iOS的keychain服务提供了一种安全的保存私密信息（密码，序列号，证书等）的方式。每个ios程序都有一个独立的keychain存储。从ios 3.0开始，跨程序分享keychain变得可行。

下面就使用keychain来实现存取用户名和密码。

苹果已经有现成的类封装好了keychain,KeychainItemWrapper.h和KeychainItemWrapper.m文件,可以在GenericKeychain实例里找到。

但是这里我只需要存取用户名和密码，就不用苹果提供的类了，自己写个简单的类来实现就好。

代码如下：

CHKeychain.h

#import <Foundation/Foundation.h>
#import <Security/Security.h>
@interface CHKeychain : NSObject
+ (void)save:(NSString *)service data:(id)data;
+ (id)load:(NSString *)service;
+ (void)delete:(NSString *)service;
@end


CHKeychain.m

#import "CHKeychain.h"
@implementation CHKeychain
+ (NSMutableDictionary *)getKeychainQuery:(NSString *)service {
    return [NSMutableDictionary dictionaryWithObjectsAndKeys:
            (id)kSecClassGenericPassword,(id)kSecClass,
            service, (id)kSecAttrService,
            service, (id)kSecAttrAccount,
            (id)kSecAttrAccessibleAfterFirstUnlock,(id)kSecAttrAccessible,
            nil];
}
+ (void)save:(NSString *)service data:(id)data {
    //Get search dictionary
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    //Delete old item before add new item
    SecItemDelete((CFDictionaryRef)keychainQuery);
    //Add new object to search dictionary(Attention:the data format)
    [keychainQuery setObject:[NSKeyedArchiver archivedDataWithRootObject:data] forKey:(id)kSecValueData];
    //Add item to keychain with the search dictionary
    SecItemAdd((CFDictionaryRef)keychainQuery, NULL);
}
+ (id)load:(NSString *)service {
    id ret = nil;
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    //Configure the search setting
    //Since in our simple case we are expecting only a single attribute to be returned (the password) we can set the attribute kSecReturnData to kCFBooleanTrue
    [keychainQuery setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    [keychainQuery setObject:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
    CFDataRef keyData = NULL;
    if (SecItemCopyMatching((CFDictionaryRef)keychainQuery, (CFTypeRef *)&keyData) == noErr) {
        @try {
            ret = [NSKeyedUnarchiver unarchiveObjectWithData:(NSData *)keyData];
        } @catch (NSException *e) {
            NSLog(@"Unarchive of %@ failed: %@", service, e);
        } @finally {
}
    }
    if (keyData)
    CFRelease(keyData);
    return ret;
}
+ (void)delete:(NSString *)service {
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    SecItemDelete((CFDictionaryRef)keychainQuery);
}
@end

首先需要定义几个字符串用来做key：

NSString * const KEY_USERNAME_PASSWORD = @"com.company.app.usernamepassword";
NSString * const KEY_USERNAME = @"com.company.app.username";
NSString * const KEY_PASSWORD = @"com.company.app.password";

把用户名和密码存入keychain：

NSMutableDictionary *usernamepasswordKVPairs = [NSMutableDictionary dictionary];
[usernamepasswordKVPairs setObject:txtfldUsername.text forKey:KEY_USERNAME];
[usernamepasswordKVPairs setObject:txtfldPassword.text forKey:KEY_PASSWORD];
[CHKeychain save:KEY_USERNAME_PASSWORD data:usernamepasswordKVPairs];


从keychain中取出用户名和密码：

NSMutableDictionary *usernamepasswordKVPairs = (NSMutableDictionary *)[CHKeychain load:KEY_USERNAME_PASSWORD];
txtfldUsername.text = [usernamepasswordKVPairs objectForKey:KEY_USERNAME];
txtfldPassword.text = [usernamepasswordKVPairs objectForKey:KEY_PASSWORD];

删除一个keychain item:

[CHKeychain delete:KEY_USERNAME_PASSWORD];

这样一个简单的使用keychain存取用户名密码的功能就做好了。

另外附上一篇写得更详细的e文：http://useyourloaf.com/blog/2010/3/29/simple-iphone-keychain-access.html