Encrypt a Realm - Swift SDK

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Overview

Considerations
Storing & Reusing Keys
Performance Impact
Encryption and Atlas Device Sync
Accessing an Encrypted Realm from Multiple Processes
Example

Overview

You can encrypt the realm database file on disk with AES-256 + SHA-2 by supplying a 64-byte encryption key when opening a realm.

Realm transparently encrypts and decrypts data with standard AES-256 encryption using the first 256 bits of the given 512-bit encryption key. Realm uses the other 256 bits of the 512-bit encryption key to validate integrity using a hash-based message authentication code (HMAC).

Warning

Do not use cryptographically-weak hashes for realm encryption keys. For optimal security, we recommend generating random rather than derived encryption keys.

Considerations

The following are key impacts to consider when encrypting a realm.

Storing & Reusing Keys

You must pass the same encryption key every time you open the encrypted realm. If you don't provide a key or specify the wrong key for an encrypted realm, the Realm SDK throws an error.

Apps should store the encryption key in the Keychain so that other apps cannot read the key.

Performance Impact

Reads and writes on encrypted realms can be up to 10% slower than unencrypted realms.

Encryption and Atlas Device Sync

You can encrypt a synced realm.

Realm only encrypts the data on the device and stores the data unencrypted in your Atlas data source. Any users with authorized access to the Atlas data source can read the data, but the following still applies:

Users must have the correct read permissions to read the synced data.
Data stored in Atlas is always encrypted at a volume (disk) level.
The transfer between client and server is always fully encrypted.

You can also enable Customer Key Management to encrypt stored Atlas data using your cloud provider's key (e.g. AWS KMS, Azure Key Vault, Google Cloud KMS).

If you need unique keys for each user of your application, you can use an OAuth provider or use one of the Realm authentication providers and an authentication trigger to create a 64-bit key and store that key in a user object.

Accessing an Encrypted Realm from Multiple Processes

You cannot open the same encrypted realm from multiple processes. Attempting to do so will throw the following error: Encrypted interprocess sharing is currently unsupported.

If multiple processes need to access a realm simultaneously, use an unencrypted realm.

As an alternative, you can store data that you want to encrypt as NSData properties on realm objects. Then, you can encrypt and decrypt individual fields.

One possible tool to encrypt and decrypt fields is Apple's CryptoKit framework. You can use Swift Crypto to simplify app development with CryptoKit.

Example

The following code demonstrates how to generate an encryption key and open an encrypted realm:

The following Swift example demonstrates how to store and retrieve a generated key from the Keychain:

// Retrieve the existing encryption key for the app if it exists or create a new one
func getKey() -> Data {
    // Identifier for our keychain entry - should be unique for your application
    let keychainIdentifier = "io.Realm.EncryptionExampleKey"
    let keychainIdentifierData = keychainIdentifier.data(using: String.Encoding.utf8, allowLossyConversion: false)!
    // First check in the keychain for an existing key
    var query: [NSString: AnyObject] = [
        kSecClass: kSecClassKey,
        kSecAttrApplicationTag: keychainIdentifierData as AnyObject,
        kSecAttrKeySizeInBits: 512 as AnyObject,
        kSecReturnData: true as AnyObject
    ]
    // To avoid Swift optimization bug, should use withUnsafeMutablePointer() function to retrieve the keychain item
    // See also: http://stackoverflow.com/questions/24145838/querying-ios-keychain-using-swift/27721328#27721328
    var dataTypeRef: AnyObject?
    var status = withUnsafeMutablePointer(to: &dataTypeRef) { SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0)) }
    if status == errSecSuccess {
        // swiftlint:disable:next force_cast
        return dataTypeRef as! Data
    }
    // No pre-existing key from this application, so generate a new one
    // Generate a random encryption key
    var key = Data(count: 64)
    key.withUnsafeMutableBytes({ (pointer: UnsafeMutableRawBufferPointer) in
        let result = SecRandomCopyBytes(kSecRandomDefault, 64, pointer.baseAddress!)
        assert(result == 0, "Failed to get random bytes")
    })
    // Store the key in the keychain
    query = [
        kSecClass: kSecClassKey,
        kSecAttrApplicationTag: keychainIdentifierData as AnyObject,
        kSecAttrKeySizeInBits: 512 as AnyObject,
        kSecValueData: key as AnyObject
    ]
    status = SecItemAdd(query as CFDictionary, nil)
    assert(status == errSecSuccess, "Failed to insert the new key in the keychain")
    return key
}
// ...
// Use the getKey() function to get the stored encryption key or create a new one
var config = Realm.Configuration(encryptionKey: getKey())
do {
    // Open the realm with the configuration
    let realm = try Realm(configuration: config)
    // Use the realm as normal
} catch let error as NSError {
    // If the encryption key is wrong, `error` will say that it's an invalid database
    fatalError("Error opening realm: \(error)")
}

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