Local Authentication on iOS
During local authentication, an app authenticates the user against credentials stored locally on the device. In other words, the user "unlocks" the app or some inner layer of functionality by providing a valid PIN, password or biometric characteristics such as face or fingerprint, which is verified by referencing local data. Generally, this is done so that users can more conveniently resume an existing session with a remote service or as a means of step-up authentication to protect some critical function.
As stated before in chapter "Mobile App Authentication Architectures": The tester should be aware that local authentication should always be enforced at a remote endpoint or based on a cryptographic primitive. Attackers can easily bypass local authentication if no data returns from the authentication process.
A variety of methods are available for integrating local authentication into apps. The Local Authentication framework provides a set of APIs for developers to extend an authentication dialog to a user. In the context of connecting to a remote service, it is possible (and recommended) to leverage the keychain for implementing local authentication.
Fingerprint authentication on iOS is known as Touch ID. The fingerprint ID sensor is operated by the SecureEnclave security coprocessor and does not expose fingerprint data to any other parts of the system. Next to Touch ID, Apple introduced Face ID: which allows authentication based on facial recognition. Both use similar APIs on an application level, the actual method of storing the data and retrieving the data (e.g. facial data or fingerprint related data is different).
Developers have two options for incorporating Touch ID/Face ID authentication:
LocalAuthentication.frameworkis a high-level API that can be used to authenticate the user via Touch ID. The app can't access any data associated with the enrolled fingerprint and is notified only whether authentication was successful.Security.frameworkis a lower level API to access keychain services. This is a secure option if your app needs to protect some secret data with biometric authentication, since the access control is managed on a system-level and can not easily be bypassed.Security.frameworkhas a C API, but there are several open source wrappers available, making access to the keychain as simple as to NSUserDefaults.Security.frameworkunderliesLocalAuthentication.framework; Apple recommends to default to higher-level APIs whenever possible.
Please be aware that using either the
LocalAuthentication.framework or the Security.framework, will be a control that can be bypassed by an attacker as it does only return a boolean and no data to proceed with. See Don't touch me that way, by David Lindner et al for more details.The Local Authentication framework provides facilities for requesting a passphrase or Touch ID authentication from users. Developers can display and utilize an authentication prompt by utilizing the function
evaluatePolicy of the LAContext class.Two available policies define acceptable forms of authentication:
deviceOwnerAuthentication(Swift) orLAPolicyDeviceOwnerAuthentication(Objective-C): When available, the user is prompted to perform Touch ID authentication. If Touch ID is not activated, the device passcode is requested instead. If the device passcode is not enabled, policy evaluation fails.deviceOwnerAuthenticationWithBiometrics(Swift) orLAPolicyDeviceOwnerAuthenticationWithBiometrics(Objective-C): Authentication is restricted to biometrics where the user is prompted for Touch ID.
The
evaluatePolicy function returns a boolean value indicating whether the user has authenticated successfully.The Apple Developer website offers code samples for both Swift and Objective-C. A typical implementation in Swift looks as follows.
let context = LAContext()
var error: NSError?
guard context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) else {
// Could not evaluate policy; look at error and present an appropriate message to user
}
context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Please, pass authorization to enter this area") { success, evaluationError in
guard success else {
// User did not authenticate successfully, look at evaluationError and take appropriate action
}
// User authenticated successfully, take appropriate action
}
The iOS keychain APIs can (and should) be used to implement local authentication. During this process, the app stores either a secret authentication token or another piece of secret data identifying the user in the keychain. In order to authenticate to a remote service, the user must unlock the keychain using their passphrase or fingerprint to obtain the secret data.
