Non-cryptographic pseudo-random number generator
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.
