iOS8指紋識別TouchID

  蘋果在2014年6月3日的WWDC2014開幕式上推出了新版iOS8系統，界面上iOS8與iOS7相比變化不大，不過在功能方面進行了完善。iOS8通知中心更加強大，支持消息直接回復操作，並支持QuickType和第三方輸入法。短信功能改進明顯，支持羣聊，發送語音、視頻，分享地理位置等。從終端用戶的角度看，iOS8的許多新功能早已出現在其他平臺中。iOS8會向第三方軟件開放TouchID訪問，這意味着可以使用該感應器登陸銀行應用等。

      第三方應用可以使用TouchID接口，意味着未來的很多應用都可以用指紋識別功能了。你可以選擇Touch ID登陸第三方應用程序，不需要輸入密碼，你的指紋數據是被保護的，在沒有被允許的情況下別的程序是訪問不到它的。

                                 

      根據蘋果的解釋，一個單一的註冊指紋與別人指紋出現隨機匹配的概率爲五萬分之一。

      蘋果聲稱“Secure Enclave”模塊系統能夠安全地管理並識別用戶的指紋，並將用戶的指紋信息獨立地保存在別的系統中，同時通過加密內存和一個硬件隨機數字密碼發生器進行管理。

      每個“Secure Enclave”是單獨設置的，不能訪問系統其他部分的，擁有自己的獨立的UID（唯一的ID），連蘋果也不知道這些UID。當設備啓動時，Touch ID會臨時創建一個祕鑰，與“Secure Enclave”的UID配合，對設備的內存空間進行加密。

      而在蘋果發佈的文件中，蘋果對A7處理器進行指紋識別授權的描述是：A7和Touch ID之間通過一個串行外設接口總線進行通信。A7處理器將數據發到“Secure Enclave”，但並不對數據內容進行讀取。加密和身份驗證都是使用Touch ID和“Secure Enclave”之間的共享密鑰。通信密鑰交換使用雙方提供的一個隨機AES密鑰，並隨機建立會話密鑰和使用AES-CCM傳輸加密。

      據瞭解：iPhone 5s中的指紋傳感器檢測到的表皮上突起的紋線。它檢測到的不是用戶手指外部的死皮指紋，這種指紋很容易被複制。iPhone 5s的指紋傳感器利用射頻信號，檢測用戶手指表面下方那一層皮膚的“活”指紋。如果手指與人的身體分離，那麼傳感器是無法檢測到這種指紋的。所以用戶不用擔心自己的指紋被複制或盜竊之後，被用於解鎖設備，因爲傳感器是無法識別這種“死”指紋的。

      最近研究了下iOS8的文檔，對指紋識別瞭解了下，並下載了一個官方提供的Demo。但是

      NS_CLASS_AVAILABLE(10_10, 8_0)

      從這句中可以看出，要想使用TouchID的接口，電腦的mac系統必須是10.10的，手機iOS系統必須是8.0，所以爲了這個Demo我也沒有升級電腦系統(畢竟還不穩定)。但根據Demo中的代碼和文檔可以看出，TouchID的基本用法。

1.首先要使用TouchID，要先導入依賴包：LocalAuthentication.framework。

2.檢查設備是否能用TouchID，返回檢查結果BOOL類型success：

LAContext *context = [[LAContext alloc] init];
    __block  NSString *msg;
    NSError *error;
    BOOL success;

    // test if we can evaluate the policy, this test will tell us if Touch ID is available and enrolled
    success = [context canEvaluatePolicy: LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&error];
    if (success) {
        msg =[NSString stringWithFormat:NSLocalizedString(@"TOUCH_ID_IS_AVAILABLE", nil)];
    } else {
        msg =[NSString stringWithFormat:NSLocalizedString(@"TOUCH_ID_IS_NOT_AVAILABLE", nil)];
    }

3.如果設備能使用TouchID，代碼塊中返回識別結果BOOL類型的success：

LAContext *context = [[LAContext alloc] init];
    __block  NSString *msg;

    // show the authentication UI with our reason string
    [context evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics localizedReason:NSLocalizedString(@"UNLOCK_ACCESS_TO_LOCKED_FATURE", nil) reply:
     ^(BOOL success, NSError *authenticationError) {
         if (success) {
             msg =[NSString stringWithFormat:NSLocalizedString(@"EVALUATE_POLICY_SUCCESS", nil)];
         } else {
             msg = [NSString stringWithFormat:NSLocalizedString(@"EVALUATE_POLICY_WITH_ERROR", nil), authenticationError.localizedDescription];
         }
     }];

4.對於檢查和識別的兩個方法在 LocalAuthentication.framework/Headers/LAContext.h 中定義的：

/// Determines if a particular policy can be evaluated.
///
/// @discussion Policies can have certain requirements which, when not satisfied, would always cause
///             the policy evaluation to fail. Examples can be a passcode set or a fingerprint
///             enrolled with Touch ID. This method allows easy checking for such conditions.
///
///             Applications should consume the returned value immediately and avoid relying on it
///             for an extensive period of time. At least, it is guaranteed to stay valid until the
///             application enters background.
///
/// @warning    Do not call this method in the reply block of evaluatePolicy:reply: because it could
///             lead to a deadlock.
///
/// @param policy Policy for which the preflight check should be run.
///
/// @param error Optional output parameter which is set to nil if the policy can be evaluated, or it
///              contains error information if policy evaluation is not possible.
///
/// @return YES if the policy can be evaluated, NO otherwise.
- (BOOL)canEvaluatePolicy:(LAPolicy)policy error:(NSError * __autoreleasing *)error;

/// Evaluates the specified policy.
///
/// @discussion Policy evaluation may involve prompting user for various kinds of interaction
///             or authentication. Actual behavior is dependent on evaluated policy, device type,
///             and can be affected by installed configuration profiles.
///
///             Be sure to keep a strong reference to the context while the evaluation is in progress.
///             Otherwise, an evaluation would be canceled when the context is being deallocated.
///
///             The method does not block. Instead, the caller must provide a reply block to be
///             called asynchronously when evaluation finishes. The block is executed on a private
///             queue internal to the framework in an unspecified threading context. Other than that,
///             no guarantee is made about which queue, thread, or run-loop the block is executed on.
///
///             Implications of successful policy evaluation are policy specific. In general, this
///             operation is not idempotent. Policy evaluation may fail for various reasons, including
///             user cancel, system cancel and others, see LAError codes.
///
/// @param policy Policy to be evaluated.
///
/// @param reply Reply block that is executed when policy evaluation finishes.
///
/// @param localizedReason Application reason for authentication. This string must be provided in correct
///                        localization and should be short and clear. It will be eventually displayed in
///                        the authentication dialog subtitle. A name of the calling application will be
///                        already displayed in title, so it should not be duplicated here.
///
/// @param success Reply parameter that is YES if the policy has been evaluated successfully or NO if
///                the evaluation failed.
///
/// @param error Reply parameter that is nil if the policy has been evaluated successfully, or it contains
///              error information about the evaluation failure.
///
/// @warning localizedReason parameter is mandatory and the call will throw NSInvalidArgumentException if
///          nil or empty string is specified.
///
/// @see LAError
///
/// Typical error codes returned by this call are:
/// @li          LAErrorUserFallback if user tapped the fallback button
/// @li          LAErrorUserCancel if user has tapped the Cancel button
/// @li          LAErrorSystemCancel if some system event interrupted the evaluation (e.g. Home button pressed).
- (void)evaluatePolicy:(LAPolicy)policy localizedReason:(NSString *)localizedReason reply:(void(^)(BOOL success, NSError *error))reply;