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dpearson2699/swift-ios-skills

Agent skills for iOS, iPadOS, Swift, SwiftUI, and modern Apple framework development.

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SKILL.mdskills/cryptokit/

name:
cryptokit
description:
Perform cryptographic operations using Apple CryptoKit. Use when hashing data with SHA256/SHA384/SHA512, generating HMAC authentication codes, encrypting with AES-GCM or ChaChaPoly, signing with P256/P384/P521/Curve25519 keys, performing ECDH key agreement, storing keys in the Secure Enclave, or migrating from CommonCrypto to CryptoKit.

CryptoKit

Apple CryptoKit provides a Swift-native API for cryptographic operations: hashing, message authentication, symmetric encryption, public-key signing, key agreement, and Secure Enclave key storage. Available on iOS 13+. Prefer CryptoKit over CommonCrypto or raw Security framework APIs in all new code targeting Swift 6.3+.

Contents

  • Hashing
  • HMAC
  • Symmetric Encryption
  • Public-Key Signing
  • Key Agreement
  • Secure Enclave
  • Common Mistakes
  • Review Checklist
  • References

Hashing

CryptoKit provides SHA256, SHA384, and SHA512 hash functions. All conform to the HashFunction protocol.

One-shot hashing

import CryptoKit

let data = Data("Hello, world!".utf8)
let digest = SHA256.hash(data: data)
let hex = digest.compactMap { String(format: "%02x", $0) }.joined()

SHA384 and SHA512 work identically -- substitute the type name.

Incremental hashing

For large data or streaming input, hash incrementally:

var hasher = SHA256()
hasher.update(data: chunk1)
hasher.update(data: chunk2)
let digest = hasher.finalize()

Digest comparison

CryptoKit digests use constant-time comparison by default. Direct == checks between digests are safe against timing attacks.

let expected = SHA256.hash(data: reference)
let actual = SHA256.hash(data: received)
if expected == actual {
    // Data integrity verified
}

HMAC

HMAC provides message authentication using a symmetric key and a hash function.

Computing an authentication code

let key = SymmetricKey(size: .bits256)
let data = Data("message".utf8)

let mac = HMAC<SHA256>.authenticationCode(for: data, using: key)

Verifying an authentication code

let isValid = HMAC<SHA256>.isValidAuthenticationCode(
    mac, authenticating: data, using: key
)

This uses constant-time comparison internally.

Incremental HMAC

var hmac = HMAC<SHA256>(key: key)
hmac.update(data: chunk1)
hmac.update(data: chunk2)
let mac = hmac.finalize()

Symmetric Encryption

CryptoKit provides two authenticated encryption ciphers: AES-GCM and ChaChaPoly. Both produce a sealed box containing the nonce, ciphertext, and authentication tag.

AES-GCM

The default choice for symmetric encryption. Hardware-accelerated on Apple silicon.

let key = SymmetricKey(size: .bits256)
let plaintext = Data("Secret message".utf8)

// Encrypt
let sealedBox = try AES.GCM.seal(plaintext, using: key)
let ciphertext = sealedBox.combined!  // nonce + ciphertext + tag

// Decrypt
let box = try AES.GCM.SealedBox(combined: ciphertext)
let decrypted = try AES.GCM.open(box, using: key)

ChaChaPoly

Use ChaChaPoly when AES hardware acceleration is unavailable or when interoperating with protocols that require ChaCha20-Poly1305 (e.g., TLS, WireGuard).

let sealedBox = try ChaChaPoly.seal(plaintext, using: key)
let combined = sealedBox.combined  // Always non-optional for ChaChaPoly

let box = try ChaChaPoly.SealedBox(combined: combined)
let decrypted = try ChaChaPoly.open(box, using: key)

Authenticated data

Both ciphers support additional authenticated data (AAD). The AAD is authenticated but not encrypted -- useful for metadata that must remain in the clear but be tamper-proof.

let header = Data("v1".utf8)
let sealedBox = try AES.GCM.seal(
    plaintext, using: key, authenticating: header
)
let decrypted = try AES.GCM.open(
    sealedBox, using: key, authenticating: header
)

SymmetricKey sizes

SizeUse
.bits128AES-128-GCM; adequate for most uses
.bits192AES-192-GCM; uncommon
.bits256AES-256-GCM or ChaChaPoly; recommended default

Generating a key

let key = SymmetricKey(size: .bits256)

To create a key from existing data:

let key = SymmetricKey(data: existingKeyData)

Public-Key Signing

CryptoKit supports ECDSA signing with NIST curves and Ed25519 via Curve25519.

