OTP_example.swift

import CommonCrypto

func otp() -> String {
    let secret = Data()  // change this

    let now = Date().timeIntervalSince1970
    let count = Int(now) / 30
    var fixedCounter = NSSwapHostLongLongToBig(UInt64(count))
    let hash = malloc(1000)!
    defer {free(hash)}

    var ctx = CCHmacContext()
    CCHmacInit(&ctx, CCHmacAlgorithm(kCCHmacAlgSHA1), (secret as NSData).bytes, secret.count)
    CCHmacUpdate(&ctx, &fixedCounter, MemoryLayout<UInt64>.size)
    CCHmacFinal(&ctx, hash)

    let offset: CChar = hash.advanced(by: 20-1).bindMemory(to: CChar.self, capacity: 1).pointee & 0x0f
    let truncatedHash = NSSwapBigIntToHost(
        hash.advanced(by: Int(offset)).bindMemory(to: UInt32.self, capacity: 1).pointee) & 0x7fffffff
    let pin = String(format: "%06u", truncatedHash % 1000000)

    return pin
}