34、iOS安全开发：构建钥匙串应用与数据加解密

iOS安全开发：构建钥匙串应用与数据加解密

1. 准备根证书

在开始构建应用之前，需要将证书转换为DER格式，因为安全框架期望的是DER格式的根证书，而非证书助手创建的PEM格式。具体操作步骤如下：
1. 打开终端（Terminal.app）。
2. 切换到 ~/Library/Application Support/Certificate Authority/<CA Name> 目录。
3. 执行以下命令：

openssl x509 -in <PEM file> −inform PEM -out root.der -outform DER

将 <PEM file> 替换为实际的文件名。

2. 创建钥匙串应用

使用Xcode创建一个新的项目，选择“Tabbed Application”模板，项目命名为“KeychainViewer”，并启用故事板（storyboards）和自动引用计数（Automatic Reference Counting）。具体步骤如下：
1. 打开Xcode，创建新项目。
2. 选择“Tabbed Application”模板。
3. 命名项目为“KeychainViewer”，启用故事板和自动引用计数。

3. 添加安全框架

项目创建完成后，需要添加安全框架（Security.framework）。操作步骤如下：
1. 在项目导航器中选择项目。
2. 在项目编辑器中选择“Keychain Viewer”目标，导航到“Build Phases”选项卡。
3. 展开“Link Binary with Libraries (3 Items)”部分，添加“Security.framework”。
4. 将“Security.framework”移动到“Frameworks”组，清理项目导航器。

4. 删除不必要的文件

替换 FirstViewController 和 SecondViewController 的实现，删除相关文件。操作步骤如下：
1. 选择 FirstViewController.h 、 FirstViewController.m 、 SecondViewController.h 和 SecondViewController.m 文件。
2. 选择“Move to Trash”删除文件。

5. 定义钥匙串项类

钥匙串API是C语言API，为了方便访问，将其封装在Objective-C类中。创建一个名为 KeychainItem 的新Objective-C类，作为 NSObject 的子类。操作步骤如下：
1. 在“Keychain Viewer”组中创建新的Objective-C类，命名为 KeychainItem 。
2. 选择 KeychainItem.h ，添加安全框架头文件的导入指令：

#import <Security/Security.h>

3. 在 KeychainItem.h 中添加以下属性：

@property (strong, nonatomic) id type;
@property (strong, nonatomic) id item;
@property (strong, nonatomic) NSDictionary *attributes;
@property (strong, nonatomic) id persistentRef;

4. 添加初始化方法：

- (id)initWithItem:(CFTypeRef)item;
- (id)initWithData:(NSData *)data options:(NSDictionary *)options;

5. 实现保存方法和获取属性值的方法：

- (BOOL)save:(NSError **)error;
- (id)valueForAttribute:(CFTypeRef)attr;

6. 实现 KeychainItem 类的方法

在 KeychainItem.m 中实现上述方法。具体代码如下：

- (id)initWithItem:(CFTypeRef)item
{
    self = [self init];
    if (self) {
        self.item = CFBridgingRelease(item);
    }
    return self;
}

- (id)initWithData:(NSData *)data options:(NSDictionary *)options
{
    return nil;
}

- (BOOL)save:(NSError **)error
{
    NSDictionary *attributes = @{
        (__bridge id)kSecValueRef : self.item,
        (__bridge id)kSecReturnPersistentRef : (id)kCFBooleanTrue
    };
    CFTypeRef cfPersistentRef;
    OSStatus status = SecItemAdd((__bridge CFDictionaryRef)attributes, &cfPersistentRef);

    if (status != errSecSuccess) {
        NSDictionary *userInfo = nil;
        switch (status) {
            case errSecParam:
                userInfo = @{ NSLocalizedDescriptionKey : NSLocalizedString(@"errorSecParam",  
                                  @"One or more parameters passed to the function were not valid.") };
                break;
            case errSecAllocate:
                userInfo = @{ NSLocalizedDescriptionKey : NSLocalizedString(@"errSecAllocate", 
                                  @"Failed to allocate memory.") };
                break;
            case errSecDuplicateItem:
                userInfo = @{ NSLocalizedDescriptionKey : NSLocalizedString(@"errSecDuplicateItem", 
                                  @"The item already exists.") };
                break;
        }
        if (*error)
            *error = [NSError errorWithDomain:NSOSStatusErrorDomain code:status userInfo:userInfo];
        return NO;
    }

    self.persistentRef = CFBridgingRelease(cfPersistentRef);
    return YES;
}

- (id)valueForAttribute：(CFTypeRef)attr
{
    return [self.attributes valueForKey:(__bridge id)attr];
}

