iOS File Protection Reference

Purpose: Comprehensive reference for file encryption and data protection APIs
Availability: iOS 4.0+ (all protection levels), latest enhancements in iOS 26
Context: Built on iOS Data Protection architecture using hardware encryption

When to Use This Skill

Use this skill when you need to:

• Protect sensitive user data at rest
• Choose appropriate FileProtectionType for files
• Understand when files are accessible/encrypted
• Debug "file not accessible" errors after device lock
• Implement secure file storage
• Compare Keychain vs file protection approaches
• Handle background file access requirements

Overview

iOS Data Protection provides hardware-accelerated file encryption tied to the device passcode. When a user sets a passcode, every file can be encrypted with keys protected by that passcode.

Key concepts:

• Files are encrypted automatically when protection is enabled
• Encryption keys are derived from device hardware + user passcode
• Files become inaccessible when device is locked (depending on protection level)
• No performance cost (hardware AES encryption)

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Protection Levels Comparison

Level	Encrypted Until	Accessible When	Use For	Background Access
complete	Device unlocked	Only while unlocked	Sensitive data (health, finances)	❌ No
completeUnlessOpen	File closed	After first unlock, while open	Large downloads, videos	✅ If already open
completeUntilFirstUserAuthentication	First unlock after boot	After first unlock	Most app data	✅ Yes
none	Never	Always	Public caches, temp files	✅ Yes

Detailed Level Descriptions

.complete

Full Description:

"The file is stored in an encrypted format on disk and cannot be read from or written to while the device is locked or booting."

Use For:

• User health data
• Financial information
• Password vaults
• Sensitive documents
• Personal photos (if app requires maximum security)

Behavior:

• Encrypted: ✅ Always
• Accessible: Only when device unlocked
• Background access: ❌ No (app can't read while locked)
• Available after boot: ❌ No (until user unlocks)

Code Example:

// ✅ CORRECT: Maximum security for sensitive data
func saveSensitiveData(_ data: Data, to url: URL) throws {
    try data.write(to: url, options: .completeFileProtection)
}

// Or set on existing file
try FileManager.default.setAttributes(
    [.protectionKey: FileProtectionType.complete],
    ofItemAtPath: url.path
)

Tradeoffs:

• ✅ Maximum security
• ❌ Can't access in background
• ❌ User sees errors if app tries to access while locked

.completeUnlessOpen

Full Description:

"The file is stored in an encrypted format on disk after it is closed."

Use For:

• Large file downloads (continue in background)
• Video files being played
• Documents being edited
• Any file that needs background access while open

Behavior:

• Encrypted: ✅ When closed
• Accessible: After first unlock, remains accessible while open
• Background access: ✅ Yes (if file was already open)
• Available after boot: ❌ No (until first unlock)

Code Example:

// ✅ CORRECT: Download in background, but encrypted when closed
func startBackgroundDownload(url: URL, destination: URL) throws {
    try Data().write(to: destination, options: .completeFileProtectionUnlessOpen)

    // Open file handle for writing
    let fileHandle = try FileHandle(forWritingTo: destination)

    // Download continues in background
    // File remains accessible because it's open
    // When closed, file becomes encrypted

    // Later, when download complete:
    try fileHandle.close()  // Now encrypted until next unlock
}

Tradeoffs:

• ✅ Good security (encrypted when not in use)
• ✅ Background access (if already open)
• ⚠️ Vulnerable while open

.completeUntilFirstUserAuthentication

Full Description:

"The file is stored in an encrypted format on disk and cannot be accessed until after the device has booted."

Use For:

• Most application data
• User preferences
• Downloaded content
• Database files
• Anything that needs background access

Behavior:

• Encrypted: ✅ Always
• Accessible: After first unlock following boot
• Background access: ✅ Yes (after first unlock)
• Available after boot: ❌ No (until user unlocks once)

This is the recommended default for most files.

Code Example:

// ✅ CORRECT: Balanced security for most app data
func saveAppData(_ data: Data, to url: URL) throws {
    try data.write(
        to: url,
        options: .completeFileProtectionUntilFirstUserAuthentication
    )
}

// ✅ This file can be accessed in background after first unlock
func backgroundTaskCanAccessFile() {
    // This works even if device is locked (after first unlock)
    let data = try? Data(contentsOf: url)
}

Tradeoffs:

• ✅ Protected during boot (device stolen while off)
• ✅ Background access (normal operation)
• ⚠️ Accessible while locked (less protection than .complete)

.none

Full Description:

"The file has no special protections associated with it."

