Nicolo - 11 months ago 84

Node.js 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

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 Source

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!