7. 实现具体的子类

安全框架中定义了四种钥匙串项类型，这里只处理证书（Certificates）和身份（Identities）两种类型，因此需要定义两个具体的子类： KeychainCertificate 和 KeychainIdentity 。
- KeychainCertificate 类 ：

- (id)init
{
    self = [super init];
    if (self) {
        self.type = [(__bridge id)kSecClassCertificate copy];
    }
    return self;
}

- (id)initWithData:(NSData *)data options:(NSDictionary *)options
{
    SecCertificateRef cert = SecCertificateCreateWithData(NULL, (__bridge CFDataRef)data);
    if (cert) {
        self = [self initWithItem:cert];
    }
    return self;
}

• KeychainIdentity 类 ：

- (id)init
{
    self = [super init];
    if (self) {
        self.type = [(__bridge id)kSecClassIdentity copy];
    }
    return self;
}

- (id)initWithData:(NSData *)data options:(NSDictionary *)options
{
    CFDataRef inPKCS12Data = (__bridge CFDataRef)data;
    CFArrayRef items = CFArrayCreate(NULL, 0, 0, NULL);
    OSStatus status = SecPKCS12Import(inPKCS12Data, (__bridge CFDictionaryRef)options, &items);
    if (status != errSecSuccess) {
        NSLog(@"Error Reading P12 file");
        abort();
    }
    CFDictionaryRef myIdentityAndTrust = CFArrayGetValueAtIndex(items, 0);
    SecIdentityRef identity =  
        (SecIdentityRef)CFDictionaryGetValue(myIdentityAndTrust, kSecImportItemIdentity);
    if (identity)
        self = [self initWithItem:identity];

    return self;
}

8. 定义视图控制器类

创建一个名为 KeychainItemsViewController 的抽象基类视图控制器，作为 UITableViewController 的子类。操作步骤如下：
1. 创建新的Objective-C类文件，命名为 KeychainItemsViewController 。
2. 在 KeychainItemsViewController.h 中添加以下属性：

@property (strong, nonatomic) NSArray *items;

3. 在 KeychainItemsViewController.m 中导入 KeychainItem.h 头文件，并更新表格视图数据源方法：

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return self.items.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"KeychainItemCell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier 
                                                            forIndexPath:indexPath];

    KeychainItem *item = self.items[indexPath.row];
    cell.textLabel.text = [item valueForAttribute:kSecAttrLabel];

    return cell;
}

4. 创建具体的子类 CertificatesViewController 和 IdentitiesViewController 。

9. 更新故事板

打开 MainStoryboard.storyboard ，删除原有的视图控制器，添加新的表格视图控制器，并进行相关设置。操作步骤如下：
1. 删除“First View Controller”和“Second View Controller”。
2. 拖动一个 UITableViewController 到编辑器面板。
3. 从标签栏控制器（Tab Bar Controller）控制拖动到表格视图控制器，选择“View Controllers”关系 segue。
4. 将表格视图控制器的标题重命名为“Identities”，使用“first.png”图像。
5. 将表格视图控制器的类从 UITableViewController 更改为 IdentitiesViewController 。
6. 选择表格视图单元格，将样式从“Custom”更改为“Basic”，设置标识符为 KeychainItemCell ，将附件更改为“Disclosure Indicator”。
7. 重复上述步骤，添加另一个表格视图控制器，标题重命名为“Certificates”，使用“second.png”图像，类更改为 CertificatesViewController 。

10. 加载钥匙串项

在 CertificatesViewController.m 和 IdentitiesViewController.m 中更新 viewDidLoad 方法，加载所有的证书和身份。操作步骤如下：
- CertificatesViewController.m ：

#import "KeychainCertificate.h"

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.items = [KeychainCertificate allKeychainCertificates];
}

• IdentitiesViewController.m ：

#import "KeychainIdentity.h"

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.items = [KeychainIdentity allKeychainIdentities];
}

