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Insecure generation of random numbers

Need​

Secure generation of random numbers

Context​

  • Usage of Swift for iOS and macOS app development
  • Usage of Foundation for building responsive and accessible web interfaces

Description​

Non compliant code​

import Foundation

func generateRandomNumber() -> Int {
    return Int(arc4random())
}

The above Swift code is a function that generates a random number using the arc4random() function provided by the Foundation framework in iOS.

This function is insecure because it does not provide a sufficient level of entropy. The arc4random() function uses a pseudorandom number generator, which means that the sequence of numbers it generates can be predicted after observing a small number of outputs.

This is a significant security vulnerability. If an attacker can predict the sequence of random numbers, they could potentially guess sensitive information such as session tokens, passwords, or encryption keys.

Additionally, the arc4random() function does not allow you to specify a range for the random number, which means that the number of possible outputs is limited by the maximum value that an Int can hold. This further reduces the entropy of the generated numbers and makes them easier to guess.

Steps​

  • Replace the use of arc4random() function with a more secure random number generator.
  • Use a cryptographically secure random number generator that provides a higher level of entropy.
  • Ensure that the random number generator used has a sufficient range to generate unpredictable numbers.
  • Consider using a library or framework that provides secure random number generation functions.
  • Test the random number generation functionality to ensure it meets the required security standards.

Compliant code​

import Foundation

func generateSecureRandomNumber() -> Int {
    var randomNum: UInt32 = 0
    let result = SecRandomCopyBytes(kSecRandomDefault, 4, &randomNum)
    if result == errSecSuccess {
        return Int(randomNum)
    } else {
        fatalError("Failed to generate secure random number")
    }
}

The previous code was using the arc4random() function to generate random numbers, which is not secure and can be predictable, leading to potential security vulnerabilities.

The updated code uses the SecRandomCopyBytes() function from the Security framework, which is a cryptographically secure random number generator. This function generates a random number with a higher level of entropy, making it more difficult for an attacker to predict.

The SecRandomCopyBytes() function is used to generate a 32-bit unsigned random number. The function takes three parameters: the first parameter is a constant kSecRandomDefault which tells the function to use the default random number generator, the second parameter is the size of the random number to generate in bytes (4 bytes for a 32-bit number), and the third parameter is a pointer to a variable where the generated random number will be stored.

If the function call is successful, it returns errSecSuccess and the generated random number is returned from the function. If the function call fails for any reason, the program will terminate with a fatal error. This is a simple way to handle errors in this case, but in a real-world application, you would likely want to handle this error in a way that is appropriate for your specific application.

References​

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