000000000 000000001011010100000000000000000000000000000000 000000000000000001101110000000000000000000000000 000000000000000000000000010010000000000000000000 000000000000000000000000000000001011011000000000 000000000000000000000000000000000000000011010011

With this simple example I could easily use parseInt(binaryString, 2) and add numbers together. But my original string is too big for this.

And I want to get a big number that can convert this number to a string ('779239339731').
I didn't find a way to do this or a BigInteger/BigNumber javascript library that allows me to pass an array of bytes to create the BigInteger object.

You can check this jsfiddle.

Can someone help me with this? Is there a way to handle such a thing in Javascript?

Ondrej Svejdar · Accepted Answer · 2013-06-18 15:04:19Z

1

I don't get why bother parsing - using the BigInteger library https://rawgithub.com/silentmatt/javascript-biginteger/master/biginteger.js

Working jsFiddle example - http://jsfiddle.net/svejdo1/sdV8L/

var exponentB64 = 'ALVuSLbT';
var exponentBytes = base64_decode(exponentB64);
var result = new BigInteger(0);
var multiplier = new BigInteger(1);
for(var i = exponentBytes.length - 1; i >= 0; i--) {
    result = result.add(multiplier.multiply(exponentBytes[i]));
    multiplier = multiplier.multiply(256);
}
document.write(result.toString());


function base64_decode(base64String) {
  var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
  var h1, h2, h3, h4, o1, o2, o3, bits, i = 0, bytes = [];

  do {
    h1 = b64.indexOf(base64String.charAt(i++));
    h2 = b64.indexOf(base64String.charAt(i++));
    h3 = b64.indexOf(base64String.charAt(i++));
    h4 = b64.indexOf(base64String.charAt(i++));

    bits = h1 << 18 | h2 << 12 | h3 << 6 | h4;

    o1 = bits >> 16 & 0xff;
    o2 = bits >> 8 & 0xff;
    o3 = bits & 0xff;

    bytes.push(o1);
    bytes.push(o2);
    bytes.push(o3);
  } while (i < base64String.length);

  return bytes;
}

edited Jun 18, 2013 at 15:04

answered Jun 18, 2013 at 14:54

Ondrej Svejdar

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2 Comments

grandouassou Over a year ago

Oh it's that easy? Let's use some real world example now: encoded string 'AK2/ss9tBm5mc4wThXztiHh5ApZDjkYA4//8vUtYtsCD6sUQ/GIoNwy7qIm4YIrr68c479nWXmIyhDVMuHZk4/Bque4xdPkNYMz5agOTweVa+NWuvWIivAZ34rHCzOVtM/Klkg/g7d0dIzYOkPfQmweyx1Bmdh2HVaKwmngvzb70d0cr7LN3YL6aDeg++w0d99INxEWMDb8s0VPVx5BHC0iOTS34JvB4vicxm2yfxzr6Y4cOJDfp1zoReogixp+zxljcMQ16dJly52tqBQH1lT8NWEtiAw8karYMRCBNKl9dn9d7FhopaAsJgZAHmZBuD6hi6IUZ6EHsUNm07xyU8S8=' expected decoded number 219337568015997254830809010245695848800620186109619871851429669559883715518752323465558437623481096892170... "Your message is too long by 461 characters..."

Ondrej Svejdar Over a year ago

I'm sorry - but this is your fault; your string contains invisible empty unicode characters 8204 and 8203; you said the string is base64, but it is not. Also down voting answers, which are providing correct output for given data - way to go (I even tested in C# using Microsoft System.Numeric.BigInteger with the same results).

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