The keychain allows saving items with the special
SecAccessControl attribute, which will allow access to the item from the keychain only after the user has passed Touch ID authentication (or passcode, if such a fallback is allowed by attribute parameters).In the following example we will save the string "test_strong_password" to the keychain. The string can be accessed only on the current device while the passcode is set (
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly parameter) and after Touch ID authentication for the currently enrolled fingers only (SecAccessControlCreateFlags.biometryCurrentSet parameter):// 1. Create the AccessControl object that will represent authentication settings
var error: Unmanaged<CFError>?
guard let accessControl = SecAccessControlCreateWithFlags(kCFAllocatorDefault,
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
SecAccessControlCreateFlags.biometryCurrentSet,
&error) else {
// failed to create AccessControl object
return
}
// 2. Create the keychain services query. Pay attention that kSecAttrAccessControl is mutually exclusive with kSecAttrAccessible attribute
var query: [String: Any] = [:]
query[kSecClass as String] = kSecClassGenericPassword
query[kSecAttrLabel as String] = "com.me.myapp.password" as CFString
query[kSecAttrAccount as String] = "OWASP Account" as CFString
query[kSecValueData as String] = "test_strong_password".data(using: .utf8)! as CFData
query[kSecAttrAccessControl as String] = accessControl
// 3. Save the item
let status = SecItemAdd(query as CFDictionary, nil)
if status == noErr {
// successfully saved
} else {
// error while saving
}
// 4. Now we can request the saved item from the keychain. Keychain services will present the authentication dialog to the user and return data or nil depending on whether a suitable fingerprint was provided or not.
// 5. Create the query
var query = [String: Any]()
query[kSecClass as String] = kSecClassGenericPassword
query[kSecReturnData as String] = kCFBooleanTrue
query[kSecAttrAccount as String] = "My Name" as CFString
query[kSecAttrLabel as String] = "com.me.myapp.password" as CFString
query[kSecUseOperationPrompt as String] = "Please, pass authorisation to enter this area" as CFString
// 6. Get the item
var queryResult: AnyObject?
let status = withUnsafeMutablePointer(to: &queryResult) {
SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
}
if status == noErr {
let password = String(data: queryResult as! Data, encoding: .utf8)!
// successfully received password
} else {
// authorization not passed
}
// 1. Create the AccessControl object that will represent authentication settings
CFErrorRef *err = nil;
SecAccessControlRef sacRef = SecAccessControlCreateWithFlags(kCFAllocatorDefault,
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly,
kSecAccessControlUserPresence,
err);
// 2. Create the keychain services query. Pay attention that kSecAttrAccessControl is mutually exclusive with kSecAttrAccessible attribute
NSDictionary* query = @{
(_ _bridge id)kSecClass: (__bridge id)kSecClassGenericPassword,
(__bridge id)kSecAttrLabel: @"com.me.myapp.password",
(__bridge id)kSecAttrAccount: @"OWASP Account",
(__bridge id)kSecValueData: [@"test_strong_password" dataUsingEncoding:NSUTF8StringEncoding],
(__bridge id)kSecAttrAccessControl: (__bridge_transfer id)sacRef
};
// 3. Save the item
OSStatus status = SecItemAdd((__bridge CFDictionaryRef)query, nil);
if (status == noErr) {
// successfully saved
} else {
// error while saving
}
// 4. Now we can request the saved item from the keychain. Keychain services will present the authentication dialog to the user and return data or nil depending on whether a suitable fingerprint was provided or not.
// 5. Create the query
NSDictionary *query = @{(__bridge id)kSecClass: (__bridge id)kSecClassGenericPassword,
(__bridge id)kSecReturnData: @YES,
(__bridge id)kSecAttrAccount: @"My Name1",
(__bridge id)kSecAttrLabel: @"com.me.myapp.password",
(__bridge id)kSecUseOperationPrompt: @"Please, pass authorisation to enter this area" };
// 6. Get the item
CFTypeRef queryResult = NULL;
OSStatus status = SecItemCopyMatching((__bridge CFDictionaryRef)query, &queryResult);
if (status == noErr){
NSData* resultData = ( __bridge_transfer NSData* )queryResult;
NSString* password = [[NSString alloc] initWithData:resultData encoding:NSUTF8StringEncoding];
NSLog(@"%@", password);
} else {
NSLog(@"Something went wrong");
}
Unlike macOS and Android, iOS does not support temporariness of an item's accessibility in the keychain: when there is no additional security check when entering the keychain (e.g.
kSecAccessControlUserPresence or similar is set), then once the device is unlocked, a key will be accessible.