• 实现 allKeychainCertificates 和 allKeychainIdentities 类方法：

+ (NSArray *)allKeychainCertificates
{
    NSMutableArray *certs = [NSMutableArray array];
    NSDictionary *query = @{
        (__bridge id)kSecClass               : (__bridge id)kSecClassCertificate,
        (__bridge id)kSecReturnRef           : (id)kCFBooleanTrue,
        (__bridge id)kSecReturnAttributes    : (id)kCFBooleanTrue,
        (__bridge id)kSecReturnPersistentRef : (id)kCFBooleanTrue,
        (__bridge id)kSecMatchLimit          : (__bridge id)kSecMatchLimitAll
    };
    CFTypeRef results = NULL;
    OSStatus status = SecItemCopyMatching((__bridge CFDictionaryRef)query, &results);
    if (status == errSecSuccess && results != NULL) {
        for (NSDictionary *result in (__bridge NSArray *)results) {
            id itemRef = [result valueForKey:(__bridge id)kSecValueRef];
            id persistentRef = [result valueForKey:(__bridge id)kSecValuePersistentRef];
            NSMutableDictionary *attrs = [NSMutableDictionary dictionaryWithDictionary:result];
            [attrs removeObjectForKey:(__bridge id)kSecValueRef];
            [attrs removeObjectForKey:(__bridge id)kSecValuePersistentRef];
            KeychainCertificate *cert =  
                [[KeychainCertificate alloc] initWithItem:(__bridge CFTypeRef)itemRef];
            cert.persistentRef = persistentRef;
            cert.attributes = attrs;
            [certs addObject:cert];
        }
    }
    return certs;
}

+ (NSArray *)allKeychainIdentities
{
    NSMutableArray *idents = [NSMutableArray array];
    NSDictionary *query = @{
        (__bridge id)kSecClass               : (__bridge id)kSecClassIdentity,
        (__bridge id)kSecReturnRef           : (id)kCFBooleanTrue,
        (__bridge id)kSecReturnAttributes    : (id)kCFBooleanTrue,
        (__bridge id)kSecReturnPersistentRef : (id)kCFBooleanTrue,
        (__bridge id)kSecMatchLimit          : (__bridge id)kSecMatchLimitAll
    };
    CFTypeRef results = NULL;
    OSStatus status = SecItemCopyMatching((__bridge CFDictionaryRef)query, &results);
    if (status == errSecSuccess && results != NULL) {
        for (NSDictionary *result in (__bridge NSArray *)results) {
            id itemRef = [result valueForKey:(__bridge id)kSecValueRef];
            id persistentRef = [result valueForKey:(__bridge id)kSecValuePersistentRef];
            NSMutableDictionary *attrs = [NSMutableDictionary dictionaryWithDictionary:result];
            [attrs removeObjectForKey:(__bridge id)kSecValueRef];
            [attrs removeObjectForKey:(__bridge id)kSecValuePersistentRef];
            KeychainIdentity *ident =  
                [[KeychainIdentity alloc] initWithItem:(__bridge CFTypeRef)itemRef];
            ident.persistentRef = persistentRef;
            ident.attributes = attrs;
            [idents addObject:ident];
        }
    }
    return idents;
}

11. 添加证书和身份文件

将之前创建的 root.cer 和 cert.p12 文件添加到项目的“Supporting Files”组中，并确保勾选“Copy items to destination group’s folder (if needed)”复选框。

12. 加载钥匙串项

在 AppDelegate.m 中实现加载证书和身份的方法。操作步骤如下：

#import "KeychainCertificate.h"
#import "KeychainIdentity.h"

@interface AppDelegate ()
- (BOOL)isAnchorCertLoaded;
- (void)addAnchorCertificate;
- (void)addIdentity;
@end

- (BOOL)application:(UIApplication *)application 
        didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    if ([self isAnchorCertLoaded]) {
        [self addIdentity];
        [self addAnchorCertificate];
    }

    return YES;
}

- (BOOL)isAnchorCertLoaded
{
    return ([[NSUserDefaults standardUserDefaults] valueForKey:@"anchor_certificate"] == nil);
}

