Cryptotokenkit

CryptoTokenKit

Access security tokens and the cryptographic assets they store using the CryptoTokenKit framework. Covers token driver extensions, smart card communication, token sessions, keychain integration, and certificate-based authentication. Targets Swift 6.3.

Platform availability: CryptoTokenKit is primarily a macOS framework. Smart card reader access (TKSmartCard, TKSmartCardSlotManager) requires macOS. Token extension APIs (TKTokenDriver, TKToken, TKTokenSession) are macOS-only. Client-side token watching (TKTokenWatcher) and keychain queries filtered by kSecAttrTokenID are available on iOS 14+/macOS 11+. NFC smart card slot sessions are available on iOS 16.4+.

Contents

• Architecture Overview
• Token Extensions
• Token Sessions
• Smart Card Communication
• Keychain Integration
• Certificate Authentication
• Token Watching
• Error Handling
• Common Mistakes
• Review Checklist
• References

Architecture Overview

CryptoTokenKit bridges hardware security tokens (smart cards, USB tokens) with macOS authentication and keychain services. The framework has two main usage modes:

Token driver extensions (macOS only) -- App extensions that make a hardware token's cryptographic items available to the system. The driver handles token lifecycle, session management, and cryptographic operations.

Client-side token access (macOS + iOS) -- Apps query the keychain for items backed by tokens. CryptoTokenKit automatically exposes token items as standard keychain entries when a token is present.

Key Types

Token Extensions

A token driver is a macOS app extension that makes a hardware token's cryptographic capabilities available to the system. The host app exists only as a delivery mechanism for the extension.

A smart card token extension has three core classes:

1. TokenDriver (subclass of TKSmartCardTokenDriver) -- entry point
2. Token (subclass of TKSmartCardToken) -- represents the token
3. TokenSession (subclass of TKSmartCardTokenSession) -- handles operations

Driver Class

import CryptoTokenKit

final class TokenDriver: TKSmartCardTokenDriver, TKSmartCardTokenDriverDelegate {
    func tokenDriver(
        _ driver: TKSmartCardTokenDriver,
        createTokenFor smartCard: TKSmartCard,
        aid: Data?
    ) throws -> TKSmartCardToken {
        return try Token(
            smartCard: smartCard,
            aid: aid,
            instanceID: "com.example.token:\(smartCard.slot.name)",
            tokenDriver: driver
        )
    }
}

Token Class

The token reads certificates and keys from hardware and populates its keychain contents:

final class Token: TKSmartCardToken, TKTokenDelegate {
    init(
        smartCard: TKSmartCard, aid: Data?,
        instanceID: String, tokenDriver: TKSmartCardTokenDriver
    ) throws {
        try super.init(
            smartCard: smartCard, aid: aid,
            instanceID: instanceID, tokenDriver: tokenDriver
        )
        self.delegate = self

        let certData = try readCertificate(from: smartCard)
        guard let cert = SecCertificateCreateWithData(nil, certData as CFData) else {
            throw TKError(.corruptedData)
        }

        let certItem = TKTokenKeychainCertificate(certificate: cert, objectID: "cert-auth")
        let keyItem = TKTokenKeychainKey(certificate: cert, objectID: "key-auth")
        keyItem?.canSign = true
        keyItem?.canDecrypt = false
        keyItem?.isSuitableForLogin = true

        self.keychainContents?.fill(with: [certItem!, keyItem!])
    }

    func createSession(_ token: TKToken) throws -> TKTokenSession {
        TokenSession(token: token)
    }
}

Info.plist and Registration

The extension's Info.plist must name the driver class:

NSExtension
  NSExtensionAttributes
    com.apple.ctk.driver-class = $(PRODUCT_MODULE_NAME).TokenDriver
  NSExtensionPointIdentifier = com.apple.ctk-tokens

Register the extension once by launching the host app as _securityagent:

sudo -u _securityagent /Applications/TokenHost.app/Contents/MacOS/TokenHost

Token Sessions

TKTokenSession manages authentication state and performs cryptographic operations via its delegate.