Use For:

• Public cache data
• Temporary files
• Non-sensitive downloads
• Thumbnails
• Only when absolutely necessary

Behavior:

• Encrypted: ❌ Never
• Accessible: ✅ Always
• Background access: ✅ Always
• Available after boot: ✅ Always

Code Example:

// ⚠️ USE SPARINGLY: Only for truly non-sensitive data
func cachePublicThumbnail(_ data: Data, to url: URL) throws {
    try data.write(to: url, options: .noFileProtection)
}

Tradeoffs:

• ✅ Always accessible
• ❌ No encryption
• ❌ Vulnerable if device is stolen

---

Setting File Protection

At File Creation

// ✅ RECOMMENDED: Set protection when writing
let sensitiveData = userData.jsonData()
try sensitiveData.write(
    to: fileURL,
    options: .completeFileProtection
)

On Existing Files

// ✅ CORRECT: Change protection on existing file
try FileManager.default.setAttributes(
    [.protectionKey: FileProtectionType.complete],
    ofItemAtPath: fileURL.path
)

Default Protection for Directory

// ✅ CORRECT: Set default protection for directory
// New files inherit this protection
try FileManager.default.setAttributes(
    [.protectionKey: FileProtectionType.completeUntilFirstUserAuthentication],
    ofItemAtPath: directoryURL.path
)

Checking Current Protection

// ✅ Check file's current protection level
func checkFileProtection(at url: URL) throws -> FileProtectionType? {
    let attributes = try FileManager.default.attributesOfItem(atPath: url.path)
    return attributes[.protectionKey] as? FileProtectionType
}

// Usage
if let protection = try? checkFileProtection(at: fileURL) {
    switch protection {
    case .complete:
        print("Maximum protection")
    case .completeUntilFirstUserAuthentication:
        print("Standard protection")
    default:
        print("Other protection")
    }
}

---

File Protection vs Keychain

Decision Matrix

Use Case	Recommended	Why
Passwords, tokens, keys	Keychain	Designed for small secrets
Small sensitive values (<few KB)	Keychain	More secure, encrypted separately
Files >1 KB	File Protection	Keychain not designed for large data
User documents	File Protection	Natural file-based storage
Structured secrets	Keychain	Query by key, access control

Code Comparison

// ✅ CORRECT: Small secrets in Keychain
let passwordData = password.data(using: .utf8)!
let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrAccount as String: "userPassword",
    kSecValueData as String: passwordData,
    kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlocked
]
SecItemAdd(query as CFDictionary, nil)

// ✅ CORRECT: Files with file protection
let userData = try JSONEncoder().encode(user)
try userData.write(to: fileURL, options: .completeFileProtection)

Keychain advantages:

• More granular access control (Face ID/Touch ID)
• Separate encryption (not tied to file system)
• Survives app deletion (if configured)

File protection advantages:

• Works with existing file operations
• Handles large data efficiently
• Automatic with minimal code

---

Background Access Considerations

iOS Background Modes and File Protection

// ❌ WRONG: .complete files can't be accessed in background
class BackgroundTask {
    func performBackgroundSync() {
        // This FAILS if file has .complete protection and device is locked
        let data = try? Data(contentsOf: sensitiveFileURL)
        // data will be nil if device locked
    }
}

// ✅ CORRECT: Use .completeUntilFirstUserAuthentication
// Files accessible in background after first unlock
try data.write(
    to: fileURL,
    options: .completeFileProtectionUntilFirstUserAuthentication
)

Handling Protection Errors

// ✅ CORRECT: Handle protection errors gracefully
func readFile(at url: URL) -> Data? {
    do {
        return try Data(contentsOf: url)
    } catch let error as NSError {
        if error.domain == NSCocoaErrorDomain &&
           error.code == NSFileReadNoPermissionError {
            // File is protected and device is locked
            print("File protected, device locked")
            return nil
        }
        throw error
    }
}

---

iCloud and File Protection

How Protection Works with iCloud

Local file protection:

• Applied to local cached copies
• Does NOT affect iCloud-stored versions
• iCloud has its own encryption (in transit and at rest)

iCloud encryption:

• All iCloud data encrypted at rest (Apple-managed keys)
• End-to-end encryption available for some data types (Advanced Data Protection)
• File protection only affects local device

// ✅ CORRECT: Protection on iCloud file affects local copy only
func saveToICloud(data: Data, filename: String) throws {
    guard let iCloudURL = FileManager.default.url(
        forUbiquityContainerIdentifier: nil
    ) else { return }

    let fileURL = iCloudURL.appendingPathComponent(filename)

