A simple utility class to store/retrieve a password in/from the iOS Keychain.

RMPasswordStore.m

//
//  RMPasswordStore.m
//
//  Created by Hermes Pique on 8/9/13.
//
//

#import <Foundation/Foundation.h>
#import <Security/Security.h>

@interface RMPasswordStore : NSObject

+ (RMPasswordStore*) defaultStore;

@property (nonatomic, copy) NSString *password;

@end

@implementation RMPasswordStore

+ (RMPasswordStore*) defaultStore
{
    static RMPasswordStore *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[[self class] alloc] init];
    });
    return sharedInstance;
}

- (void)setPassword:(NSString *)password
{
    NSMutableDictionary *searchDictionary = [RMPasswordStore dictionaryForPasswordSearch];
    OSStatus status;
    NSData *ignore;
    if (SecItemCopyMatching((CFDictionaryRef)searchDictionary, (CFTypeRef *)&ignore) == errSecSuccess)
    { // Update
        if (password == nil)
        {
            status = SecItemDelete((CFDictionaryRef)searchDictionary);
        }
        else
        {
            NSMutableDictionary *updateDictionary = [NSMutableDictionary dictionary];
            NSData *passwordData = [password dataUsingEncoding:NSUTF8StringEncoding];
            [updateDictionary setObject:passwordData forKey:(id)kSecValueData];
            status = SecItemUpdate((CFDictionaryRef)searchDictionary, (CFDictionaryRef)updateDictionary);
        }
        if (status != errSecSuccess)
        {
            NSLog(@"RMPasswordStore: failed to update password with error %ld", status);
        }
    }
    else if (password != nil)
    { // Add
        NSData *passwordData = [password dataUsingEncoding:NSUTF8StringEncoding];
        [searchDictionary setObject:passwordData forKey:(id)kSecValueData];
        status = SecItemAdd((CFDictionaryRef)searchDictionary, NULL);
        if (status != errSecSuccess)
        {
            NSLog(@"RMPasswordStore: failed to add password with error %ld", status);
        }
    }
}

-(NSString*)password
{
    NSMutableDictionary *searchDictionary = [RMPasswordStore dictionaryForPasswordSearch];
    [searchDictionary setObject:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
    [searchDictionary setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    
    NSData *passwordData = nil;
    OSStatus status = SecItemCopyMatching((CFDictionaryRef)searchDictionary, (CFTypeRef *)&passwordData);
    if (status != errSecSuccess)
    {
        NSLog(@"RMPasswordStore: failed to get password with error %ld", status);
        return nil;
    }
    NSString *password = [[NSString alloc] initWithData:passwordData encoding:NSUTF8StringEncoding];
    return [password autorelease];
}

#pragma mark - Private

+ (NSMutableDictionary *)dictionaryForPasswordSearch
{
    NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
    [dictionary setObject:(id)kSecClassGenericPassword forKey:(id)kSecClass];
    
    static NSString *identifier = @"password";
    NSData *encodedIdentifier = [identifier dataUsingEncoding:NSUTF8StringEncoding];
    
    [dictionary setObject:encodedIdentifier forKey:(id)kSecAttrGeneric];
    [dictionary setObject:encodedIdentifier forKey:(id)kSecAttrAccount];

    NSString *serviceName = [NSBundle mainBundle].bundleIdentifier;
    [dictionary setObject:serviceName forKey:(id)kSecAttrService];
    
    return dictionary;
}

@end

I wanted to store a password securely in the simplest manner possible:

[RMPasswordStore defaultStore].password = @"ThePassword"; // to add/update
[RMPasswordStore defaultStore].password = nil; // to remove