Cryptokit Agent Skill

CryptoKit

Apple CryptoKit provides a Swift-native API for cryptographic operations:

hashing, message authentication, symmetric encryption, public-key signing,

key agreement, HPKE, quantum-secure key encapsulation/signing, and Secure

Enclave-backed keys. Most core primitives are available on iOS 13+; check

availability for HPKE (iOS 17+) and SHA-3 / post-quantum APIs (iOS 26+).

Prefer CryptoKit over CommonCrypto or raw Security framework APIs for new

cryptographic primitive code targeting Swift 6.3+.

Contents

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

Hashing

Use SHA256/SHA384/SHA512 on iOS 13+; SHA3_256/SHA3_384/SHA3_512 require iOS 26+. All conform to HashFunction.

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()

SHA-3 availability

Use SHA-3 only behind an availability check unless the deployment target is

iOS 26+:

if #available(iOS 26.0, *) {
    let digest = SHA3_256.hash(data: data)
}

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

Compare CryptoKit digest values directly. Do not convert digests to

strings or arrays for security-sensitive equality checks.

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

HMAC

Use HMAC when a protocol requires keyed message authentication; verify with isValidAuthenticationCode rather than comparing serialized values yourself.

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
)

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
)

Use .bits256 as the default SymmetricKey size for AES-256-GCM or

ChaChaPoly. 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 keys support DER, PEM, X9.63, and raw representations. See

references/cryptokit-patterns.md for

serialization examples.

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

| Curve | Signature Scheme | Key Size | Typical Use |

|---|---|---|---|

| P256 | ECDSA | 256-bit | General purpose; Secure Enclave support |

| P384 | ECDSA | 384-bit | Higher security requirements |

| P521 | ECDSA | 521-bit | Maximum NIST security level |

| Curve25519 | Ed25519 | 256-bit | Fast; 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:

| Method | Standard | Use |

|---|---|---|

| hkdfDerivedSymmetricKey | HKDF (RFC 5869) | Recommended default |

| x963DerivedSymmetricKey | ANSI X9.63 | Interop with X9.63 systems |

Always provide a non-empty sharedInfo string to bind the derived key

to a specific protocol context.

HPKE

HPKE is available on iOS 17+ for public-key encryption workflows. Prefer it over

hand-rolled ECDH + HKDF + AEAD protocols when encrypting to a recipient public key.

let info = Data("my-protocol-v1".utf8)
let recipientKey = Curve25519.KeyAgreement.PrivateKey()
var sender = try HPKE.Sender(
    recipientKey: recipientKey.publicKey,
    ciphersuite: .Curve25519_SHA256_ChachaPoly,
    info: info
)
let encapsulatedKey = sender.encapsulatedKey
let ciphertext = try sender.seal(
    plaintext,
    authenticating: Data("metadata".utf8)
)

var recipient = try HPKE.Recipient(
    privateKey: recipientKey,
    ciphersuite: .Curve25519_SHA256_ChachaPoly,
    info: info,
    encapsulatedKey: encapsulatedKey
)

HPKE.Sender and HPKE.Recipient are stateful; keep them as var, send

encapsulatedKey alongside the ciphertext, and open messages in the same

order they were sealed. See references/cryptokit-patterns.md

for ciphersuite selection and post-quantum HPKE.

Post-Quantum CryptoKit

iOS 26+ adds quantum-secure APIs:

• Key encapsulation: MLKEM768, MLKEM1024
• Hybrid HPKE: XWingMLKEM768X25519 with .XWingMLKEM768X25519_SHA256_AES_GCM_256
• Digital signatures: MLDSA65, MLDSA87
• Secure Enclave variants: SecureEnclave.MLKEM768, SecureEnclave.MLKEM1024,

SecureEnclave.MLDSA65, SecureEnclave.MLDSA87

Use hybrid mechanisms for migration when both classical and quantum-secure

resistance matter. Account for much larger public keys, ciphertexts, and

signatures than P256 or Curve25519.

Secure Enclave

The Secure Enclave provides hardware-backed key storage. Private keys

never leave the hardware. For classical elliptic-curve CryptoKit, Secure

Enclave supports P256 signing and key agreement. On iOS 26+ supported

hardware, CryptoKit also exposes Secure Enclave ML-KEM key encapsulation

and ML-DSA signing types.

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

Use SecAccessControl with .privateKeyUsage when the key requires biometric

or passcode-gated use. Keep detailed Keychain policy decisions in the

swift-security domain.

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 = sharedSecret.withUnsafeBytes { bytes in
    SymmetricKey(data: Data(bytes))
}

// 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(rawKeyData, 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
• [ ] HPKE used instead of custom ECDH+HKDF+AEAD for recipient public-key encryption on iOS 17+
• [ ] SHA-3 and post-quantum APIs guarded with iOS 26+ availability
• [ ] 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 documentation: HPKE
• Apple documentation: Quantum-secure workflows
• Apple sample: Performing Common Cryptographic Operations
• Apple sample: Storing CryptoKit Keys in the Keychain