Using a cryptographically weak pseudo-random number generator to generate a security-sensitive value, such as a password, makes it easier for an attacker to predict the value.

Pseudo-random number generators generate a sequence of numbers that only approximates the properties of random numbers. The sequence is not truly random because it is completely determined by a relatively small set of initial values, the seed. If the random number generator is cryptographically weak, then this sequence may be easily predictable through outside observations.

Use a cryptographically secure pseudo-random number generator if the output is to be used in a security-sensitive context. As a rule of thumb, a value should be considered "security-sensitive" if predicting it would allow the attacker to perform an action that they would otherwise be unable to perform. For example, if an attacker could predict the random password generated for a new user, they would be able to log in as that new user.

For JavaScript on the NodeJS platform, crypto.getRandomBytes provides a cryptographically secure pseudo-random byte generator. Note that the conversion from bytes to numbers can introduce bias that breaks the security.

For JavaScript in the browser, crypto.getRandomValues provides a cryptographically secure pseudo-random number generator.

The following examples show different ways of generating a password.

In the first case, we generate a fresh password by appending a random integer to the end of a static string. The random number generator used (Math.random) is not cryptographically secure, so it may be possible for an attacker to predict the generated password.

In the second example, a cryptographically secure random number generator is used for the same purpose. In this case, it is much harder to predict the generated integers.

  • Wikipedia: Pseudo-random number generator.
  • Mozilla Developer Network: Crypto: getRandomValues().
  • NodeJS: crypto.randomBytes
    •