NIST curves: P256, P384, P521

let signingKey = P256.Signing.PrivateKey()
let publicKey = signingKey.publicKey

// Sign
let signature = try signingKey.signature(for: data)

// Verify
let isValid = publicKey.isValidSignature(signature, for: data)

P384 and P521 use the same API -- substitute the curve name.

NIST key representations:

// Export
let der = signingKey.derRepresentation
let pem = signingKey.pemRepresentation
let x963 = signingKey.x963Representation
let raw = signingKey.rawRepresentation

// Import
let restored = try P256.Signing.PrivateKey(derRepresentation: der)

Curve25519 / Ed25519

let signingKey = Curve25519.Signing.PrivateKey()
let publicKey = signingKey.publicKey

// Sign
let signature = try signingKey.signature(for: data)

// Verify
let isValid = publicKey.isValidSignature(signature, for: data)

Curve25519 keys use rawRepresentation only (no DER/PEM/X9.63).

Choosing a curve

CurveSignature SchemeKey SizeTypical Use
P256ECDSA256-bitGeneral purpose; Secure Enclave support
P384ECDSA384-bitHigher security requirements
P521ECDSA521-bitMaximum NIST security level
Curve25519Ed25519256-bitFast; simple API; no Secure Enclave

Use P256 by default. Use Curve25519 when interoperating with Ed25519-based protocols.

Key Agreement

Key agreement lets two parties derive a shared symmetric key from their public/private key pairs using ECDH.

ECDH with P256

// Alice
let aliceKey = P256.KeyAgreement.PrivateKey()

// Bob
let bobKey = P256.KeyAgreement.PrivateKey()

// Alice computes shared secret
let sharedSecret = try aliceKey.sharedSecretFromKeyAgreement(
    with: bobKey.publicKey
)

// Derive a symmetric key using HKDF
let symmetricKey = sharedSecret.hkdfDerivedSymmetricKey(
    using: SHA256.self,
    salt: Data("salt".utf8),
    sharedInfo: Data("my-app-v1".utf8),
    outputByteCount: 32
)

Bob computes the same sharedSecret using his private key and Alice's public key. Both derive the same symmetricKey.

ECDH with Curve25519

let aliceKey = Curve25519.KeyAgreement.PrivateKey()
let bobKey = Curve25519.KeyAgreement.PrivateKey()

let sharedSecret = try aliceKey.sharedSecretFromKeyAgreement(
    with: bobKey.publicKey
)

let symmetricKey = sharedSecret.hkdfDerivedSymmetricKey(
    using: SHA256.self,
    salt: Data(),
    sharedInfo: Data("context".utf8),
    outputByteCount: 32
)

Key derivation functions

SharedSecret is not directly usable as a SymmetricKey. Always derive a key using one of:

MethodStandardUse
hkdfDerivedSymmetricKeyHKDF (RFC 5869)Recommended default
x963DerivedSymmetricKeyANSI X9.63Interop with X9.63 systems

Always provide a non-empty sharedInfo string to bind the derived key to a specific protocol context.

Secure Enclave

The Secure Enclave provides hardware-backed key storage. Private keys never leave the hardware. Only P256 signing and key agreement are supported for ECDH operations. Post-quantum key types (MLKEM, MLDSA) are also available in the Secure Enclave on supported hardware.

