如何解密使用cbc_encrypt加密的内容 （Linux GCC）

Question

我想根据我在perl中所做的事情用C语言做一个加密/解密程序。编译后的perl程序是2MB，所以我想如果我用C语言编写它，它的可执行大小会更小。

我的问题是，虽然我让它加密，但我无法解密它。自从我上次使用C以来，已经很久了，所以我忘记了很多东西。有人请启发我在这里做错了什么？谢谢。

/*
    ============================================================================
    Name        : test-c.c
    Description : Testing Project
                  Trying to do a C version of this perl code:
                    my $cipher = Crypt::CBC->new( -key => $salt_key, -cipher => 'DES' -header => 'none');
                    my $enc_text = $cipher->encrypt_hex($text);
    Requires    : -lcrypt
    References  :
      Function: cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)
      GNU C Library: DES Encryption (http://www.gnu.org/software/libc/manual/html_node/DES-Encryption.html#DES-Encryption)
      cbc_crypt (http://unix.derkeiler.com/Newsgroups/comp.unix.programmer/2012-10/msg00023.html)
    ============================================================================
    */

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <rpc/des_crypt.h>

    int main(void) {
        char key[]     = "aBcDeFg1";
        char pass[]    = "mypass1234test";
        char encbuff[] = "87654321";
        char decbuff[] = "87645321";
        int buffsize;
        int result;

        des_setparity(key);

        /* Encrypt pass, result is in encbuff */
        buffsize = sizeof(pass);
        /* Add to pass to ensure size is divisable by 8. */
        while (buffsize % 8) {
            pass[buffsize++] = '\0';
        }

        printf("Encrypted: ");
        result = cbc_crypt(key, pass, buffsize, DES_ENCRYPT | DES_SW, encbuff);
        if (DES_FAILED(result) || strcmp(encbuff, "") == 0) {
            if(strcmp(encbuff, "") == 0) {
                printf("*** Null Output ***\n");
            } else {
                printf("*** Encryption Error ***\n");
            }
        } else {
            printf("%s\n", encbuff);
        }

        /* Decrypt encbuff, result is in decbuff */

        /* FIXME: Decryption doesn't work:
            Encrypted: ,�7&���8
            Decrypted: *** Decryption Error ***
        */
        buffsize = sizeof(encbuff);
        /* Add to pass to ensure size is divisable by 8. */
        while (buffsize % 8) {
            encbuff[buffsize++] = '\0';
        }

        printf("Decrypted: ");
        result = cbc_crypt(key, encbuff, buffsize, DES_DECRYPT | DES_SW, decbuff);
        if(DES_FAILED(result) || strcmp(decbuff, "") == 0) {
            if(strcmp(encbuff, "") == 0) {
                printf("*** Null Output ***\n");
            } else {
                printf("*** Decryption Error ***\n");
            }
        } else {
            printf("%s\n", decbuff);
        }

        return 0;
    }

Answer 1

正如@Brian_Sidebotham所注意到的，使用＜code＞char str[10]；buffsize=大小（str）；。。。str[buffsize]=“\0”在c中不是一件好事。在数组末尾之外写入可能会触发未定义的行为。它可能会工作，或者提供错误的结果，或者由于分段错误而停止。因此，首先要做的是使用更大的缓冲区。例如：

char key[9];
sprintf(key,"12345678");

char password[420];
sprintf(password,"i am not going to write my password here.");

请注意，它是9，而不是8，为空终止字符< code>\0留出了一些空间，该字符由< code>sprintf()自动添加以构建有效的c字符串。

而且，< code>cbc_crypt()的最后一个参数不是输出。它不是加密的字符串，而是用于链接的8字节初始化向量。这样，如果您需要加密多个字符串，它可以防止明文中的规则出现在密文中。因为要解密密文，所以必须提供相同的初始化向量。加密就地执行:输入字符串< code>pass将被加密的文本覆盖。

