axiom-keychain-ref

name	axiom-keychain-ref
description	Use when needing SecItem function signatures, keychain attribute constants, item class uniqueness constraints, accessibility level details, SecAccessControlCreateFlags, kSecReturn behavior per class, LAContext keychain integration, or OSStatus error codes. Covers complete keychain API surface.
license	MIT

Keychain Services API Reference

Comprehensive API reference for iOS/macOS Keychain Services: SecItem CRUD functions, item class attributes, uniqueness constraints, accessibility levels, access control flags, biometric integration, and error codes.

Quick Reference

// Add a generic password
let addQuery: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "[e-mail]",
    kSecValueData as String: "secret".data(using: .utf8)!
]
let addStatus = SecItemAdd(addQuery as CFDictionary, nil)

// Read a generic password
let readQuery: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "[e-mail]",
    kSecReturnData as String: true,
    kSecMatchLimit as String: kSecMatchLimitOne
]
var result: AnyObject?
let readStatus = SecItemCopyMatching(readQuery as CFDictionary, &result)
let data = result as? Data

// Update a generic password
let updateQuery: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "[e-mail]"
]
let updateAttributes: [String: Any] = [
    kSecValueData as String: "newSecret".data(using: .utf8)!
]
let updateStatus = SecItemUpdate(updateQuery as CFDictionary, updateAttributes as CFDictionary)

// Delete a generic password
let deleteQuery: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "[e-mail]"
]
let deleteStatus = SecItemDelete(deleteQuery as CFDictionary)

---

SecItem Functions

SecItemAdd

func SecItemAdd(_ attributes: CFDictionary, _ result: UnsafeMutablePointer<CFTypeRef?>?) -> OSStatus

attributes dictionary accepts: Item class + item attributes + value properties + return type properties.

Does NOT accept search properties (kSecMatch*). Providing kSecMatchLimit in an add query is an error.

result: Pass nil if you don't need the added item back. Pass a pointer to receive the item in the format specified by kSecReturn* keys. Pass nil in most cases — requesting the result back forces an extra read.

SecItemCopyMatching

func SecItemCopyMatching(_ query: CFDictionary, _ result: UnsafeMutablePointer<CFTypeRef?>?) -> OSStatus

query dictionary accepts: Item class + item attributes + search properties + return type properties.

Does NOT accept value properties (kSecValueData) as search criteria.

result: The type depends on which kSecReturn* keys are set:

• kSecReturnData alone → CFData
• kSecReturnAttributes alone → CFDictionary
• kSecReturnRef alone → SecKey / SecCertificate / SecIdentity
• kSecReturnPersistentRef alone → CFData (persistent reference)
• Multiple kSecReturn* keys → CFDictionary containing requested types
• kSecMatchLimit = kSecMatchLimitAll → CFArray of the above

SecItemUpdate

func SecItemUpdate(_ query: CFDictionary, _ attributesToUpdate: CFDictionary) -> OSStatus

query dictionary accepts: Item class + item attributes + search properties. Used to find items to update.

attributesToUpdate dictionary accepts: Item attributes + value properties. These are applied to matched items. Does NOT accept item class or search properties.

Updating kSecValueData replaces the stored secret. Updating attributes (e.g., kSecAttrLabel) changes metadata without touching the secret.

SecItemDelete

func SecItemDelete(_ query: CFDictionary) -> OSStatus

query dictionary accepts: Item class + item attributes + search properties.

On macOS, deletes ALL matching items by default (implicit kSecMatchLimitAll). On iOS, also deletes all matches. There is no confirmation — deletion is immediate.

---

Item Classes

kSecClassGenericPassword

General-purpose secret storage. The most commonly used class.

Attribute	Key	Type
Service	kSecAttrService	CFString
Account	kSecAttrAccount	CFString
Access Group	kSecAttrAccessGroup	CFString
Accessible	kSecAttrAccessible	CFString (constant)
Synchronizable	kSecAttrSynchronizable	CFBoolean
Label	kSecAttrLabel	CFString
Description	kSecAttrDescription	CFString
Comment	kSecAttrComment	CFString
Generic	kSecAttrGeneric	CFData
Creator	kSecAttrCreator	CFNumber (FourCharCode)
Type	kSecAttrType	CFNumber (FourCharCode)
Creation Date	kSecAttrCreationDate	CFDate (read-only)
Modification Date	kSecAttrModificationDate	CFDate (read-only)

kSecClassInternetPassword

URL-associated credentials. Rarely needed — most apps use generic passwords.