- (void)addAnchorCertificate
{
    NSString *rootCertificatePath = [[NSBundle mainBundle] pathForResource:@"root" ofType:@"der"];
    NSData *data = [[NSData alloc] initWithContentsOfFile:rootCertificatePath];
    KeychainCertificate *cert = [[KeychainCertificate alloc] initWithData:data options:nil];
    if (cert) {
        NSError *error;
        BOOL saved = [cert save:&error];
        if (!saved) {
            NSLog(@"Error Saving Certificate: %@", [error localizedDescription]);
            abort();
        }
        NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
        [userDefaults setObject:cert.persistentRef forKey:@"anchor_certificate"];
        [userDefaults synchronize];
    }
}

- (void)addIdentity
{
    NSString *identityPath = [[NSBundle mainBundle] pathForResource:@"cert" ofType:@"p12"];
    NSData *data = [[NSData alloc] initWithContentsOfFile:identityPath];
    NSString *password = @"@Super2048Cat";
    KeychainIdentity *ident =  
        [[KeychainIdentity alloc] initWithData:data 
                                       options:@{ (__bridge id)kSecImportExportPassphrase : password }];
    if (ident) {
        NSError *error;
        BOOL saved = [ident save:&error];
        if (!saved) {
            NSLog(@"Error Saving Identity: %@", [error localizedDescription]);
            abort();
        }
    }
}

13. 添加详细视图

创建一个名为 KeychainItemViewController 的新Objective-C类，作为 UIViewController 的子类，用于显示钥匙串项的详细信息。操作步骤如下：
1. 在 KeychainItemViewController.h 中添加以下属性和方法：

@class KeychainItem;

@property (strong, nonatomic) KeychainItem *item;
@property (weak, nonatomic) IBOutlet UITextView *textView;
- (IBAction)done:(id)sender;

2. 在 KeychainItemViewController.m 中实现 done: 方法：

- (IBAction)done:(id)sender
{
    [self dismissViewControllerAnimated:YES completion:nil];
}

3. 在故事板中添加 UIViewController 场景，设置背景颜色为黑色，添加 UITextView 和 UIButton ，并进行相关设置。
4. 使用身份检查器（Identity Inspector）将视图控制器的类更改为 KeychainItemViewController 。
5. 控制拖动设置 textView 输出口和 done: 动作。
6. 在 KeychainItemViewController.m 中实现 viewWillAppear 方法，填充文本视图：

- (void)viewWillAppear:(BOOL)animated
{
    if (self.item) {
        NSMutableString *itemInfo = [NSMutableString string];
        [itemInfo appendFormat:@"AccessGroup: %@\n", 
                               [self.item valueForAttribute:kSecAttrAccessGroup]];
        [itemInfo appendFormat:@"CreationDate: %@\n", 
                               [self.item valueForAttribute:kSecAttrCreationDate]];
        [itemInfo appendFormat:@"CertificateEncoding: %@\n", 
                               [self.item valueForAttribute:kSecAttrCertificateEncoding]];
        [itemInfo appendFormat:@"CreationDate: %@\n", [self.item valueForAttribute:kSecClass]];
        [itemInfo appendFormat:@"Issuer: %@\n", [self.item valueForAttribute:kSecAttrIssuer]];
        [itemInfo appendFormat:@"Label: %@\n", [self.item valueForAttribute:kSecAttrLabel]];
        [itemInfo appendFormat:@"ModificationDate: %@\n", 
                               [self.item valueForAttribute:kSecAttrModificationDate]];
        [itemInfo appendFormat:@"Accessible: %@\n", [self.item valueForAttribute:kSecAttrAccessible]];
        [itemInfo appendFormat:@"PublicKeyHash: %@\n", 
                               [self.item valueForAttribute:kSecAttrPublicKeyHash]];
        [itemInfo appendFormat:@"SerialNumber: %@\n", 
                               [self.item valueForAttribute:kSecAttrSerialNumber]];
        [itemInfo appendFormat:@"Subject: %@\n", [self.item valueForAttribute:kSecAttrSubject]];
        self.textView.text = itemInfo;
    }
}

7. 在 CertificatesViewController.m 和 IdentitiesViewController.m 中实现 prepareForSegue:sender: 方法，设置钥匙串项：
   - CertificatesViewController.m ：

#import "KeychainItemViewController.h"

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"CertificateSegue"]) {
        NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
        KeychainCertificate *cert = self.items[indexPath.row];
        KeychainItemViewController *kivc = [segue destinationViewController];
        kivc.item = cert;
    }
}