最后，确保您的字符串足够大，因为加密文本似乎比明文稍大。

以下代码应该可以正常工作。它由 gcc 主编纂.c -o 主

/*
    ============================================================================
    Name        : test-c.c
    Description : Testing Project
                  Trying to do a C version of this perl code:
                    my $cipher = Crypt::CBC->new( -key => $salt_key, -cipher => 'DES' -header => 'none');
                    my $enc_text = $cipher->encrypt_hex($text);
    Requires    : -lcrypt
    References  :
      Function: cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)
      GNU C Library: DES Encryption (http://www.gnu.org/software/libc/manual/html_node/DES-Encryption.html#DES-Encryption)
      cbc_crypt (http://unix.derkeiler.com/Newsgroups/comp.unix.programmer/2012-10/msg00023.html)
    ============================================================================
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <rpc/des_crypt.h>

#define BUFFSIZE 420
int main(void) {
    //  char key[]     = "aBcDeFg1";
    char key[9];
    sprintf(key,"aBcDeFg1");
    //char pass[]    = "mypass1234test";
    char pass[BUFFSIZE];
    sprintf(pass,"mypass1234test");

    //  char encbuff[] = "87654321";
    char ivec[9];
    sprintf(ivec,"87654321");
    //  char decbuff[] = "87645321";
    char ivectemp[9];
    strcpy(ivectemp,ivec);
    int buffsize;
    int result;

    des_setparity(key);

    /* Encrypt pass, result is in encbuff */
    buffsize = strlen(pass);
    printf("buffsize is %d\n",buffsize);
    /* Add to pass to ensure size is divisable by 8. */
    while (buffsize % 8 && buffsize<BUFFSIZE) {
        pass[buffsize++] = '\0';
    }
    printf("pass is %s\n",pass);
    printf("buffsize is %d\n",buffsize);
    printf("Encrypted: ");
    result = cbc_crypt(key, pass, buffsize, DES_ENCRYPT | DES_SW, ivectemp);
    if (DES_FAILED(result) || strcmp(pass, "") == 0) {
        if(strcmp(pass, "") == 0) {
            printf("*** Null Output ***\n");
        } else {
            printf("*** Encryption Error ***\n");
        }
    } else {
        printf("%s\n", pass);
    }

    /* Decrypt encbuff, result is in decbuff */

    /* FIXME: Decryption doesn't work:
            Encrypted: ,�7&���8
            Decrypted: *** Decryption Error ***
     */
    buffsize = strlen(pass);
    printf("buffsize is %d\n",buffsize);
    /* Add to pass to ensure size is divisable by 8. */
    while (buffsize % 8 && buffsize<BUFFSIZE) {
        pass[buffsize++] = '\0';
    }
    printf("buffsize is %d\n",buffsize);
    printf("Decrypted: ");
    //keeping the same initialization vector for decrypting, encrypt has altered ivectemp
    strcpy(ivectemp,ivec);
    result = cbc_crypt(key, pass, buffsize, DES_DECRYPT | DES_SW, ivectemp);
    if(DES_FAILED(result) || strcmp(pass, "") == 0) {
        if(strcmp(pass, "") == 0) {
            printf("*** Null Output ***\n");
        } else {
            printf("*** Decryption Error ***\n");
        }
    } else {
        printf("%s\n",pass);
    }

    return 0;
}

现在…关于安全的几句话…根据维基百科上关于数据加密标准NBS DES的页面：

对于许多应用程序来说，DES现在被认为是不安全的。这主要是由于56位密钥尺寸太小；1999年1月，distributed.net和电子前沿基金会合作，在22小时15分钟内公开破解了一个DES密钥

例如，您可以通过openssl库切换到AES的实现。请参阅如何在Openssl中使用AES进行加密，或使用C中的Openssl库进行简单AES加密解密，或使用Openssl C进行AES（AES-bc-128、AES-cb-192、AES-bc-256）加密/解密。