I've written this class to generate a random password and was wondering if there's anything I could improve upon here and whether or not this a safe (most random) method of generating a password.

There's only one function in here which can be used to create an alphanumerical password of a specified length. It can also be overloaded to be more specific in which characters it will use.

The code works as it's supposed to.

import UIKit

class codeGen: NSObject {
    
    private let base: String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    private let lowerChars: String = "abcdefghijklmnopqrstuvwxyz"
    private let numberChars: String = "1234567890"
    private let specialChars: String = "!@#$%^&*"
    
    func generate(length: Int) -> String
    {
        let letters : NSString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        let len = UInt32(letters.length)
        
        var result = ""
        
        for _ in 0 ..< length {
            let rand = arc4random_uniform(len)
            var nextChar = letters.character(at: Int(rand))
            result += NSString(characters: &nextChar, length: 1) as String
        }
        
        return result
    }
    
    func generate(length: Int, lowers: Bool, numbers: Bool, specials: Bool) -> String
    {
        var letters = base
        
        if (lowers == true)
        {
            letters += lowerChars
        }
        if (numbers == true)
        {
            letters += numberChars
        }
        if (specials == true)
        {
            letters += specialChars
        }
        
        let NSletters : NSString = letters as NSString
        let len = UInt32(NSletters.length)
        
        var result = ""
        
        for _ in 0 ..< length {
            let rand = arc4random_uniform(len)
            var nextChar = NSletters.character(at: Int(rand))
            result += NSString(characters: &nextChar, length: 1) as String
        }
        
        return result
    }

}