Tutorial: Threaded RSA Functionality

Threaded RSA Functionality

Threaded RSA Key Generation and Decryption

View the RSA demo page

CowCrypt supports RSA encryption / decryption, as well as a FIPS 186-4 compliant secure key generator. RSA cryptography involves mathematics with very large prime numbers. Depending on the browser and CPU, these operations can be resource-intensive (slow). Because JavaScript is typically single-threaded, executing these operations synchronously can cause the browser to become unresponsive while it waits for the functions to complete. This can lead to an apparently hung browser, or the dreaded "a script on this page is taking too long" warning box.

Fortunately, many modern browsers support the Web Workers API, a new part of the HTML 5 spec. This allows us to execute resource-intensive operations in a separate thread, which runs asynchronously and "phones home" to the main browser thread when the work is complete. Using Web Workers, CowCrypt can do expensive RSA operations in the background without impacting perceived browser performance or responsiveness.

Table of Contents


  • Browser support for the Web Workers API
  • Browser support for window.crypto.getRandomValues (for key generation)
  • CowCrypt's crypto_math.js (for key generation), plus rsa.js and convert.js (for encryption / decryption)

Generating an RSA key

An RSA key is built up from several values. We'll generate all of these:

  • e: The public exponent
  • p: First private prime factor
  • q: Second private prime factor
  • n: The public modulus, the product of p × q. (The security of RSA depends on it being difficult to factor p and q out of this number)
  • Φ(n): (phi of n) The product of (p - 1) × (q - 1). Used to compute d
  • u: Multiplicative inverse of the lesser(p, q) mod greater(p, q)
  • d: The private exponent

Generate the public exponent e

First, we generate our public exponent e. This can be any odd integer greater than 2^16 and less than 2^256. It doesn't have to be random (65537 will do just fine), but if you want a random value, CowCrypt has you covered:


// Generate random public exponent
var e = crypto_math.get_random_public_exponent();


This returns a BigInt value (not an integer). If you're wondering what the hell that is, read on.

A note on BigInt values

RSA uses huge integers, sometimes upwards of 2048 bits. JavaScript's native support for integers tops out at 53 bits, so we're using a slightly modified version of Leemon's excellent BigInt.js library to give us those extra bits. Leemon's code is super fast, but it's kind of ugly, so we've wrapped it up inside crypto_math.js to make it feel a bit more object-oriented.

CowCrypt's RSA functions expect BigInt values. If you have a big number, say, 812345834502348952793, and you want to convert it to a BigInt, you can parse it from a string of digits:


var big_int = new BigInt().parse("812345834502348952793");


Keep in mind: if you're going to be clever and choose your own public exponent e, you'll need to parse it as a BigInt, or else EVERYTHING WILL FAIL.

Generating the large primes p and q

CowCrypt's RSA key generation supports 2048- and 3072-bit key sizes. These keys are constructed using the product of two large prime numbers p and q, which must be 1024- or 1536-bit BigInts (respective to the key sizes). CowCrypt uses an algorithm based on the Miller-Rabin probabilistic primality test defined in FIPS 186-4. This generates numbers which are probably prime, within a miniscule margin of error (less than 2 to the negative 80th power). While other algorithms exist that generate provably prime numbers, they are much slower, and as we'll see, the Miller-Rabin method is already slow enough. In any case, it's secure enough for US government use.

Depending on the hardware used, these primes can take between a few seconds to over a minute to generate. We're going to use the Web Workers API to do this in a separate thread, so as not to freeze up the browser. To set this up, we'll define a function to create the Worker thread and listen for events from it!!