final class TokenSession: TKSmartCardTokenSession, TKTokenSessionDelegate {
    func tokenSession(
        _ session: TKTokenSession,
        supports operation: TKTokenOperation,
        keyObjectID: TKToken.ObjectID,
        algorithm: TKTokenKeyAlgorithm
    ) -> Bool {
        switch operation {
        case .signData:
            return algorithm.isAlgorithm(.rsaSignatureDigestPKCS1v15SHA256)
                || algorithm.isAlgorithm(.ecdsaSignatureDigestX962SHA256)
        case .decryptData:
            return algorithm.isAlgorithm(.rsaEncryptionOAEPSHA256)
        case .performKeyExchange:
            return algorithm.isAlgorithm(.ecdhKeyExchangeStandard)
        default:
            return false
        }
    }

    func tokenSession(
        _ session: TKTokenSession,
        sign dataToSign: Data,
        keyObjectID: TKToken.ObjectID,
        algorithm: TKTokenKeyAlgorithm
    ) throws -> Data {
        let smartCard = try getSmartCard()
        return try smartCard.withSession {
            try performCardSign(smartCard: smartCard, data: dataToSign, keyID: keyObjectID)
        }
    }

    func tokenSession(
        _ session: TKTokenSession,
        decrypt ciphertext: Data,
        keyObjectID: TKToken.ObjectID,
        algorithm: TKTokenKeyAlgorithm
    ) throws -> Data {
        let smartCard = try getSmartCard()
        return try smartCard.withSession {
            try performCardDecrypt(smartCard: smartCard, data: ciphertext, keyID: keyObjectID)
        }
    }
}

PIN Authentication

Return a TKTokenAuthOperation from beginAuthFor: to prompt the user for PIN entry before cryptographic operations:

func tokenSession(
    _ session: TKTokenSession,
    beginAuthFor operation: TKTokenOperation,
    constraint: Any
) throws -> TKTokenAuthOperation {
    let pinAuth = TKTokenSmartCardPINAuthOperation()
    pinAuth.pinFormat.charset = .numeric
    pinAuth.pinFormat.minPINLength = 4
    pinAuth.pinFormat.maxPINLength = 8
    pinAuth.smartCard = (session as? TKSmartCardTokenSession)?.smartCard
    pinAuth.apduTemplate = buildVerifyAPDU()
    pinAuth.pinByteOffset = 5
    return pinAuth
}

Smart Card Communication

TKSmartCard provides low-level APDU communication with smart cards connected via readers (macOS-only).

Discovering Card Readers

import CryptoTokenKit

func discoverSmartCards() {
    guard let slotManager = TKSmartCardSlotManager.default else {
        print("Smart card services unavailable")
        return
    }

    for slotName in slotManager.slotNames {
        slotManager.getSlot(withName: slotName) { slot in
            guard let slot else { return }
            if slot.state == .validCard, let card = slot.makeSmartCard() {
                communicateWith(card: card)
            }
        }
    }
}

Sending APDU Commands

Use send(ins:p1:p2:data:le:) for structured APDU communication. Always wrap calls in withSession:

func selectApplication(card: TKSmartCard, aid: Data) throws {
    try card.withSession {
        let (sw, response) = try card.send(
            ins: 0xA4, p1: 0x04, p2: 0x00, data: aid, le: nil
        )
        guard sw == 0x9000 else {
            throw TKError(.communicationError)
        }
    }
}

For raw APDU bytes or non-standard formats, use transmit(_:reply:) with manual beginSession/endSession lifecycle management.

NFC Smart Card Sessions (iOS 16.4+)

On supported iOS devices, create NFC smart card sessions to communicate with contactless smart cards:

func readNFCSmartCard() {
    guard let slotManager = TKSmartCardSlotManager.default,
          slotManager.isNFCSupported() else { return }

    slotManager.createNFCSlot(message: "Hold card near iPhone") { session, error in
        guard let session else { return }
        defer { session.end() }

        guard let slotName = session.slotName,
              let slot = slotManager.slotNamed(slotName),
              let card = slot.makeSmartCard() else { return }
        // Communicate with the NFC card using card.send(...)
    }
}

Keychain Integration

When a token is present, CryptoTokenKit exposes its items as standard keychain entries. Query them using the kSecAttrTokenID attribute:

import Security

func findTokenKey(tokenID: String) throws -> SecKey {
    let query: [String: Any] = [
        kSecClass as String: kSecClassKey,
        kSecAttrTokenID as String: tokenID,
        kSecReturnRef as String: true
    ]
    var result: CFTypeRef?
    let status = SecItemCopyMatching(query as CFDictionary, &result)
    guard status == errSecSuccess, let key = result else {
        throw TKError(.objectNotFound)
    }
    return key as! SecKey
}

Use kSecReturnPersistentRef instead of kSecReturnRef to obtain a persistent reference that survives across app launches. The reference becomes invalid when the token is removed -- handle errSecItemNotFound by prompting the user to reinsert the token.

Query certificates the same way with kSecClass: kSecClassCertificate.

Certificate Authentication

Token Key Requirements

For user login, the token must contain at least one key capable of signing with: EC signature digest X962, RSA signature digest PSS, or RSA signature digest PKCS1v15.