Attribute	Key	Type
Server	kSecAttrServer	CFString
Protocol	kSecAttrProtocol	CFString (constant)
Port	kSecAttrPort	CFNumber
Path	kSecAttrPath	CFString
Account	kSecAttrAccount	CFString
Authentication Type	kSecAttrAuthenticationType	CFString (constant)
Security Domain	kSecAttrSecurityDomain	CFString
Accessible	kSecAttrAccessible	CFString (constant)
Access Group	kSecAttrAccessGroup	CFString
Synchronizable	kSecAttrSynchronizable	CFBoolean
Label	kSecAttrLabel	CFString
Comment	kSecAttrComment	CFString
Creator	kSecAttrCreator	CFNumber (FourCharCode)
Type	kSecAttrType	CFNumber (FourCharCode)

kSecClassCertificate

X.509 certificates. Typically managed by the system, not app code.

Attribute	Key	Type
Subject	kSecAttrSubject	CFData (read-only)
Issuer	kSecAttrIssuer	CFData (read-only)
Serial Number	kSecAttrSerialNumber	CFData (read-only)
Subject Key ID	kSecAttrSubjectKeyID	CFData (read-only)
Public Key Hash	kSecAttrPublicKeyHash	CFData (read-only)
Certificate Type	kSecAttrCertificateType	CFNumber
Certificate Encoding	kSecAttrCertificateEncoding	CFNumber
Label	kSecAttrLabel	CFString
Access Group	kSecAttrAccessGroup	CFString
Synchronizable	kSecAttrSynchronizable	CFBoolean

kSecClassKey

Cryptographic keys (RSA, EC, AES). Used for encryption, signing, key agreement.

Attribute	Key	Type
Key Class	kSecAttrKeyClass	CFString (constant)
Application Label	kSecAttrApplicationLabel	CFData
Application Tag	kSecAttrApplicationTag	CFData
Key Type	kSecAttrKeyType	CFString (constant)
Key Size in Bits	kSecAttrKeySizeInBits	CFNumber
Effective Key Size	kSecAttrEffectiveKeySize	CFNumber
Permanent	kSecAttrIsPermanent	CFBoolean
Sensitive	kSecAttrIsSensitive	CFBoolean
Extractable	kSecAttrIsExtractable	CFBoolean
Label	kSecAttrLabel	CFString
Access Group	kSecAttrAccessGroup	CFString
Synchronizable	kSecAttrSynchronizable	CFBoolean
Token ID	kSecAttrTokenID	CFString

kSecClassIdentity

A digital identity is a certificate paired with its private key. Not a distinct storage class — the system synthesizes it from a matching certificate and key. You cannot add a kSecClassIdentity item directly; add the certificate and key separately. Queries return an identity when both halves share the same kSecAttrPublicKeyHash.

See Quinn "The Eskimo!"'s technote: "SecItem: Pitfalls and Best Practices" (forums/thread/724013) — digital identities are a virtual join, not a stored item.

---

Uniqueness Constraints Per Class

Each keychain item is uniquely identified by a subset of its attributes. Adding a second item with the same primary key returns errSecDuplicateItem (-25299). Use SecItemUpdate to modify existing items.

Class	Primary Key Attributes
Generic Password	kSecAttrService + kSecAttrAccount + kSecAttrAccessGroup + kSecAttrSynchronizable
Internet Password	kSecAttrServer + kSecAttrPort + kSecAttrProtocol + kSecAttrAuthenticationType + kSecAttrPath + kSecAttrAccount + kSecAttrAccessGroup + kSecAttrSynchronizable
Certificate	kSecAttrCertificateType + kSecAttrIssuer + kSecAttrSerialNumber + kSecAttrAccessGroup + kSecAttrSynchronizable
Key	kSecAttrApplicationLabel + kSecAttrApplicationTag + kSecAttrKeyType + kSecAttrKeySizeInBits + kSecAttrEffectiveKeySize + kSecAttrKeyClass + kSecAttrAccessGroup + kSecAttrSynchronizable
Identity	N/A (virtual join of certificate + key)

Consequence: If you store tokens for multiple users under the same kSecAttrService without unique kSecAttrAccount values, SecItemAdd returns errSecDuplicateItem for the second user.

