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The C Standard function rand() (available in stdlib.h) does not have good random number properties. The numbers generated by rand() have a comparatively short cycle , and the numbers may can be predictable.

Noncompliant Code Example

The following noncompliant code generates an ID with a numeric part produced by calling the rand() function. The IDs produced are predictable and have limited randomness.

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Compliant Solution (POSIX)

A In this compliant solution, a better pseudorandom number generator is the random() function. While the low-dozen bits generated by rand() go through a cyclic cyclical pattern, all the bits generated by random() are usable.

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Although not specified by POSIX, arc4random() is an option on systems that support it. From the The arc4random(3) manual page :says that

arc4random() fits into a middle ground not covered by other subsystems such as the strong, slow, and resource expensive random devices described in random(4) versus the fast but poor quality interfaces described in rand(3), random(3), and drand48(3).

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability really matters and speed is not an issue, such as in the creation of strong cryptographic keys, use a true entropy source such as /dev/random or a hardware device capable of generating random numbers should be used. Note that the /dev/random device may block for a long time if there are not enough events going on to generate sufficient entropy.

Compliant Solution (Windows)

OnIn the compliant solution, on Windows platforms, the [{{CryptGenRandom()}}|http://msdn2.microsoft.com/en-us/library/aa379942.aspx] function may be used to generate cryptographically strong random numbers.  It is important to note that the exact details of the implementation are unknown, and it is unknown what source of entropy the {{CryptGenRandom()}} uses.  FromThe the Microsoft Developer Network {{CryptGenRandom()}} reference \[[MSDN|AA. Bibliography#MSDN]\]: says,

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If an application has access to a good random source, it can fill the {{pbBuffer}} buffer with some random data before calling {{CryptGenRandom()}}. The CSP \[cryptographic service provider\] then uses this data to further randomize its internal seed. It is acceptable to omit the step of initializing the {{pbBuffer}} buffer before calling {{CryptGenRandom()}}.

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Risk Assessment

Using the rand() function leads could lead to possibly predictable random numbers.

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\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 7.20.2.1, "The rand function"
\[[MITRE 072008a|AA. Bibliography#MITRE 0708a]\] [CWE ID 327 |http://cwe.mitre.org/data/definitions/327.html], "Use of a Broken or Risky Cryptographic Algorithm,"." 
\[[MITRE 2008b|AA. Bibliography#MITRE 08b]\] [CWE ID 330|http://cwe.mitre.org/data/definitions/330.html], "Use of Insufficiently Random Values."
\[[MSDN 2010|AA. Bibliography#MSDN 10]\] "[CryptGenRandom Function|http://msdn.microsoft.com/en-us/library/aa379942.aspx]."

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MSC23-CPP. Ensure objects are fully initialized before allowing access.      49. Miscellaneous (MSC)      MSC31-CPP. Ensure that return values are compared against the proper type