最近项目需要存储用户的唯一标识符，但是由于如果用户重装APP，获取到的又会是一个新的UDID。查询了一系列资料下来，可以用Keychain进行存储UDID，然后就算重装了APP，也能从Keychain中读取出之前存储的UDID。

Keychain存储在iOS系统的内部数据库中,由系统进行管理和加密，哪怕APP被删除了，它存储在Keychain中的数据也不会被删除。

So，Keychain使用走起～
首先一定要在Capabilities中打开Keychain，如图：

6B836BEE-1CDC-4D49-8E7E-50490DE2E953.png

好吧，不打开的话进行Keychain操作将会返回一个34018的错误码。。

当然别忘记引入import <Security/Security.h>

Keychain存储的流程如图所示：

B9C180AB-88D0-44B6-905E-37802CC13B6E.png

简单来说，如果要存储一个password，需要先遍历Keychain，看它的password是否已经存在于Keychain中，存在的话就更新它的值，不存在就存储。
所以这就使用到苹果提供的方法：

// 查询
OSStatus SecItemCopyMatching(CFDictionaryRef query, CFTypeRef *result);

// 添加
OSStatus SecItemAdd(CFDictionaryRef attributes, CFTypeRef *result);

// 更新
OSStatus SecItemUpdate(CFDictionaryRef query, CFDictionaryRef attributesToUpdate);

// 删除
OSStatus SecItemDelete(CFDictionaryRef query)

接下来看看操作Keychain常用的key-value.

kSecClass:有五个值，分别为
      kSecClassGenericPassword(通用密码－－也是接下来使用的)、
      kSecClassInternetPassword(互联网密码)、
      kSecClassCertificate(证书)、
      kSecClassKey(密钥)、
      kSecClassIdentity(身份)

kSecAttrService:服务
kSecAttrServer:服务器域名或IP地址
kSecAttrAccount:账号
kSecAttrAccessGroup: 可以在应用之间共享keychain中的数据
kSecMatchLimit:返回搜索结果，kSecMatchLimitOne（一个）、kSecMatchLimitAll（全部）
//

这是基于CoreFundation框架的操作，通过带有以上key－value值的字典进行操作，所以写起来还是有些繁琐的。所幸苹果提供了一个源码Demo。demo中使用service和account以及accessGroup进行password的存储，也就是一个kSecClassGenericPassword类型的Keychain表，一个account对应一个password，可以很方便在APP中进行Key-Value类型的存储。

不过由于它是使用Swift编写的，在我的小项目中不适合引入混编，所以我就按着demo 的逻辑抄了个OC版的出来。个人看来，OC的还是好懂点，去除了各种try catch，思路上会清晰很多。

初始化方法：

- (instancetype)initWithSevice:(NSString *)service account :(NSString *)account accessGroup:(NSString *)accessGroup {
    if (self = [super init]) {
        _service = service;
        _account = account;
        _accessGroup = accessGroup;//可以在不同的app当中共享
    }
    return self;
}

主要方法有：

- (void)savePassword:(NSString *)password;
- (BOOL)deleteItem;

- (NSString *)readPassword;

//返回当前accessGroup下的service的所有Keychain Item
+ (NSArray *)passwordItemsForService:(NSString *)service accessGroup:(NSString *)accessGroup;

（具体代码在后面贴吧，防止影响篇幅，有兴趣的朋友也可以下demo看看）

使用起来就是：
存储：

    KeychainWrapper *wrapper = [[KeychainWrapper alloc] initWithSevice:kKeychainService account:self.accountField.text accessGroup:kKeychainAccessGroup];
    [wrapper savePassword:self.passwordField.text];

读取当前account 的password：

KeychainWrapper *item = [[KeychainWrapper alloc] initWithSevice:service account:acount accessGroup:accessGroup];
[item readPassword];

返回当前accessGroup下的service的所有Keychain：

NSArray *keychains = [KeychainWrapper passwordItemsForService:kKeychainService accessGroup:kKeychainAccessGroup];

踩坑：OSStatus状态错误码：

• 50: 请求参数错误。我就曾经把kSecAttrService写成了kSecAttrServer，crash到哭了。。。
• 34018:前面说的Capabilities...

