Overview

The use of Non-cryptographic Pseudo-Random Number Generators (PRNGs) as a source for security can be very dangerous, since they are predictable.

Consequences

• Authentication: Potentially a weak source of random numbers could weaken the encryption method used for authentication of users. In this case, a password could potentially be discovered.

Exposure period

• Design through Implementation: It is important to realize that if one is utilizing randomness for important security, one should use the best random numbers available.

Platform

• Languages: All languages.

• Operating platforms: All platforms.

Required resources

Any

Severity

High

Likelihood of exploit

Medium

Avoidance and mitigation

• Design through Implementation: Use functions or hardware which use a hardware-based random number generation for all crypto. This is the recommended solution. Use CyptGenRandom on Windows, or hw_rand() on Linux.

Discussion

Often a pseudo-random number generator (PRNG) is not designed for cryptography. Sometimes a mediocre source of randomness is sufficient or preferable for algorithms which use random numbers. Weak generators generally take less processing power and/or do not use the precious, finite, entropy sources on a system.

Examples

In C\C++:

srand(time())
int randNum = rand();

In Java:

Random r = new Random();

For a given seed, these "random number" generators will produce a reliable stream of numbers. Therefore, if an attacker knows the seed or can guess it easily, he will be able to reliably guess your random numbers.