 *	Creates a Web Worker to generate a probable prime of a specified
 *	security length. Executes a callback upon completion.
 *	@param {BigInt} e			The pre-selected public exponent
 *	@param {Number} nlen		(2048 or 3072) The desired security length
 *	@param {function} callback	Callback function to execute on completion
 *	@param {BigInt} [p]			OPTIONAL value for p. If this is passed in,
 *								then we're obviously generating q (and some
 *								additional constraints will be used)
var generate_prime_threaded = function(e, nlen, callback, p)
	// If no value for p is passed in, default it to false
	p || (p = false);

	// Define the Web Worker
	var worker = new Worker('crypto_math.js');

	// Listen for event messages passed back from the worker
	worker.addEventListener('message', function(e) {
		var data = e.data;

		switch (data.cmd)
			// we can only get "secure" random numbers from the main thread
			case 'get_csprng_random_values':
				var _bitlen = data.request.bits;
				var _rand = crypto_math.get_csprng_random_values(_bitlen);
					cmd: 'put_csprng_random_values',
					response: {
						random_values: _rand

			// handle an error in the generation
			case 'put_error':
				console.log('OMG AN ERROR HAS OCCURRED! ', data.error.msg);

			// log data to the console (for debugging purposes)
			case 'put_console_log':

			// GOT OUR PROBABLE PRIME NUMBER! Callback and kill the thread
			case 'put_probable_prime':
	}, false);

	// Send the get_probable_prime command to start the process
		cmd: 'get_probable_prime',
		request: {
			e: e,
			nlen: nlen,
			p: p		// either a BigInt or false


Let's pick this apart starting from the top. First, notice that this function takes an optional input parameter p. We'll use this same function to generate both p and q. So when we pass p into it, then the Worker knows that it should generate q, which has some additional constraints on it (based on the value p).

Next, the worker is defined and an event listener is attached that performs specific actions when the worker sends a "message" event to the main thread. See below for a full list of the messages that can be sent to or from the worker, but most of these are self-explanatory: "puterror" throws an error. "putconsolelog" writes debug data to the JavaScript console. "putprobable_prime" is called upon completion, and it kills the worker thread and executes the callback function.

The only real oddball message is "getcsprngrandomvalues". This is sent when the worker needs some cryptographically-secure random numbers (it turns out that being able to get random numbers is pretty important when you're looking for random prime numbers). When the main thread gets this message, it gets some random numbers from the cryptomath.getcsprngrandomvalues method, and then sends them back to the worker via the "putcsprngrandomvalues" message. You might wonder why the worker doesn't just get the random numbers on its own without bothering the main thread. Well it's because cryptomath.getcsprngrandomvalues depends on window.crypto.getRandomValues, and worker threads have no access to the window object. So the worker has to phone home to the main thread and ask the main thread to get its random values for it. Ugly, but effective.

After the event listener is defined, a message is sent to the worker with the "getprobableprime" command, telling it to start the generation process.

Next, we wire up a series of callback functions to handle our random primes p and q, and ultimately generate the private key:


// define our desired security length (either 2048 or 3072)
var nlen = 2048;

// define our private key variables
var p, q, n, phi_n, d, u;

// callback function after p is generated
var generate_p_complete = function(prime)
	p = prime;

	// call the prime generator again to generate q (passing in p)
	generate_prime_threaded(e, nlen, generate_q_complete, p);

// callback function after q is generated, completes the key generation
var generate_q_complete = function(prime)
	q = prime;

	// compute the rest of the private key using e, p and q
	var inverse_data = crypto_math.compute_rsa_key_inverse_data(e, p, q);

	n		= inverse_data.n;
	phi_n	= inverse_data.phi_n;
	d		= inverse_data.d;
	u		= inverse_data.u;

	// The order of p and q may have been swapped, such that p < q
	p		= inverse_data.p;
	q		= inverse_data.q;

	// Pass your values into a CowCrypt RSAKey object! lolz
	var key	= new RSAKey({e: e, n: n, d: d, p: p, q: q, u: u});


// call our prime generator, callback to generate_p_complete on completion
generate_prime_threaded(e, nlen, generate_p_complete);


This is pretty easy. We first define our callback functions for primes p and q. Then we call generateprimethreaded, passing in the generatepcomplete callback, and away it goes. The process ends up looking like this:

generateprimethreaded => generatepcomplete => generateprimethreaded(p) => generateqcomplete

Compute the private key from p and q

Once we have primes p and q, we call cryptomath.computersakeyinverse_data to generate the rest of the private key values, and that's it! You now have a full RSA public/private key pair and you're ready to encrypt and decrypt data!!

Threaded RSA Decryption

We don't have to worry about using a Worker thread for encryption, because the public exponent is usually relatively small. But decryption can be pretty slow. Fortunately this is a bit more straightforward than key generation!