---

Attribute Constants Reference

Identity Attributes

Constant	Type	Used By
kSecAttrService	CFString	GenericPassword
kSecAttrAccount	CFString	GenericPassword, InternetPassword
kSecAttrServer	CFString	InternetPassword
kSecAttrLabel	CFString	All classes
kSecAttrDescription	CFString	GenericPassword, InternetPassword
kSecAttrComment	CFString	GenericPassword, InternetPassword
kSecAttrGeneric	CFData	GenericPassword

Security Attributes

Constant	Type	Used By
kSecAttrAccessible	CFString (constant)	All classes
kSecAttrAccessControl	SecAccessControl	All classes
kSecAttrAccessGroup	CFString	All classes
kSecAttrSynchronizable	CFBoolean	All classes

kSecAttrAccessible and kSecAttrAccessControl are mutually exclusive. Setting both is an error — kSecAttrAccessControl includes an accessibility level in its creation.

Token Attributes

Constant	Type	Purpose
kSecAttrTokenID	CFString	Bind key to hardware token
kSecAttrTokenIDSecureEnclave	CFString (value)	Secure Enclave — EC keys only (256-bit)

Key Metadata Attributes

Constant	Type	Values
kSecAttrKeyType	CFString	kSecAttrKeyTypeRSA, kSecAttrKeyTypeECSECPrimeRandom
kSecAttrKeySizeInBits	CFNumber	256 (EC), 2048/4096 (RSA)
kSecAttrKeyClass	CFString	kSecAttrKeyClassPublic, kSecAttrKeyClassPrivate, kSecAttrKeyClassSymmetric
kSecAttrApplicationTag	CFData	App-defined tag for key lookup
kSecAttrApplicationLabel	CFData	SHA-1 hash of public key (auto-generated)

---

Search Properties

Used in SecItemCopyMatching, SecItemUpdate (query parameter), and SecItemDelete queries.

Constant	Type	Purpose
kSecMatchLimit	CFString or CFNumber	Max results — kSecMatchLimitOne, kSecMatchLimitAll, or CFNumber for explicit integer limits (e.g., limit to 5 results)
kSecMatchCaseInsensitive	CFBoolean	Case-insensitive string attribute matching

kSecMatchLimit Defaults

The default depends on context and is a common source of bugs:

Function	Default	Behavior
SecItemCopyMatching	kSecMatchLimitOne	Returns first match
SecItemDelete	All matches	Deletes every matching item

Always set kSecMatchLimit explicitly in SecItemCopyMatching to make intent clear. For SecItemDelete, omitting kSecMatchLimit deletes all matches — this is by design, not a bug.

---

Return Type Properties

Control what SecItemCopyMatching and SecItemAdd return. Set in the query dictionary.

Constant	Returns	Result Type
kSecReturnData	The secret (password bytes, key data)	CFData
kSecReturnAttributes	Item metadata dictionary	CFDictionary
kSecReturnRef	Keychain object reference	SecKey, SecCertificate, or SecIdentity
kSecReturnPersistentRef	Persistent reference (survives app relaunch)	CFData

Return Type Behavior Per Class

Class	kSecReturnData	kSecReturnRef
Generic Password	Password bytes	N/A (no ref type)
Internet Password	Password bytes	N/A (no ref type)
Certificate	DER-encoded certificate data	SecCertificate
Key	Key data (if extractable)	SecKey
Identity	N/A	SecIdentity

Multiple Return Types

When multiple kSecReturn* keys are true, the result is a CFDictionary with keys:

• kSecValueData → the data
• kSecValueRef → the ref
• kSecValuePersistentRef → the persistent ref
• Plus all attribute keys if kSecReturnAttributes is true

When kSecMatchLimitAll is set, the result is a CFArray of the above.

---

Value Type Properties

Used to provide or extract values in add, query, and update dictionaries.

Constant	Type	Purpose
kSecValueData	CFData	The secret (password, key material)
kSecValueRef	SecKey / SecCertificate / SecIdentity	Keychain object reference
kSecValuePersistentRef	CFData	Persistent reference to an item

Behavior Per Operation

Property	SecItemAdd	SecItemCopyMatching	SecItemUpdate
kSecValueData	Sets the secret	Not valid as search criteria	Replaces the secret
kSecValueRef	Adds the referenced object	Finds by reference	Not valid
kSecValuePersistentRef	Not valid	Finds by persistent ref	Not valid

---

Accessibility Constants

Controls when keychain items are readable. Set via kSecAttrAccessible.

