cryptoKit OTP

cryptoKitOTP.swift

import CryptoKit
import CommonCrypto
import Foundation

let period = TimeInterval(30)
let digits = 6
let secret = base32Decode(value: "5FAA5JZ7WHO5WDNN")! // Change this.
var counter = UInt64(Date().timeIntervalSince1970 / period).bigEndian

func cryptoKitOTP() {
    // Generate the key based on the counter.
    let key = SymmetricKey(data: Data(bytes: &counter, count: MemoryLayout.size(ofValue: counter)))
    let hash = HMAC<Insecure.SHA1>.authenticationCode(for: secret, using: key)

    var truncatedHash = hash.withUnsafeBytes { ptr -> UInt32 in
        let offset = ptr[hash.byteCount - 1] & 0x0f

        let truncatedHashPtr = ptr.baseAddress! + Int(offset)
        return truncatedHashPtr.bindMemory(to: UInt32.self, capacity: 1).pointee
    }

    truncatedHash = UInt32(bigEndian: truncatedHash)
    truncatedHash = truncatedHash & 0x7FFF_FFFF
    truncatedHash = truncatedHash % UInt32(pow(10, Float(digits)))

    print("CryptoKit OTP value: \(String(format: "%0*u", digits, truncatedHash))")
}

cryptoKitOTP()