Is Math.random() cryptographically secure?

How good are algorithms used in Javascript Math.random() in different browsers? Is it okay to use it for generating salts and one-time passwords?

How many bits from one random I can use?

Nope; JavaScript's Math.random() function is not a cryptographically-secure random number generator. You are better off using the JavaScript Crypto Library's Fortuna implementation which is a strong pseudo-random number generator (have a look at src/js/Clipperz/Crypto/PRNG.js), or the Web Crypto API for getRandomValues

• Here is a detailed explanation: How trustworthy is javascript's random implementation in various browsers?
• Here is how to generate a good crypto grade random number: Secure random numbers in javascript?

It is not secure at all, and in some cases was so predictable you could rebuild internal state of the PRNG, deduct the seed and thus could use it to track people across websites even if they didn't use cookies, hid behind onion routing etc...

2022 edit since this answer still gets upvotes: use Crypto.getRandomValues if you need a cryptographic RNG in JavaScript

• http://landing2.trusteer.com/sites/default/files/Temporary_User_Tracking_in_Major_Browsers.pdf a 2008 paper exposing the user tracking possibilities of the browser weak PRNG

• http://dl.packetstormsecurity.net/papers/general/Google_Chrome_3.0_Beta_Math.random_vulnerability.pdf a later (2009) Chrome vulnerability, as the problem was already well known

As of March 2013, window.crypto.getRandomValues is an "experimental technology" available since Chrome 11 and Firefox 21 that lets you get cryptographically random values. Also, see getRandomValues from the lastest W3C Web Cryptography API draft.

Description:

If you provide an integer-based TypedArray (i.e. Int8Array, Uint8Array, Int16Array, Uint16Array, Int32Array, or Uint32Array), the function is going fill the array with cryptographically random numbers. The browser is supposed to be using a strong (pseudo) random number generator. The method throws the QuotaExceededError if the requested length is greater than 65536 bytes.

Example:

var array = new Uint32Array(10);
window.crypto.getRandomValues(array);

console.log("Your lucky numbers:");
for (var i = 0; i < array.length; i++) {
    console.log(array[i]);
}

Also, an answer to How random is JavaScript's Math.random? refers to Temporary user tracking in major browsers and Cross-domain information leakage and attacks from 2008 which discusses how the JavaScript Math.random() function leaks information.

Update: For current browser support status, check out the Modern.IE Web Crypto API section, which also links to the Chrome, Firefox, and Safari bug reports.

Because you cannot know the exact implementation of the browser (except for closed user groups like for your business intranet) I would generally consider the RNG weak.

Even if you can identify the browser you don't know if the browser itself or any other browser's agent ID is manipulated. If you can you should generate the number on the server.

Even if you include a good PRNG in your JavaScript your server cannot know whether the request from the client originates from an unmodified script. If the number goes into your database and/or is used as a cryptographic tool it is no good idea to trust the data from the client at all. That is true not only for validity (You do validate all data coming from the client, don't you?) but also for general properties like randomness.