Constant	Available When	Survives Backup	Syncs via iCloud
kSecAttrAccessibleWhenUnlocked	Device unlocked	Yes	Yes (default)
kSecAttrAccessibleAfterFirstUnlock	After first unlock until reboot	Yes	Yes
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly	Device unlocked + passcode set	No	No
kSecAttrAccessibleWhenUnlockedThisDeviceOnly	Device unlocked	No	No
kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly	After first unlock until reboot	No	No

Default: kSecAttrAccessibleWhenUnlocked for new items.

ThisDeviceOnly variants: Item is not included in encrypted backups and does not sync via iCloud Keychain. Use for device-bound secrets (biometric-gated tokens, Secure Enclave keys).

WhenPasscodeSetThisDeviceOnly: Item is deleted if the user removes their passcode. Use for secrets that must not survive passcode removal.

AfterFirstUnlock: Available in the background after the user unlocks once post-reboot. Required for background fetch, push notification handlers, and background URLSession completions.

Deprecated (do not use): kSecAttrAccessibleAlways, kSecAttrAccessibleAlwaysThisDeviceOnly.

---

SecAccessControlCreateFlags

Fine-grained access control for keychain items. Created with SecAccessControlCreateWithFlags and set via kSecAttrAccessControl.

All Flags

Flag	Purpose
.userPresence	Any biometric OR device passcode
.biometryAny	Any enrolled biometric (survives new enrollment)
.biometryCurrentSet	Current biometric set only (invalidated if biometrics change)
.devicePasscode	Device passcode required
.privateKeyUsage	Required for Secure Enclave key signing operations
.applicationPassword	App-provided password (in addition to other factors)
.watch	Paired Apple Watch can satisfy authentication
.or	Combine flags with logical OR (any one satisfies)
.and	Combine flags with logical AND (all must satisfy)

Creating Access Control

var error: Unmanaged<CFError>?
guard let accessControl = SecAccessControlCreateWithFlags(
    kCFAllocatorDefault,
    kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
    [.biometryCurrentSet, .or, .devicePasscode],
    &error
) else {
    let nsError = error!.takeRetainedValue() as Error
    fatalError("Failed to create access control: \(nsError)")
}

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "auth-token",
    kSecAttrAccessControl as String: accessControl,
    kSecValueData as String: tokenData
]
let status = SecItemAdd(query as CFDictionary, nil)

Flag Combinations

Combination	Meaning
[.biometryAny]	Any enrolled fingerprint/face
[.biometryCurrentSet]	Current fingerprint/face set (re-enroll invalidates)
[.biometryCurrentSet, .or, .devicePasscode]	Biometric OR passcode fallback
[.biometryCurrentSet, .and, .applicationPassword]	Biometric AND app password
[.privateKeyUsage]	Secure Enclave key operations (sign, decrypt)
[.biometryAny, .or, .watch]	Biometric OR paired Watch

.biometryAny vs .biometryCurrentSet: Use .biometryCurrentSet for high-security items (banking tokens). If the user enrolls a new fingerprint, the item becomes inaccessible — your app must re-authenticate and re-store. Use .biometryAny for convenience items where new enrollment should not invalidate access.

---

LocalAuthentication Integration

LAContext with Keychain

Pre-evaluate biometrics with LAContext, then pass the context to the keychain query to avoid a second biometric prompt.

import LocalAuthentication

let context = LAContext()
context.localizedReason = "Access your credentials"

var authError: NSError?
guard context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &authError) else {
    // Biometrics unavailable — handle error or fall back to passcode
    return
}

context.evaluatePolicy(
    .deviceOwnerAuthenticationWithBiometrics,
    localizedReason: "Authenticate to access credentials"
) { success, error in
    guard success else { return }

    let query: [String: Any] = [
        kSecClass as String: kSecClassGenericPassword,
        kSecAttrService as String: "com.example.app",
        kSecAttrAccount as String: "auth-token",
        kSecReturnData as String: true,
        kSecMatchLimit as String: kSecMatchLimitOne,
        kSecUseAuthenticationContext as String: context
    ]
    var result: AnyObject?
    let status = SecItemCopyMatching(query as CFDictionary, &result)
}

LAContext Keychain Keys

Key	Type	Purpose
kSecUseAuthenticationContext	LAContext	Reuse authenticated context (avoids double prompt)
kSecUseAuthenticationUI	CFString	Control UI behavior: kSecUseAuthenticationUIAllow (default), kSecUseAuthenticationUIFail, kSecUseAuthenticationUISkip

kSecUseAuthenticationUIFail: Returns errSecInteractionNotAllowed instead of showing the biometric prompt. Use to check if an item exists without triggering UI.

