iOS Generate Random string

+(NSString *) generateRandomString

{

    NSString *letters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    int len = 8;

    NSMutableString *randomString = [NSMutableString stringWithCapacity: len];

    

    for (int i=0; i<len; i++) {

        [randomString appendFormat: @"%C", [letters characterAtIndex: arc4random_uniform([letters length])]];

    }

    

    return randomString;

}