Insecure Randomness

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This article includes content generously donated to OWASP by Fortify.JPG.

Last revision (mm/dd/yy): 02/20/2009

Vulnerabilities Table of Contents

Description

Standard pseudo-random number generators cannot withstand cryptographic attacks.

Insecure randomness errors occur when a function that can produce predictable values is used as a source of randomness in security-sensitive context.

Computers are deterministic machines, and as such are unable to produce true randomness. Pseudo-Random Number Generators (PRNGs) approximate randomness algorithmically, starting with a seed from which subsequent values are calculated.

There are two types of PRNGs: statistical and cryptographic. Statistical PRNGs provide useful statistical properties, but their output is highly predictable and forms an easy to reproduce numeric stream that is unsuitable for use in cases where security depends on generated values being unpredictable. Cryptographic PRNGs address this problem by generating output that is more difficult to predict. For a value to be cryptographically secure, it must be impossible or highly improbable for an attacker to distinguish between it and a truly random value. In general, if a PRNG algorithm is not advertised as being cryptographically secure, then it is probably a statistical PRNG and should not be used in security-sensitive contexts.

Risk Factors

TBD

Examples

The following code uses a statistical PRNG to create a URL for a receipt that remains active for some period of time after a purchase.

	String GenerateReceiptURL(String baseUrl) {
		Random ranGen = new Random();
		ranGen.setSeed((new Date()).getTime());
		return(baseUrl + Gen.nextInt(400000000) + ".html");
	}

This code uses the Random.nextInt() function to generate "unique" identifiers for the receipt pages it generates. Because Random.nextInt() is a statistical PRNG, it is easy for an attacker to guess the strings it generates. Although the underlying design of the receipt system is also faulty, it would be more secure if it used a random number generator that did not produce predictable receipt identifiers, such as a cryptographic PRNG.


Related Attacks


Related Vulnerabilities


Related Controls


Related Technical Impacts


References

TBD