totp.swift

totp.swift

import CryptoKit
import Foundation

extension Data {
    init?(base32Encoded string: String) {
        let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
        var bytes: [UInt8] = []
        var accum = 0
        var bits = 0 // # of valid bits in `accum`

        for character in string {
            guard let stringIndex = alphabet.firstIndex(of: character) else {
                print("Invalid character: \(character)")
                return nil
            }

            let index = alphabet.distance(from: alphabet.startIndex, to: stringIndex)

            accum = (accum << 5) | index
            bits += 5

            if bits >= 8 {
                bytes.append(UInt8(truncatingIfNeeded: accum >> (bits - 8)))
                bits -= 8
            }
        }

        self.init(bytes)
    }
}

func totp(secret: Data, period: TimeInterval = 30, digits: Int = 6) -> String {
    var counter = UInt64(Date().timeIntervalSince1970 / period).bigEndian
    let counterData = withUnsafeBytes(of: &counter) { Array($0) }
    let hash = HMAC<Insecure.SHA1>.authenticationCode(for: counterData, using: SymmetricKey(data: secret))

    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)))

    return (String(format: "%0*u", digits, truncatedHash))
}

print(totp(secret: Data(base32Encoded: "JBSWY3DPEHPK3PXP")!)) // Test here: https://totp.danhersam.com/