Vigenere 加密

Question

我写了一些代码，里面使用了 Vigenere 加密。这是一个用于加密/解密任何文件的简单程序。

#include<stdio.h>
/*
LANGUAGE: C.
STANDARD: C89.

ABOUT PROGRAM:
This is a simple program for encrypting/decrypting any files.
The size of source file coincide with size of result file.
For encryption of file are use any string key. For decrypting, 
you must to use the same key, which was used for encryption.

NOTES:
The Vigenere encryption are used in it. 
Info at the site: http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher.
This simple algorithm is often used at commercial products. The 
Vigenere's algorithm are using a string key, and 'XOR' for 
encrypting/decrypting information.

WARNING!
The use of this method, doesn't give 100% of a warranty 
for protection of your information. Don't create the keys, 
consisting of identical characters, for example" "aaaaa", 
"zzz", "xxxx" e.t.c. - it is very feeble protection!
Don't forget your encrypting keys... :)

SYNTAX OF USING:

vigenere StringKey SourceFileName ResultFileName

where:
vigenere - program name;
StringKey - string key for encrypting/decrypting;
SourceFileName - source file name;
ResultFileName - result file name;

EXAMPLE OF USING:
vigenere "G5$hj4*df7f3+x" "c:\temp\source.txt" "c:\temp\result.txt"

*/
int main(int argc, char *args[]){
    /****************************************************/
    /* All variables must be defined on top in function, otherwise 
    some compilers can't compile this code (for example - MS 
    Visual Studio 2012. */
    char ch; /* The next char for encrypting/decrypting. */
    char *x; /* String key. */
    FILE *srcFile; /* Source file. */
    FILE *trgFile; /* Result file. */   
    /****************************************************/

    /* The first argument always is a program file name. */
    if (4 != argc)
        return 1; /* Invalid arguments count. */

    if (!*args[1] || !*args[2] || !*args[3])
        return 2; /* Contains the empty argument. */

    x = args[1];
    if ((srcFile = fopen(args[2], "rb")) != NULL){
        if ((trgFile = fopen(args[3], "wb")) != NULL){          
            while((ch = getc(srcFile)) != EOF){
                if(!*x++)
                    x = args[1];
                putc((ch ^= *x), trgFile);
            }           
            fclose(trgFile);
        }
        else
            return 4;  /* Result file wasn't created. */
        fclose(srcFile);
    }
    else
        return 3; /* Source file wasn't opened. */
    return 0; /* Successful operation. */
}

但这段代码并不总是能正常工作。我不明白为什么会发生。我对每个字节进行异或。我已经在such TXT files 上测试了这段代码。我的错在哪里？

Answer 1

 char ch;

 /* ...  */

 while((ch = getc(srcFile)) != EOF)

ch 必须是 int。 EOF 被定义为否定的int。

Answer 2

除了 ouah 的回答，指针值的增量看起来是关闭的。

您的 if 声明 if(!*x++) 不好，原因有两个：

1. 通过在实际 XOR 操作之前执行增量，您将跳过初始循环中键的第一个字符。
2. 如果您已经到达空终止字符，则增加指针是没有意义的。

更好的代码是：

while((ch = getc(srcFile)) != EOF){
    putc((ch ^= *x), trgFile);
    if(!*++x)
        x = args[1];
}