Time-based OTP 2FA Token

TOTP.swift

// import HMAC in bridging header:
// #import <CommonCrypto/CommonHMAC.h>

import Foundation

enum OTPAlgorithm: String {
    case sha1, sha256, sha512

    var hmacAlgorithm: CCHmacAlgorithm {
        switch self {
        case .sha1:   return CCHmacAlgorithm(kCCHmacAlgSHA1)
        case .sha256: return CCHmacAlgorithm(kCCHmacAlgSHA256)
        case .sha512: return CCHmacAlgorithm(kCCHmacAlgSHA512)
        }
    }

    var digestLength: Int {
        switch self {
        case .sha1:   return Int(CC_SHA1_DIGEST_LENGTH)
        case .sha256: return Int(CC_SHA256_DIGEST_LENGTH)
        case .sha512: return Int(CC_SHA512_DIGEST_LENGTH)
        }
    }
}

struct Token {
    let id: UUID
    let label: String
    let issuer: String
    let secret: Data // should be stored in keychain
    let algorithm: OTPAlgorithm
    let digits: Int
    let period: TimeInterval

    init(label: String, issuer: String, secret: String, algorithm: OTPAlgorithm = .sha1,
         digits: Int = 6, period: TimeInterval = 30) {
        self.id = UUID()
        self.label = label
        self.issuer = issuer
        self.secret = secret.base32DecodedData! // https://github.com/norio-nomura/Base32
        self.algorithm = algorithm
        self.digits = digits // 6 or 8
        self.period = period
    }

    func code(at time: Date = Date()) -> String {
        let secret = self.secret as NSData
        var counter = UInt64(time.timeIntervalSince1970 / period).bigEndian
        var digest = [UInt8](repeating: 0, count: algorithm.digestLength)
        CCHmac(algorithm.hmacAlgorithm, secret.bytes, secret.length, &counter, MemoryLayout<UInt64>.size, &digest)
        let offset = Int(digest.last! & 0x0f)
        let truncated = UnsafePointer<UInt8>(digest).advanced(by: offset)
            .withMemoryRebound(to: UInt32.self, capacity: MemoryLayout<UInt32>.size)
            { $0.pointee.bigEndian & 0x7fffffff }
        return String(truncated).suffix(digits).padding(toLength: digits, withPad: "0", startingAt: 0)
    }
}