当我们在系统里“删除”了一个文件时，并不意味着这个文件就一定从磁盘上清除了，很多优秀的文件恢复软件都可以恢复被删除的文件，这在一定程度上就带来了隐私泄露的隐患。好在现在很多软件，比如360、电脑管家等等软件都集成了文件粉碎的实用功能。今天介绍一种以前被用于美国国防部的机密文件销毁算法，并附上实现的代码（C）。

美国国防部DOD5220.22M文件销毁标准包括以下三步:

  1. 将文件先用0×00覆盖，再用0×01覆盖，如此重复三次；
  2. 将文件用一个随机值覆盖；
  3. 将文件名改为一个单字符文件名，最后删除之。

此算法虽然已经不再被美国国防部采用，但也足够应付一般的环境，主流文件恢复软件恢复的可能性还有待验证。

/*
*    File Destroyer 文件安全销毁
*
*    Copyright (C) 2015 Chaobs
*
*    This program is free software: you can redistribute it and/or modify
*    it under the terms of the GNU General Public License as published by
*    the Free Software Foundation, either version 3 of the License, or
*    (at your option) any later version.
*
*    This program is distributed in the hope that it will be useful,
*    but WITHOUT ANY WARRANTY; without even the implied warranty of
*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
*    GNU General Public License for more details.
*
*    You should have received a copy of the GNU General Public License
*    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*
*    E-mail: chaobs@outlook.com
*    Blog: www.cnblogs.com/chaobs
*
*    用法: file-destroyer [filename1] <filename2>...
*
*     算法介绍：
*     基于美国国防部DOD5220.22M标准进行文件销毁，包括以下三步:
*     （1）将文件先用0x00覆盖，再用0x01覆盖，如此重复三次；
*     （2）将文件用一个随机值覆盖；
*     （3）将文件名改为一个单字符文件名，最后删除之。
*
*     算法可靠性验证：
*     此算法虽然已经不再被美国国防部采用，但也足够应付一般的环境，对于主流文件恢复软件恢复的可能性还有待验证。
*
*/
 
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
void notice(int i, char *s);    
/* print short notice */
 
void wipe(FILE *f, char c);    
/* core func */
 
long file_size(FILE *f);    
/* get the size of a file */
 
int require(int c, char *s[]);
 
int main(int argc, char *argv[])
{
    int i, j;
    char ch;
 
    FILE *f;
 
    notice(1, argv[0]);
 
    if (argc < 2) {
        
/* too few arguments */
        notice(2, argv[0]);
        exit(0);
    }
 
    if (!require(argc, argv)) {
        fprintf(stderr, "Cancel Operating.\n");
        exit(0);    
/* cancel */
    }
 
    srand(time(NULL));    
/* randomize */
 
    for (i = 1; i < argc; ++i) {
        
/* process each file */
 
        if ((f = fopen(argv[i], "r+b")) == NULL) {
/* fail to open file */
            fprintf(stderr, "Error when open %s:\n", argv[i]);
            exit(0);
        }
 
        for (j = 0; j < 3; ++j) {
            
/* DOD5220.22M Step 1 */
            wipe(f, 0x00);
            wipe(f, 0x01);
        }
 
        wipe(f, rand() % 256);    
/* Step 2 */
 
        if (rename(argv[i], "C")) {
            
/* Step 3*/
            fprintf(stderr, "Error when rename %s\n", argv[i]);
            exit(0);
 
            
/* XXX:文件名冲突的解决？可以考虑使用tmpnam()吗？*/
        }
 
        remove("C"); 
/* XXX:如果是一个符号连接怎样保证删除的是真正的文件？ */
        fclose(f);
    }
 
    printf("Done! Destroy %d files\n", argc - 1);
 
    return 0;
}
 
/* implementation */
void notice(int i, char *s)
{
    if (i == 1) {
        printf("\nFile Destroyer Copyright (C) 2015 Chaobs\n");
        printf("This program comes with ABSOLUTELY NO WARRANTY.\n");
        printf("This is free software, and you are welcome to redistribute under certain conditions.\n\n");
    } else {
        fprintf(stderr, "Usage: %s [filename1] <filename2> ...\n", s);
    }
}
 
void wipe(FILE *f, char c)
{
    long i;
    long len = file_size(f); 
/*获得文件长度*/
 
    for (i = 0; i < len; ++i)
        fwrite(&c, 1, 1, f);    
/* overwrite */
}
 
long file_size(FILE *f)
{
    long len;
    fseek(f, 0, SEEK_END);    
/* jump to the and of file */
    len = ftell(f);
    fseek(f, 0, SEEK_SET);    
/* restore */
    return len; 
/*感谢网友冰尘醉*/
}
 
int require(int c, char *s[])
{
    int i;
    char ch;
    for (i = 1; i < c; ++i) {
        
/* FIXME: the comfirm function can make mistakes and
* it is not convenient even can't work in some cases.
*/
        printf("Do you want to destroy %s ?(y/n) ", s[i]);
        ch = getchar();
        getchar(); 
/* '\n' */
        if (ch == 'n')
            return 0;
    }
 
    return 1;
}