Availability check

guard SecureEnclave.isAvailable else {
    // Fall back to software keys
    return
}

Creating a Secure Enclave signing key

let privateKey = try SecureEnclave.P256.Signing.PrivateKey()
let publicKey = privateKey.publicKey  // Standard P256.Signing.PublicKey

let signature = try privateKey.signature(for: data)
let isValid = publicKey.isValidSignature(signature, for: data)

Access control

Require biometric authentication to use the key:

let accessControl = SecAccessControlCreateWithFlags(
    nil,
    kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
    [.privateKeyUsage, .biometryCurrentSet],
    nil
)!

let privateKey = try SecureEnclave.P256.Signing.PrivateKey(
    accessControl: accessControl
)

Persisting Secure Enclave keys

The dataRepresentation is an encrypted blob that only the same device's Secure Enclave can restore. Store it in the Keychain.

// Export
let blob = privateKey.dataRepresentation

// Restore
let restored = try SecureEnclave.P256.Signing.PrivateKey(
    dataRepresentation: blob
)

Secure Enclave key agreement

let seKey = try SecureEnclave.P256.KeyAgreement.PrivateKey()
let peerPublicKey: P256.KeyAgreement.PublicKey = // from peer

let sharedSecret = try seKey.sharedSecretFromKeyAgreement(
    with: peerPublicKey
)

Common Mistakes

1. Using the shared secret directly as a key

// DON'T
let badKey = SymmetricKey(data: sharedSecret)

// DO -- derive with HKDF
let goodKey = sharedSecret.hkdfDerivedSymmetricKey(
    using: SHA256.self,
    salt: salt,
    sharedInfo: info,
    outputByteCount: 32
)

2. Reusing nonces

// DON'T -- hardcoded nonce
let nonce = try AES.GCM.Nonce(data: Data(repeating: 0, count: 12))
let box = try AES.GCM.seal(data, using: key, nonce: nonce)

// DO -- let CryptoKit generate a random nonce (default behavior)
let box = try AES.GCM.seal(data, using: key)

3. Ignoring authentication tag verification

// DON'T -- manually strip tag and decrypt
// DO -- always use AES.GCM.open() or ChaChaPoly.open()
// which verifies the tag automatically

4. Using Insecure hashes for security

// DON'T -- MD5/SHA1 for integrity or security
import CryptoKit
let bad = Insecure.MD5.hash(data: data)

// DO -- use SHA256 or stronger
let good = SHA256.hash(data: data)

Insecure.MD5 and Insecure.SHA1 exist only for legacy compatibility (checksum verification, protocol interop). Never use them for new security-sensitive operations.

5. Storing symmetric keys in UserDefaults

// DON'T
UserDefaults.standard.set(key.rawBytes, forKey: "encryptionKey")

// DO -- store in Keychain
// See references/cryptokit-patterns.md for Keychain storage patterns

6. Not checking Secure Enclave availability

// DON'T -- crash on simulator or unsupported hardware
let key = try SecureEnclave.P256.Signing.PrivateKey()

// DO
guard SecureEnclave.isAvailable else { /* fallback */ }
let key = try SecureEnclave.P256.Signing.PrivateKey()

Review Checklist

  • Using CryptoKit, not CommonCrypto or raw Security framework
  • SHA256+ for hashing; no MD5/SHA1 for security purposes
  • HMAC verification uses isValidAuthenticationCode (constant-time)
  • AES-GCM or ChaChaPoly for symmetric encryption; 256-bit keys
  • Nonces are random (default) -- not hardcoded or reused
  • Authenticated data (AAD) used where metadata needs integrity
  • SharedSecret derived via HKDF, not used directly
  • sharedInfo parameter is non-empty and context-specific
  • Secure Enclave availability checked before use
  • Secure Enclave key dataRepresentation stored in Keychain
  • Private keys not logged, printed, or serialized unnecessarily
  • Symmetric keys stored in Keychain, not UserDefaults or files
  • Encryption export compliance considered (ITSAppUsesNonExemptEncryption)

References

  • Extended patterns (key serialization, Insecure module, Keychain integration, AES key wrapping, HPKE): references/cryptokit-patterns.md
  • Apple documentation: CryptoKit
  • Apple sample: Performing Common Cryptographic Operations
  • Apple sample: Storing CryptoKit Keys in the Keychain

skills

cryptokit

SKILL.md

CHANGELOG.md

README.md

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