• IdentitiesViewController.m ：

#import "KeychainItemViewController.h"

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"IdentitySegue"]) {
        NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
        KeychainIdentity *ident = self.items[indexPath.row];
        KeychainItemViewController *kivc = [segue destinationViewController];
        kivc.item = ident;
    }
}

以下是整个流程的mermaid流程图：

graph LR
    A[准备根证书] --> B[创建钥匙串应用]
    B --> C[添加安全框架]
    C --> D[删除不必要的文件]
    D --> E[定义钥匙串项类]
    E --> F[实现具体的子类]
    F --> G[定义视图控制器类]
    G --> H[更新故事板]
    H --> I[加载钥匙串项]
    I --> J[添加证书和身份文件]
    J --> K[加载钥匙串项]
    K --> L[添加详细视图]

通过以上步骤，我们完成了一个简单的iOS钥匙串应用的开发，实现了证书和身份的加载、显示和管理。接下来，我们将进一步增强数字身份的功能，实现数据的加密和解密。

iOS安全开发：构建钥匙串应用与数据加解密

14. 增强数字身份功能

为了实现数据的加密和解密，需要在 KeychainIdentity.h 中添加两个方法：

- (NSData *)encrypt:(NSData *)data;
- (NSData *)decrypt:(NSData *)data;

这两个方法分别用于对传入的 NSData 对象进行加密和解密，并返回处理后的 NSData 对象。

同时，为了判断数字身份是否可信任，需要声明一些私有属性和方法：

@interface KeychainIdentity ()
@property (assign, nonatomic, readonly, getter=isTrusted) BOOL trusted;
@property (assign, nonatomic, readonly) SecTrustRef trust;
@property (assign, nonatomic, readonly) SecCertificateRef anchorCertificate;
@property (assign, nonatomic, readonly) SecCertificateRef certificate;
- (BOOL)recoverTrust;
@end

在实现文件中合成这些属性：

@synthesize trusted = _trusted;
@synthesize trust = _trust;
@synthesize anchorCertificate = _anchorCertificate;
@synthesize certificate = _certificate;

更新 init 方法来初始化这些属性：

- (id)init
{
    self = [super init];
    if (self) {
        self.type = [(__bridge id)kSecClassIdentity copy];
        _trusted = NO;
        _trust = NULL;
    }
    return self;
}

实现 dealloc 方法来释放信任和证书引用：

- (void)dealloc
{
    if (_trust)
        CFRelease(_trust);
    if (_certificate)
        CFRelease(_certificate);

    if (_anchorCertificate)
        CFRelease(_anchorCertificate);
}

15. 实现信任相关方法

• 加载锚证书 ：

- (SecCertificateRef)anchorCertificate
{
    if (_anchorCertificate == NULL) {
        id persistentRef =  
            [[NSUserDefaults standardUserDefaults] objectForKey:@"anchor_certificate"];
        NSDictionary *query = @{
                (__bridge id)kSecClass               : (__bridge id)kSecClassCertificate,
                (__bridge id)kSecValuePersistentRef  : persistentRef,
                (__bridge id)kSecReturnRef           : (id)kCFBooleanTrue,
                (__bridge id)kSecMatchLimit          : (__bridge id)kSecMatchLimitOne
        };
        OSStatus status =  
            SecItemCopyMatching((__bridge CFDictionaryRef)query, (CFTypeRef *)&_anchorCertificate);
        if (status != errSecSuccess || _anchorCertificate == NULL) {
            NSLog(@"Error loading Anchor Certificate");
            abort();
        }
    }
    return _anchorCertificate;
}

• 加载数字身份证书 ：

- (SecCertificateRef)certificate
{
    if (_certificate == NULL) {
        OSStatus status =  
            SecIdentityCopyCertificate((__bridge SecIdentityRef)self.item, &_certificate);
        if (status != errSecSuccess) {
            NSLog(@"Error retrieving Identity Certificate");
            return NULL;
        }
    }
    return _certificate;
}

