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)。
算法介绍:
美国国防部DOD5220.22M文件销毁标准包括以下三步:
- 将文件先用0x00覆盖,再用0x01覆盖,如此重复三次;
- 将文件用一个随机值覆盖;
- 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,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] ... * * 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] ...\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; }
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