brute-force-chances.js

/**
 * Bit of code you can run in your developer toolbar or on https://jsfiddle.net .
 * 
 * Fill in the characteristics of your own 'public file' solution and acceptance criteria.
 */
(function(){
    'use strict';

    // dec=10,hex=16,[A-Z0-9]=36, etc.
var CHARACTER_COMBINATIONS = 16,
    NUMBER_OF_CHARACTERS= 6,
    TOTAL_COMBINATIONS = Math.pow(CHARACTER_COMBINATIONS, NUMBER_OF_CHARACTERS),
    // How many files do you expect to host?
    EXPECTED_NUMBER_OF_FILES=1,
    // At what percentage does it become unacceptable.
    MINIMUM_PERCENTAGE=50,
    // How many tries per second could an attacker get away with?
    TRIES_PER_SECOND=700,
    // A years worth of seconds
    MAX_SECONDS = 60 * 60 * 24 * 365;

var minSeconds = 0,
    maxSeconds = MAX_SECONDS,
    currentSeconds,
    currentPercentage,
    triesAtThisTime;

// Do binary search in the given range.
while (minSeconds <= maxSeconds) {
    currentSeconds = (minSeconds + maxSeconds) / 2 | 0;
    triesAtThisTime = TRIES_PER_SECOND * currentSeconds;

    if ((TOTAL_COMBINATIONS - triesAtThisTime) <= EXPECTED_NUMBER_OF_FILES) {
        currentPercentage = 100;
    }
    else {
        currentPercentage = EXPECTED_NUMBER_OF_FILES / (TOTAL_COMBINATIONS - triesAtThisTime) * 100;
    }

    if (currentPercentage < MINIMUM_PERCENTAGE) {
        minSeconds = currentSeconds + 1;
    }
    else if (currentPercentage > MINIMUM_PERCENTAGE) {
        maxSeconds = currentSeconds - 1;
    }
}

if (currentSeconds >= MAX_SECONDS) {
    console.log('Above acceptance criteria');
}
else {
    console.log('Unacceptable. A brute force attack achieves ' + MINIMUM_PERCENTAGE + '% chance of a hit in ' + currentSeconds + ' seconds');
}

})();