Security Tip: Cryptographically Secure Randomness

[Tip#19] Because all randomness should be cryptographically secure.

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This week I wanted to remind you all about using cryptographically secure randomness. It’s something I see developers get wrong all the time in my security audits, and in many cases it can open the door to vulnerabilities in your apps, in ways you don’t expect.

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Cryptographically Secure Randomness

Computers are terrible at generating randomness. Give a computer the exact same inputs and it will produce the exact same outputs. If you can figure out the starting point (“seed”), you can figure out the values. This is a huge problem when you consider that cryptography depends on generating unpredictable random keys. If you can predict the key used to encrypt something, you can decrypt it.

So how do computers get around this? Computers use entropy to help generate randomness, which can be collected from all sorts of sources, including user activity and general noise detected within the hardware. Cloudflare have even taken it a step further, and have an entire wall filled with Lava Lamps which generates a significant amount of entropy for use in cryptography.

Although it’s not truly random, it’s good enough to be considered “cryptographically secure”².

This brings us to PHP and Laravel. When we’re generating random values in PHP and Laravel, we always want to be using cryptographically secure randomness - which is actually pretty simple. You just need to use the right methods.

Secure Functions

PHP 7 & 8 includes two cryptographically secure methods by default, which you should always use when you need a random value.

random_int()³ generates random numbers:

$number = random_int($min, $max);

random_bytes()⁴ generates random bytes, which you can use to generate random strings. Laravel includes the Str::random()⁵ helper, which uses random_bytes() internally to generate friendly random strings for you:

use Illuminate\Support\Str;
 
$random = Str::random($length);

Unsafe Functions

The following functions are not cryptographically secure and should not be used except when securely is not a concern⁶:

md5(time())
md5(microtime())
md5($model->id . time())
md5(rand(...).time())
sha1(time())
rand($min, $max)
mt_rand($min, $max)
tempnam(...)
md5(rand($min, $max))
base64_encode(...)
base64_encode(rand(...))
md5(base64_encode(time()))

str_shuffle(...)
shuffle()
array_rand()

All of these can be guessed if the attacker has enough time and motivation, and the time-based ones are absolutely trivial.

Note about shuffle() and array_rand(): they don’t use a cryptographically secure random number, so they aren’t safe for any secure purposes or anything that requires true randomness. There is the possibility that their order can be predilected, and if this is a concern, you need to use a cryptographically secure shuffling method.

Laravel’s Arr::shuffle() uses shuffle() internally, so this advice applies to that too.

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2. The chances of you replicating the exact inputs to produce the same entropy is practically (although not completely) impossible.

3. https://www.php.net/manual/en/function.random-int.php

4. https://www.php.net/manual/en/function.random-bytes.php

5. https://laravel.com/docs/9.x/helpers#method-str-random

6. I’ve seen every single one of these in use on production systems, and a few more variants on the same theme. It’s incredible how often md5() gets used for something that’s supposed to be secure.