file.m

    NSString *bundlePath = [[NSBundle mainBundle] bundlePath];
    NSLog(@"bundlePath is %@",bundlePath);
    
    NSString *documentDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentationDirectory, NSUserDomainMask, YES) lastObject];
    NSLog(@"documentDirectory is %@", documentDirectory);
    
    NSString *libraryDirectory = [NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES) lastObject];
    NSLog(@"libraryDirectory is %@", libraryDirectory);
    
    NSString *cacheDirectory = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
    NSLog(@"cacheDirectory is %@", cacheDirectory);
    
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *filePathName = [documentDirectory stringByAppendingPathComponent:@"1.txt"];
    NSError *error = nil;
    
    NSDictionary *fileAttributes = [fileManager attributesOfItemAtPath:filePathName
                                                                 error:&error];
    
    if (!error) {
        NSDate *dateFileCreated = [fileAttributes valueForKey:NSFileCreationDate];
        NSString *fileType = [fileAttributes valueForKey:NSFileType];
        NSLog(@"This %@ file was created on %@", fileType, dateFileCreated);
    }
    
    NSArray *listOfFiles = [fileManager contentsOfDirectoryAtPath:documentDirectory error:&error];
    if (!error) {
        NSLog(@"Contents of Documentation directory: %@",listOfFiles);
    }
    NSArray *listOfSubPaths = [fileManager subpathsOfDirectoryAtPath:documentDirectory error:&error];
    if (!error) {
        NSLog(@"Sub paths of Documentation directory: %@",listOfSubPaths);
    }
    
    
    
    
//
//  ViewController.m
//  FileEncrypt
//
//  Created by yxj on 5/20/15.
//  Copyright (c) 2015 yxj. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
        NSString *documentDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentationDirectory, NSUserDomainMask, YES) lastObject];
    NSLog(@"%@",documentDirectory);

    NSString *filePathName = [documentDirectory stringByAppendingPathComponent:@"1.jpg"];

    
    //把待上传的文件读入NSData
    NSData *fileData = [[NSData alloc] initWithContentsOfFile:filePathName];
    [self.imageView setImage:[UIImage imageWithData:fileData]];

    NSMutableData *afterFile = [[NSMutableData alloc] init];
    [afterFile appendData:fileData];
    NSMutableData *keyContainer = [[NSMutableData alloc] init];
    NSLog(@"file: %@",fileData);
    
    //跳过文件头，均匀的抽取，存到一个字符串中，作为key
    
    
    //key的结构是offset占4个字节，length占4个字节，content占自己的大小，因为我采用均匀抽取所以每一段的大小都是固定的。

//    [keyContainer appendData:[NSData dataWithBytes:&offset length:sizeof(offset)]];
//    NSLog(@"%@",keyContainer);
    int PERCENTAGE = 1;
    int ex_location = 0;
    int ex_length = 0;
    ex_length = PERCENTAGE;
    
    int fileHeaderLength = 16;
    
    for(ex_location += fileHeaderLength; ex_location+ex_length < fileData.length; ex_location += 100-ex_length){
        [keyContainer appendData:[NSData dataWithBytes:&ex_location length:sizeof(ex_location)]];
        [keyContainer appendData:[NSData dataWithBytes:&ex_length length:sizeof(ex_length)]];
        [keyContainer appendData:[fileData subdataWithRange:NSMakeRange(ex_location, ex_length)]];
        [afterFile resetBytesInRange:NSMakeRange(ex_location, ex_length)];
    }
//    NSLog(@"key: %@",keyContainer);
    NSLog(@"file: %@",afterFile);
    [self.imageView setImage:[UIImage imageWithData:afterFile]];
//    [afterFile writeToFile:filePathName atomically:YES];
    //把抽取了的位置填0
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end