Generate random number in range with SecRandomCopyBytes

Question

I'm using SecRandomCopyBytes for generate a secure random number.

Is there a way to specify a "range"?

I need to obtain the same behaviour of this Java piece of code:

SecureRandom r = new SecureRandom();
char x = (char)(r.nextInt(26) + 'a');

Any tips will appreciate!

UPDATE

Seeing that I made a silly question I feel compelled to share the solution, made extending Int type:

public extension Int {
  /**
  Create a random num Int in range
  :param: lower number Int
  :param: upper number Int
  :return: random number Int
  */
  public static func random(#min: Int, max: Int) -> Int {
     return Int(arc4random_uniform(UInt32(max - min + 1))) + min
  }

  /**
  Create a secure random num Int in range
  :param: lower number Int
  :param: upper number Int
  :return: random number Int
  */
  public static func secureRandom(#min: Int, max: Int) -> Int {
    if max == 0 {
        NSException(name: "secureRandom", reason: "max number must be > 0", userInfo: nil).raise()
    }
    var randomBytes = UnsafeMutablePointer<UInt8>.alloc(8)
    SecRandomCopyBytes(kSecRandomDefault, 8, randomBytes)
    let randomNumber = unsafeBitCast(randomBytes, UInt.self)
    return Int(randomNumber) % max + min
  }
}

This solution doesn't return random number between min..max but in the range min..(min+max) I would rather suggest using Swift's 4.2 implementation: https://stackoverflow.com/a/39768472/1245231 — petrsyn, Dec 20 '18 at 17:28

petrsyn · Answer 1 · 2018-12-20T17:32:17.747

2

Edit: Swift 4.2 allows to generate random numbers using Int class:

let randomNumber = Int.random(in: 0 ..< 26)

The random source should be cryptographically secure SystemRandomNumberGenerator

See also:

Old answer: iOS 9 introduced GameplayKit which provides SecureRandom.nextInt() Java equivalent.

To use it in Swift 3:

import GameplayKit

// get random Int
let randomInt = GKRandomSource.sharedRandom().nextInt(upperBound: 26)

See this answer for more info: https://stackoverflow.com/a/31215189/1245231

edited Dec 20 '18 at 17:32

answered Sep 29 '16 at 11:01

petrsyn

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Not crypto safe according to: https://developer.apple.com/documentation/gameplaykit/gkrandom – Ivan Aug 07 '18 at 06:20
@Ivan Swift 4.2 should use crypto secure random source. Please see the edited answer. – petrsyn Dec 13 '18 at 22:33

score 0 · Accepted Answer · answered Jun 11 '15 at 14:37

0

You can always specify a range by applying modulo and addition, check this pseudocode:

// random number in the range 1985-2014 
r = rand() % 30 + 1985

answered Jun 11 '15 at 14:37

nburk

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Oh.. I just realized I made the stupidest question in the story! – Luca Davanzo Jun 11 '15 at 14:43
haha no worries :D sometimes the instinct let's you post a question before thinking it through ;) – nburk Jun 11 '15 at 14:46
This also happens after hours of code-porting! Kill me! – Luca Davanzo Jun 11 '15 at 14:51
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This solution will result in a biased random number generator unless your max argument happens to be divisible by UInt8.max. This problem is commonly referred to as modulo bias. See this post for a great explanation, and a more secure solution: http://stackoverflow.com/a/10984975 – Lockyer Aug 20 '16 at 00:52

Generate random number in range with SecRandomCopyBytes

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