Web/API/Crypto/getRandomValues

The Crypto.getRandomValues() method lets you get cryptographically strong random values. The array given as the parameter is filled with random numbers (random in its cryptographic meaning).

To guarantee enough performance, implementations are not using a truly random number generator, but they are using a pseudo-random number generator seeded with a value with enough entropy. The pseudo-random number generator algorithm (PRNG) may vary across user agents, but is suitable for cryptographic purposes. Implementations are required to use a seed with enough entropy, like a system-level entropy source.

getRandomValues() is the only member of the Crypto interface which can be used from an insecure context.

Syntax

typedArray = cryptoObj.getRandomValues(typedArray);

Parameters

typedArray: An integer-based TypedArray, that is an Int8Array, a Uint8Array, an Int16Array, a Uint16Array, an Int32Array, or a Uint32Array. All elements in the array are overwritten with random numbers.

Return value

The same array passed as typedArray but with its contents replaced with the newly generated random numbers. Note that typedArray is modified in-place, and no copy is made.

Exceptions

This method can throw an exception under error conditions.

DOMException (name: QuotaExceededError); The requested length exceeds 65,536 bytes.

Usage notes

Don't use getRandomValues() to generate encryption keys. Instead, use the generateKey() method. There are a few reasons for this; for example, getRandomValues() is not guaranteed to be running in a secure context.

There is no minimum degree of entropy mandated by the Web Cryptography specification. User agents are instead urged to provide the best entropy they can when generating random numbers, using a well-defined, efficient pseudorandom number generator built into the user agent itself, but seeded with values taken from an external source of pseudorandom numbers, such as a platform-specific random number function, the Unix /dev/urandom device, or other source of random or pseudorandom data.

Examples

/* Assuming that window.crypto.getRandomValues is available */

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]);
}

Specification

Specification	Status	Comment
Web Cryptography API	Recommendation	Initial definition

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`getRandomValues()`	Chrome Full support 11	Edge Full support 12	Firefox Full support 26	IE Full support 11	Opera Full support 15	Safari Full support 6.1	WebView Android Full support ≤37	Chrome Android Full support 18	Firefox Android Full support 26	Opera Android Full support 14	Safari iOS Full support 6.1	Samsung Internet Android Full support 1.0

Legend

Full support: Full support