Vigenere加密

Question

我编写了一些代码，并在其中使用了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. */
}

但是这段代码并不总是很好用。我不明白为什么会这样。我为每个字节做XOR。我在such TXT files上测试了这段代码。我的错误在哪里？

Answer 1

 char ch;

 /* ...  */

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

ch必须是int。 EOF被定义为负int。

Answer 2

除了ouah的答案，指针值增量看起来很近。

你的if声明，if(!*x++)，有两个原因：

1. 通过在实际的XOR操作之前执行增量，您将在初始循环中跳过键的第一个字符。
2. 如果已经到达空终止字符，则递增指针是没有意义的。

更好的代码是：

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