    // This protection applies to local cached copy
    try data.write(to: fileURL, options: .completeFileProtection)

    // iCloud has separate encryption for cloud storage
}

---

Common Patterns

Pattern 1: Default Protection for New Apps

// ✅ RECOMMENDED: Set default protection at app launch
func configureDefaultFileProtection() {
    let fileManager = FileManager.default

    let directories: [FileManager.SearchPathDirectory] = [
        .documentDirectory,
        .applicationSupportDirectory
    ]

    for directory in directories {
        guard let url = fileManager.urls(
            for: directory,
            in: .userDomainMask
        ).first else { continue }

        try? fileManager.setAttributes(
            [.protectionKey: FileProtectionType.completeUntilFirstUserAuthentication],
            ofItemAtPath: url.path
        )
    }
}

// Call during app initialization
func application(_ application: UIApplication, didFinishLaunchingWithOptions...) {
    configureDefaultFileProtection()
    return true
}

Pattern 2: Encrypting Database Files

// ✅ CORRECT: Protect SwiftData/SQLite database
let appSupportURL = FileManager.default.urls(
    for: .applicationSupportDirectory,
    in: .userDomainMask
)[0]

let databaseURL = appSupportURL.appendingPathComponent("app.sqlite")

// Set protection before creating database
try? FileManager.default.setAttributes(
    [.protectionKey: FileProtectionType.completeUntilFirstUserAuthentication],
    ofItemAtPath: appSupportURL.path
)

// Now create database - it inherits protection
let container = try ModelContainer(
    for: MyModel.self,
    configurations: ModelConfiguration(url: databaseURL)
)

Pattern 3: Downgrading Protection for Background Tasks

// ⚠️ SOMETIMES NECESSARY: Lower protection for background access
func enableBackgroundAccess(for url: URL) throws {
    try FileManager.default.setAttributes(
        [.protectionKey: FileProtectionType.completeUntilFirstUserAuthentication],
        ofItemAtPath: url.path
    )
}

// Only do this if:
// 1. Background access is truly required
// 2. Data sensitivity allows it
// 3. You've considered security tradeoffs

---

Debugging File Protection Issues

Issue: File Not Accessible in Background

Symptom: Background tasks fail to read files

// Debug: Check current protection
if let protection = try? FileManager.default.attributesOfItem(
    atPath: url.path
)[.protectionKey] as? FileProtectionType {
    print("Protection: \(protection)")
    if protection == .complete {
        print("❌ Can't access in background when locked")
    }
}

Solution: Use .completeUntilFirstUserAuthentication instead

Issue: Files Inaccessible After Restart

Symptom: App can't access files immediately after device reboot

Cause: Using .complete or .completeUntilFirstUserAuthentication (works as designed)

Solution: This is expected behavior. Either:

1. Wait for user to unlock device
2. Handle gracefully with appropriate UI
3. Use .none for files that must be accessible (security tradeoff)

---

Entitlements

File protection generally works without special entitlements, but some features require:

Data Protection Entitlement

<!-- Required for: .complete protection level -->
<key>com.apple.developer.default-data-protection</key>
<string>NSFileProtectionComplete</string>

When needed:

• Using .complete protection
• Some iOS versions for any protection (check documentation)

How to add:

1. Xcode → Target → Signing & Capabilities
2. "+ Capability" → Data Protection
3. Select protection level

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Quick Reference Table

Scenario	Recommended Protection	Accessible When Locked?	Background Access?
User health data	.complete	❌ No	❌ No
Financial records	.complete	❌ No	❌ No
Most app data	.completeUntilFirstUserAuthentication	✅ Yes (after first unlock)	✅ Yes
Downloads (large files)	.completeUnlessOpen	✅ While open	✅ While open
Database files	.completeUntilFirstUserAuthentication	✅ Yes	✅ Yes
Downloaded images	.completeUntilFirstUserAuthentication	✅ Yes	✅ Yes
Public caches	.none	✅ Yes	✅ Yes
Temp files	.none	✅ Yes	✅ Yes

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Related Skills

• axiom-storage — Decide when to use file protection vs other security measures
• axiom-storage-management-ref — File lifecycle, purging, and disk management
• axiom-storage-diag — Debug file access issues

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Last Updated: 2025-12-12
Skill Type: Reference
Minimum iOS: 4.0 (all protection levels)
Latest Updates: iOS 26