• 创建信任引用 ：

- (SecTrustRef)trust
{
    if (_trust == NULL) {
        SecPolicyRef policy = SecPolicyCreateBasicX509();
        NSArray *certs = @[ (__bridge id)self.certificate, (__bridge id)self.anchorCertificate ];
        OSStatus status =  
            SecTrustCreateWithCertificates((__bridge CFTypeRef)certs, policy, &_trust);
        if (status != errSecSuccess) {
            NSLog(@"Error Creating Trust from Certificate");
            return NULL;
        }
    }
    return _trust;
}

• 判断是否可信任 ：

- (BOOL)isTrusted
{
    if (_trust == NULL) {
        SecTrustResultType trustResult;
        OSStatus status = SecTrustEvaluate(self.trust, &trustResult);
        if (status == errSecSuccess) {
            switch (trustResult) {
                case kSecTrustResultInvalid:
                case kSecTrustResultDeny:
                case kSecTrustResultFatalTrustFailure:
                case kSecTrustResultOtherError:
                    _trusted = NO;
                    break;
                case kSecTrustResultProceed:
                case kSecTrustResultConfirm:
                case kSecTrustResultUnspecified:
                    _trusted = YES;
                    break;
                case kSecTrustResultRecoverableTrustFailure:
                    _trusted = [self recoverTrust];
                    break;
            }
        }
        else
            _trusted = NO;
    }
    return _trusted;
}

• 恢复信任 ：

- (BOOL)recoverTrust
{
    NSArray *anchorCerts = @[ (__bridge id)self.anchorCertificate ];
    SecTrustSetAnchorCertificates(self.trust, (__bridge CFArrayRef)anchorCerts);
    SecTrustSetAnchorCertificatesOnly(self.trust, NO);
    SecTrustResultType trustResult;
    OSStatus status = SecTrustEvaluate(self.trust, &trustResult);
    if (status == errSecSuccess) {
        switch (trustResult) {
            case kSecTrustResultInvalid:
            case kSecTrustResultDeny:
            case kSecTrustResultFatalTrustFailure:
            case kSecTrustResultOtherError:
            case kSecTrustResultRecoverableTrustFailure:
                return NO;
                break;
            case kSecTrustResultProceed:
            case kSecTrustResultConfirm:
            case kSecTrustResultUnspecified:
                return YES;
                break;
        }
    }
    return NO;
}

16. 实现加密和解密方法

• 加密方法 ：

- (NSData *)encrypt:(NSData *)data
{
    if (!self.isTrusted)
        return nil;

    SecKeyRef publicKey = SecTrustCopyPublicKey(self.trust);
    size_t keyBlockSize = SecKeyGetBlockSize(publicKey);
    size_t bufferSize = keyBlockSize*sizeof(uint8_t);

    uint8_t *srcBuffer = malloc(bufferSize);
    size_t srcBufferLen = keyBlockSize - 11;

    uint8_t *buffer = malloc(bufferSize);
    size_t bufferLen = keyBlockSize;

    NSMutableData *result = [[NSMutableData alloc] init];

    NSRange range = NSMakeRange(0, keyBlockSize);
    while (range.location < data.length) {
        memset(srcBuffer, 0x0, bufferSize);
        memset(buffer, 0x0, bufferSize);

        if (NSMaxRange(range) > data.length)
            range.length = data.length - range.location;

        [data getBytes:srcBuffer range:range];
        OSStatus status =  
            SecKeyEncrypt(publicKey, kSecPaddingPKCS1, srcBuffer, srcBufferLen, buffer, &bufferLen);
        if (status != errSecSuccess) {
            NSLog(@"Error Encrypting Data");
            free(buffer);
            free(srcBuffer);
            free(publicKey);
            return nil;
        }

        [result appendBytes:buffer length:bufferLen];
        range.location += srcBufferLen;
    }

    free(buffer);
    free(srcBuffer);
    free(publicKey);

    return result;
}

• 解密方法 ：

- (NSData *)decrypt:(NSData *)data
{
    if (!self.isTrusted)
        return nil;

    SecKeyRef privateKey;
    OSStatus status =  SecIdentityCopyPrivateKey((__bridge SecIdentityRef)self.item, &privateKey);
    if (status != errSecSuccess && privateKey != NULL) {
        CFRelease(privateKey);
        privateKey = NULL;
        return nil;
    }

    size_t keyBlockSize = SecKeyGetBlockSize(privateKey);
    size_t bufferSize = keyBlockSize*sizeof(uint8_t);

    uint8_t *srcBuffer = malloc(bufferSize);

    uint8_t *buffer = malloc(bufferSize);
    size_t bufferLen = keyBlockSize;

