how to secure few numbers in string swift code example

Example: random string swift

func randomString(of length: Int) -> String {
    let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    var s = ""
    for _ in 0 ..< length {
        s.append(letters.randomElement()!)
    }
    return s
}