Node.js - How to generate random numbers in specific range using crypto.randomBytes

Question

How can I generate random numbers in a specific range using crypto.randomBytes?

I want to be able to generate a random number like this:

console.log(random(55, 956)); // where 55 is minimum and 956 is maximum

and I'm limited to use crypto.randomBytes only inside random function to generate random number for this range.

I know how to convert generated bytes from randomBytes to hex or decimal but I can't figure out how to get a random number in a specific range from random bytes mathematically.

Answer 1

To generate random number in a certain range you can use the following equation

Math.random() * (high - low) + low

But you want to use crypto.randomBytes instead of Math.random() this function returns a buffer with randomly generated bytes. In turn, you need to convert the result of this function from bytes to decimal. this can be done using biguint-format package. To install this package simply use the following command:

npm install biguint-format --save

Now you need to convert the result of crypto.randomBytes to decimal, you can do that as follow:

var x= crypto.randomBytes(1);
return format(x, 'dec');

Now you can create your random function which will be as follow:

var crypto = require('crypto'),
    format = require('biguint-format');

function randomC (qty) {
    var x= crypto.randomBytes(qty);
    return format(x, 'dec');
}
function random (low, high) {
    return randomC(4)/Math.pow(2,4*8-1) * (high - low) + low;
}
console.log(random(50,1000));

Answer 2

Thanks to answer from @Mustafamg and huge help from @CodesInChaos I managed to resolve this issue. I made some tweaks and increase range to maximum 256^6-1 or 281,474,976,710,655. Range can be increased more but you need to use additional library for big integers, because 256^7-1 is out of Number.MAX_SAFE_INTEGER limits.

If anyone have same problem feel free to use it.

var crypto = require('crypto');

/*
Generating random numbers in specific range using crypto.randomBytes from crypto library
Maximum available range is 281474976710655 or 256^6-1
Maximum number for range must be equal or less than Number.MAX_SAFE_INTEGER (usually 9007199254740991)
Usage examples:
cryptoRandomNumber(0, 350);
cryptoRandomNumber(556, 1250425);
cryptoRandomNumber(0, 281474976710655);
cryptoRandomNumber((Number.MAX_SAFE_INTEGER-281474976710655), Number.MAX_SAFE_INTEGER);

Tested and working on 64bit Windows and Unix operation systems.
*/

function cryptoRandomNumber(minimum, maximum){
	var distance = maximum-minimum;
	
	if(minimum>=maximum){
		console.log('Minimum number should be less than maximum');
		return false;
	} else if(distance>281474976710655){
		console.log('You can not get all possible random numbers if range is greater than 256^6-1');
		return false;
	} else if(maximum>Number.MAX_SAFE_INTEGER){
		console.log('Maximum number should be safe integer limit');
		return false;
	} else {
		var maxBytes = 6;
		var maxDec = 281474976710656;
		
		// To avoid huge mathematical operations and increase function performance for small ranges, you can uncomment following script
		/*
		if(distance<256){
			maxBytes = 1;
			maxDec = 256;
		} else if(distance<65536){
			maxBytes = 2;
			maxDec = 65536;
		} else if(distance<16777216){
			maxBytes = 3;
			maxDec = 16777216;
		} else if(distance<4294967296){
			maxBytes = 4;
			maxDec = 4294967296;
		} else if(distance<1099511627776){
			maxBytes = 4;
			maxDec = 1099511627776;
		}
		*/
		
		var randbytes = parseInt(crypto.randomBytes(maxBytes).toString('hex'), 16);
		var result = Math.floor(randbytes/maxDec*(maximum-minimum+1)+minimum);
		
		if(result>maximum){
			result = maximum;
		}
		return result;
	}
}

So far it works fine and you can use it as really good random number generator, but I strictly not recommending using this function for any cryptographic services. If you will, use it on your own risk.

All comments, recommendations and critics are welcome!