代码：

- (void)savePassword:(NSString *)password {
    NSData *encodePasswordData = [password dataUsingEncoding:NSUTF8StringEncoding];
    
    // if password was existed, update
    NSString *originPassword = [self readPassword];
    
    if (originPassword.length > 0) {
        NSMutableDictionary *updateAttributes = [NSMutableDictionary dictionary];
        updateAttributes[(__bridge id)kSecValueData] = encodePasswordData;
        
        NSMutableDictionary *query = [self keychainQueryWithService:_service account:_account accessGroup:_accessGroup];
        OSStatus statusCode = SecItemUpdate(
                                           (__bridge CFDictionaryRef)query,
                                           (__bridge CFDictionaryRef)updateAttributes);
        NSAssert(statusCode == noErr, @"Couldn't update the Keychain Item." );
    }else{
        // else , add
        NSMutableDictionary *attributes = [self keychainQueryWithService:_service account:_account accessGroup:_accessGroup];
        attributes[(__bridge id)kSecValueData] = encodePasswordData;
        
        OSStatus status = SecItemAdd((__bridge CFDictionaryRef)attributes, nil);
        
        NSAssert(status == noErr, @"Couldn't add the Keychain Item.");
    }
}

- (NSString *)readPassword {
    NSMutableDictionary *attributes = [self keychainQueryWithService:_service account:_account accessGroup:_accessGroup];
    attributes[(__bridge id)kSecMatchLimit] = (__bridge id)(kSecMatchLimitOne);
    attributes[(__bridge id)kSecReturnAttributes] = (__bridge id _Nullable)(kCFBooleanTrue);
    attributes[(__bridge id)kSecReturnData] = (__bridge id _Nullable)(kCFBooleanTrue);
    
    CFMutableDictionaryRef queryResult = nil;
    OSStatus keychainError = noErr;
    keychainError = SecItemCopyMatching((__bridge CFDictionaryRef)attributes,(CFTypeRef *)&queryResult);
    if (keychainError == errSecItemNotFound) {
        if (queryResult) CFRelease(queryResult);
        return nil;
    }else if (keychainError == noErr) {
        
        if (queryResult == nil){return nil;}
        
        NSMutableDictionary *resultDict = (__bridge NSMutableDictionary *)queryResult;
        NSData *passwordData = resultDict[(__bridge id)kSecValueData];
        
        NSString *password = [[NSString alloc] initWithData:passwordData encoding:NSUTF8StringEncoding];
        
        return password;
    }else
    {
        NSAssert(NO, @"Serious error.\n");
        if (queryResult) CFRelease(queryResult);
    }
    
    return nil;
}

- (NSMutableDictionary *)keychainQueryWithService:(NSString *)service account:(NSString *)account accessGroup:(NSString *)accessGroup {
    
    NSMutableDictionary *query = [NSMutableDictionary dictionary];
    
    query[(__bridge id)kSecClass] = (__bridge id)kSecClassGenericPassword;
    
    query[(__bridge id)kSecAttrService] = service;
    
    query[(__bridge id)kSecAttrAccount] = account;
    
    query[(__bridge id)kSecAttrAccessGroup] = accessGroup;
    
    return query;
}

+ (NSArray *)passwordItemsForService:(NSString *)service accessGroup:(NSString *)accessGroup {
    NSMutableDictionary *query = [[[self alloc]init] keychainQueryWithService:service account:nil accessGroup:accessGroup];
    
    query[(__bridge id)kSecMatchLimit] = (__bridge id)kSecMatchLimitAll;
    query[(__bridge id)kSecReturnAttributes] = (__bridge id _Nullable)(kCFBooleanTrue);
    query[(__bridge id)kSecReturnData] = (__bridge id _Nullable)(kCFBooleanFalse);
    
    CFMutableArrayRef queryResult = nil;
    OSStatus status = noErr;
    status = SecItemCopyMatching((__bridge CFDictionaryRef)query,(CFTypeRef *)&queryResult);
    
    if (status == errSecItemNotFound) {
        return @[];
    }else if (status == noErr) {
        
        NSArray<NSDictionary *> *resultArray = (__bridge NSArray<NSDictionary *> *)queryResult;
        
        if (resultArray.count == 0){return @[];}
        
        NSMutableArray *passwordItems = [NSMutableArray array];
        
        for (NSDictionary *result in resultArray) {
            NSString *acount = result[(__bridge id)kSecAttrAccount];
            
            if (acount.length > 0) {
                KeychainWrapper *item = [[KeychainWrapper alloc] initWithSevice:service account:acount accessGroup:accessGroup];
                [passwordItems addObject:item];
            }
        }
        
        return passwordItems;
    }else {
        NSAssert(NO, @"Serious error.\n");
        if (queryResult) CFRelease(queryResult);
        return @[];
    }
}

- (BOOL)deleteItem {
    // Delete the existing item from the keychain.
    NSMutableDictionary *query = [self keychainQueryWithService:self.service account:self.account accessGroup:self.accessGroup];
    
    OSStatus status = SecItemDelete((__bridge CFDictionaryRef)query);
    
    if (status != noErr && status != errSecItemNotFound) {
        return NO;
    }
    return true;
}

demo
参考：
https://developer.apple.com/library/prerelease/content/documentation/Security/Conceptual/keychainServConcepts/iPhoneTasks/iPhoneTasks.html#//apple_ref/doc/uid/TP30000897-CH208-SW1
http://www.360doc.com/content/15/0708/15/20918780_483580050.shtml
https://my.oschina.net/w11h22j33/blog/206713