For keychain unlock, the token needs:

• 256-bit EC key (kSecAttrKeyTypeECSECPrimeRandom) supporting ecdhKeyExchangeStandard, or
• 2048/3072/4096-bit RSA key (kSecAttrKeyTypeRSA) supporting rsaEncryptionOAEPSHA256 decryption

Smart Card Authentication Preferences (macOS)

Configure in the com.apple.security.smartcard domain (MDM or systemwide):

Trust levels: 0 = trust all, 1 = validity + issuer, 2 = + soft revocation, 3 = + hard revocation.

Token Watching

TKTokenWatcher monitors token insertion and removal. Available on both macOS and iOS 14+.

import CryptoTokenKit

final class TokenMonitor {
    private let watcher = TKTokenWatcher()

    func startMonitoring() {
        for tokenID in watcher.tokenIDs {
            print("Token present: \(tokenID)")
            if let info = watcher.tokenInfo(forTokenID: tokenID) {
                print("  Driver: \(info.driverName ?? "unknown")")
                print("  Slot: \(info.slotName ?? "unknown")")
            }
        }

        watcher.setInsertionHandler { [weak self] tokenID in
            print("Token inserted: \(tokenID)")
            self?.watcher.addRemovalHandler({ removedTokenID in
                print("Token removed: \(removedTokenID)")
            }, forTokenID: tokenID)
        }
    }
}

Error Handling

CryptoTokenKit operations throw TKError. Key error codes:

Common Mistakes

DON'T: Query token keychain items without checking token presence

// WRONG -- query may fail if token was removed
let key = try findTokenKey(tokenID: savedTokenID)

// CORRECT -- verify the token is still present first
let watcher = TKTokenWatcher()
guard watcher.tokenIDs.contains(savedTokenID) else {
    promptUserToInsertToken()
    return
}
let key = try findTokenKey(tokenID: savedTokenID)

DON'T: Assume smart card APIs work on iOS

// WRONG -- TKSmartCardSlotManager.default is nil on iOS
let manager = TKSmartCardSlotManager.default!  // Crashes on iOS

// CORRECT -- guard availability
guard let manager = TKSmartCardSlotManager.default else {
    print("Smart card services unavailable on this platform")
    return
}

DON'T: Skip session management for card communication

// WRONG -- sending commands without a session
card.transmit(apdu) { response, error in /* may fail */ }

// CORRECT -- use withSession or beginSession/endSession
try card.withSession {
    let (sw, response) = try card.send(
        ins: 0xCA, p1: 0x00, p2: 0x6E, data: nil, le: 0
    )
}

DON'T: Ignore status words in APDU responses

// WRONG -- assuming success
let (_, response) = try card.send(ins: 0xA4, p1: 0x04, p2: 0x00, data: aid, le: nil)

// CORRECT -- check status word
let (sw, response) = try card.send(ins: 0xA4, p1: 0x04, p2: 0x00, data: aid, le: nil)
guard sw == 0x9000 else {
    throw SmartCardError.commandFailed(statusWord: sw)
}

DON'T: Hard-code blanket algorithm support

The supports delegate method must reflect what the hardware actually implements. Returning true unconditionally causes runtime failures when the system attempts unsupported operations.

Review Checklist

• Platform availability verified (TKSmartCard macOS-only, TKTokenWatcher iOS 14+)
• Token extension target uses NSExtensionPointIdentifier = com.apple.ctk-tokens
• com.apple.ctk.driver-class set to the correct driver class in Info.plist
• Extension registered via _securityagent launch during installation
• TKTokenSessionDelegate checks specific algorithms, not blanket true
• Smart card sessions opened and closed (withSession or beginSession/endSession)
• APDU status words checked after every send call
• Token presence verified via TKTokenWatcher before keychain queries
• TKError cases handled with appropriate user feedback
• Keychain contents populated with correct objectID values
• TKTokenKeychainKey capabilities (canSign, canDecrypt) match hardware
• Certificate trust level configured appropriately for deployment environment
• errSecItemNotFound handled for persistent references when token is removed

References

• Extended patterns (PIV commands, TLV parsing, generic token drivers, APDU helpers, secure PIN): references/cryptotokenkit-patterns.md
• CryptoTokenKit framework
• TKTokenDriver
• TKToken
• TKTokenSession
• TKSmartCard
• TKSmartCardSlotManager
• TKTokenWatcher
• Authenticating Users with a Cryptographic Token
• Using Cryptographic Assets Stored on a Smart Card
• Configuring Smart Card Authentication