LAPolicy Types

Policy	Requires
.deviceOwnerAuthenticationWithBiometrics	Face ID or Touch ID only
.deviceOwnerAuthentication	Biometrics or passcode fallback

BiometryType Detection

let context = LAContext()
var error: NSError?
context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error)

switch context.biometryType {
case .faceID:    // Face ID available
case .touchID:   // Touch ID available
case .opticID:   // Optic ID available (visionOS)
case .none:      // No biometric hardware
@unknown default: break
}

LAError Codes

Error	Code	Cause
.authenticationFailed	-1	User failed authentication
.userCancel	-2	User tapped Cancel
.userFallback	-3	User tapped "Enter Password"
.systemCancel	-4	System cancelled (app backgrounded)
.passcodeNotSet	-5	No passcode configured
.biometryNotAvailable	-6	Hardware unavailable or restricted
.biometryNotEnrolled	-7	No biometrics enrolled
.biometryLockout	-8	Too many failed attempts

---

OSStatus Error Codes

Common keychain OSStatus values and their root causes.

Error	Code	Description	Common Cause
errSecSuccess	0	Operation succeeded	—
errSecDuplicateItem	-25299	Item already exists	Adding with same primary key — use SecItemUpdate instead
errSecItemNotFound	-25300	No matching item	Wrong query attributes or item never stored
errSecInteractionNotAllowed	-25308	UI prompt blocked	Item requires auth but device locked, or kSecUseAuthenticationUIFail set
errSecAuthFailed	-25293	Authentication failed	Wrong password, failed biometric, or ACL denied
errSecMissingEntitlement	-34018	Missing keychain entitlement	App lacks keychain-access-groups entitlement — common in unit test targets
errSecNoSuchAttr	-25303	Attribute not found	Querying an attribute not valid for the item class
errSecParam	-50	Invalid parameter	Malformed query dictionary — check for type mismatches (e.g., String where Data expected)
errSecAllocate	-108	Memory allocation failed	System resource exhaustion
errSecDecode	-26275	Unable to decode data	Corrupted item or encoding mismatch
errSecNotAvailable	-25291	Keychain not available	No keychain database (rare — Simulator reset or corrupted install)

Interpreting OSStatus in Swift

let status = SecItemAdd(query as CFDictionary, nil)
if status != errSecSuccess {
    let message = SecCopyErrorMessageString(status, nil) as? String ?? "Unknown error"
    print("Keychain error \(status): \(message)")
}

-34018 on Test Targets

Unit test runners (XCTest) often lack the keychain-access-groups entitlement. Workarounds:

1. Add a Host Application to the test target (Xcode → Test Target → General → Host Application)
2. Use an in-memory mock for unit tests, real keychain for integration tests only

---

Keychain Sharing

Access Groups

Items are isolated per app by default. To share between apps or extensions:

1. Enable "Keychain Sharing" capability in Xcode
2. Add shared access group identifiers
3. Set kSecAttrAccessGroup when adding items

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.shared",
    kSecAttrAccount as String: "shared-token",
    kSecAttrAccessGroup as String: "TEAMID.com.example.shared",
    kSecValueData as String: tokenData
]

The access group format is $(TeamIdentifierPrefix)$(GroupIdentifier). Items without an explicit access group default to the app's first access group in its entitlements.

iCloud Keychain Sync

Set kSecAttrSynchronizable to true to sync via iCloud Keychain:

let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app",
    kSecAttrAccount as String: "sync-token",
    kSecAttrSynchronizable as String: true,
    kSecValueData as String: tokenData
]

Synchronizable items cannot use ThisDeviceOnly accessibility levels or SecAccessControl. They must use kSecAttrAccessibleWhenUnlocked or kSecAttrAccessibleAfterFirstUnlock.

When querying, kSecAttrSynchronizable defaults to kSecAttrSynchronizableAny (returns both local and synced items). Set explicitly to true or false to filter.

---

Resources

WWDC: 2013-709, 2014-711, 2020-10147

Docs: /security/keychain_services, /localauthentication, /security/secaccesscontrolcreateflags, /security/secitemadd(::)

Skills: axiom-code-signing-ref, axiom-app-attest