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index.html: generate unique pad names 

Etherpad-lite relies on the user's browser to generate a random pad
name, but the current solution is not safe against collisions. In order
to generate unique pad names, the following modifications are made:

* use a PRNG instead of Math.random() and ensure uniform distribution
  when selecting chars.

* choose the pad name length to achieve a specific number of bits of
  security.

Closes: #3516
pull/3519/head
drebs 2018-11-23 08:18:03 -02:00 committed by muxator
parent 7df26840cb
commit 39fbc37dd8
1 changed files with 14 additions and 3 deletions
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17
src/templates/index.html
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@ -187,12 +187,23 @@
function randomPadName()
{
var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
var string_length = 10;
// the number of distinct chars (64) is chosen to ensure that
// the selection will be uniform when using the PRNG below
var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_";
// the length of the pad name is chosen to get 120-bit security:
// log2(64^20) = 120
var string_length = 20;
// make room for 8-bit integer values that span from 0 to 255.
var randomarray = new Uint8Array(string_length);
// use browser's PRNG to generate a "unique" sequence
var cryptoObj = window.crypto || window.msCrypto; // for IE 11
cryptoObj.getRandomValues(randomarray);
var randomstring = '';
for (var i = 0; i < string_length; i++)
{
var rnum = Math.floor(Math.random() * chars.length);
// instead of writing "Math.floor(randomarray[i]/256*64)"
// we can save some cycles.
var rnum = Math.floor(randomarray[i]/4);
randomstring += chars.substring(rnum, rnum + 1);
}
return randomstring;