In this example, we'll assume that the encrypted data is base64-encoded.



var worker = new Worker('/library/crypto_math.js');

worker.addEventListener('message', function(e) {
	var data = e.data;

	switch (data.cmd) {
		case 'put_rsa_decrypt':
}, false);

	cmd: 'get_rsa_decrypt',
	request: {
		ciphertext: convert.base64.decode(ciphertext),
		n: n,	// must be a BigInt
		d: d	// must be a BigInt

var decryption_complete_callback = function(decrypted)
	// Manually undo the PKCS1 v1.5 padding
	decrypted = new PKCS1_v1_5().decode(decrypted);



In the above code, we define our worker, add an event listener that calls the decryptioncompletecallback function when a "putrsadecrypt" message is received. We send the worker the "getrsadecrypt" command message to start the decryption process, and define the decryptioncompletecallback function, which un-pads the decrypted plaintext at the end of the process. Simple enough!

crypto_math.js Worker Messaging Reference

Due to the threaded nature of Web Workers, we can't explicitly call functions between a worker and the main browser thread. Instead, the crypto_math.js worker listens for and handles "message" events, and the main thread (your JavaScript code) must do the same. Refer to the examples above to see how these listeners are defined in the main thread. This section defines all of the messages that can be sent to a worker thread, as well as the messages a worker thread can send to the main thread.

Messages sent to the crypto_math.js worker

getprobableprime: This message kicks off the Miller-Rabin random prime generation process. The following properties must be passed into the request object:

  • e: (BigInt) Required public exponent e
  • nlen: (Number, either 2048 or 3072) The desired security bit length
  • [p]: (Optional BigInt) The random prime p (used to constrain q)

getrsaencrypt: This message tells the worker to encrypt RSA plaintext given a public key. The following properties must be passed into the request object:

  • plaintext: (ASCII-encoded binary string) The plaintext to encrypt, for your security this should be EME PKCS1 v1.5-encoded
  • n: (BigInt) The modulus n
  • e: (BigInt) The public exponent e

getrsadecrypt: This message tells the worker to decrypt RSA ciphertext given a private key. The following properties must be passed into the request object:

  • ciphertext: (ASCII-encoded binary string) The ciphertext to decrypt
  • n: (BigInt) The modulus n
  • d: (BigInt) The private exponent d

putcsprngrandom_values: This returns random values from the cryptomath.getcsprngrandomvalues method to the worker. The worker will request these from the main thread via the "getcsprngrandom_values" message (see below). The following properties must be passed into the response object:

  • randomvalues: (Uint32Array) Result of cryptomath.getcsprngrandomvalues for the bit length specified in the "getcsprngrandomvalues" method.

Messages sent from the crypto_math.js worker

putprobableprime: This message contains the probable prime generated by the worker after it receives the "getprobableprime" message from the main thread. The following properties are passed in the response object:

  • prime: (BigInt) A probable prime

putrsadecrypt: This message contains the decrypted plaintext computed after the worker gets the "getrsadecrypt" mssage from the main thread. The following properties are passed in the response object:

  • plaintext: (ASCII-encoded binary string) Decrypted plaintext. This will most likely still have PKCS1 v1.5 padding, so it's up to you to un-pad!

putconsolelog: This is used for debugging. Since the worker can't write directly to the JavaScript console, it sends this to ask the main thread to do it instead. The following properties are passed in the response object:

  • msg: (String) the message you will hopefully be kind enough to console.log

put_error: Sent when an error occurs. It's up to the main thread to listen for this and handle the errors. The following properties are passed in the response object:

  • msg: (String) The error message
  • code: (Number) An integer error message

The following error codes are currently defined:

  • 1: Unsupported nlen security length (must be 2048 or 3072)
  • 2: Invalid public exponent e
  • 3: Probable prime generation failure (giving up after too many tries)

getcsprngrandom_values: The worker thread sends this message when it needs cryptographically-secure random values. Since only the main thread can generate these (due to reliance on the window object), the main thread must call cryptomath.getcsprngrandomvalues, passing in the requested bit length, and return the result to the worker in a "putcsprngrandom_values" message. The following properties are passed in the request object:

  • bits: (Number) the requested number of randomly generated bits