    NSMutableData *result = [[NSMutableData alloc] init];

    NSRange range = NSMakeRange(0, keyBlockSize);
    while (range.location < data.length) {
        memset(srcBuffer, 0x0, bufferSize);
        memset(buffer, 0x0, bufferSize);

        if (NSMaxRange(range) > data.length)
            range.length = data.length - range.location;

        [data getBytes:srcBuffer range:range];
        OSStatus status =  
            SecKeyDecrypt(privateKey, kSecPaddingPKCS1, srcBuffer, keyBlockSize, buffer, &bufferLen);
        if (status != errSecSuccess) {
            NSLog(@"Error Decrypting Data");
            free(buffer);
            free(srcBuffer);
            free(privateKey);
            return nil;
        }

        [result appendBytes:buffer length:bufferLen];
        range.location += keyBlockSize;
    }

    free(buffer);
    free(srcBuffer);
    free(privateKey);

    return result;
}

17. 创建身份视图控制器

创建一个名为 IdentityViewController 的 KeychainItemViewController 子类，用于处理数字身份的加密和解密操作。
- 在 IdentityViewController.h 中声明按钮的输出口和动作：

@property (weak, nonatomic) IBOutlet UIButton *cryptButton;
- (IBAction)crypt:(id)sender;

• 在 IdentityViewController.m 中添加私有属性和方法：

@interface IdentityViewController () {
    NSData *_encryptedData;
}
- (void)encrypt;
- (void)decrypt;
@end

• 实现 crypt: 动作方法：

- (IBAction)crypt:(id)sender
{
    if (_encryptedData)
        [self decrypt];
    else
        [self encrypt];
}

• 实现 encrypt 方法：

- (void)encrypt
{
    KeychainIdentity *ident = (KeychainIdentity *)self.item;
    NSData *data = [self.textView.text dataUsingEncoding:NSUTF8StringEncoding];
    _encryptedData = [ident encrypt:data];
    if (_encryptedData == nil) {
        NSLog(@"Encryption Failed");
        return;
    }

    self.textView.text = [_encryptedData description];
    [self.cryptButton setTitle:@"Decrypt" forState:UIControlStateNormal];
}

• 实现 decrypt 方法：

- (void)decrypt
{
    KeychainIdentity *ident = (KeychainIdentity *)self.item;
    NSData *data = [ident decrypt:_encryptedData];
    if (data == nil) {
        NSLog(@"Decryption Failed");
        return;
    }

    NSString *decryptedString = [[NSString alloc] initWithBytes:[data bytes]  
                                                         length:[data length]  
                                                       encoding:NSUTF8StringEncoding];
    _encryptedData = nil;
    self.textView.text = decryptedString;
    [self.cryptButton setTitle:@"Encrypt" forState:UIControlStateNormal];
}

18. 更新故事板以使用新的视图控制器

• 打开 MainStoryboard.storyboard ，找到 IdentitiesViewController 旁边的 KeychainItemViewController 。
• 将其类从 KeychainItemViewController 更改为 IdentityViewController 。
• 拖动一个 UIButton 到视图的右下角，使用蓝色指南将其与 Done 按钮对齐，并将标签更改为 Encrypt 。
• 控制拖动从视图控制器图标到 Encrypt 按钮，绑定到 cryptButton 输出口。
• 控制拖动从 Encrypt 按钮到视图控制器图标，绑定到 crypt: 动作。

19. 测试应用

构建并运行应用程序，选择数字身份以打开 IdentityViewController 。点击 Encrypt 按钮对文本进行加密，然后点击 Decrypt 按钮对文本进行解密。

以下是增强数字身份功能部分的mermaid流程图：

graph LR
    A[添加加密和解密方法] --> B[实现信任相关方法]
    B --> C[实现加密方法]
    B --> D[实现解密方法]
    C --> E[创建身份视图控制器]
    D --> E
    E --> F[更新故事板]
    F --> G[测试应用]

通过以上步骤，我们完成了iOS钥匙串应用的开发，并实现了数字身份的数据加密和解密功能，进一步增强了应用的安全性。在实际开发中，可以根据需求对应用进行更多的优化和扩展。