c - 手册 "encryption"只输出0到文件

Question

我的作业(不是家庭作业，只是“如果你能做到就试试”的事情)是使用位操作来加密和解密 .txt 文件。

这是程序。它成功打开文件进行读/写，但将所有 0 和空格放入 output.txt 文件而不是预期的“加密”文本。我猜这个问题来自于对数据类型或 putc() 的根本误解。我知道它输出一个 unsigned char，但我的教授说 unsigned char 只不过是一个 unsigned int——不确定这是否完全正确，或者它是否是一种教学简化。非常感谢您的帮助。

#include <stdio.h>
#include <ctype.h>
#include <string.h>

#define NUMARG 3
#define INFILEARG 1
#define OUTFILEARG 2

int main(int argc, char *argv[]){
    /* Function prototypes */
    unsigned int encryptDecrypt(unsigned int x, unsigned int ed);
    const char *get_filename_ext(const char *filename);

    FILE *finp;
    FILE *foutp;

    //ed for encryption/decryption choice
    unsigned int ed, c;
    const char *ext;

    //Check for errors in argument number and file opening.
    if(argc != NUMARG){
        printf("You have to put the input and output files after the 
                program name.\n");
        return(1);
    }

    if( (finp = fopen(argv[INFILEARG], "r")) == NULL ){
         printf("Couldn't open %s for reading.\n", argv[INFILEARG]);
         return(1);
    }

    if( (foutp = fopen(argv[OUTFILEARG], "w")) == NULL){
        printf("Couldn't open %s for writing.\n", argv[OUTFILEARG]);
        return(1);
    }


    //Get and check file extension.
    ext = get_filename_ext(argv[INFILEARG]);
    if(strcmp(ext, "txt")){
        printf("Input file is not a .txt file.\n");
        return(1);
    }

    ext = get_filename_ext(argv[OUTFILEARG]);
    if(strcmp(ext, "txt")){
        printf("Output file is not a .txt file.\n");
        return(1);
    }

    //Get command to encrypt or decrypt.
    do{
        printf("Enter e to encrypt, d to decrypt: ");
        ed = getchar();
    } while(ed != 'e' && ed != 'd');


    //Send characters to output file.
    while((c = getc(finp)) != EOF ){
         putc(encryptDecrypt(c, ed), foutp);
    }


    // Close files.
    if (fclose(finp) == EOF){
         printf("Error closing input file.\n");
    }

    if (fclose(foutp) == EOF){
        printf("Error closing output file.\n");
    }

    if ( ed == 'e'){
        printf("Encrypted data written.\n");
    } else {
         printf("Data decrypted.\n");
    }

    return 0;
}

const char *get_filename_ext(const char *filename) {
    const char *dot = strrchr(filename, '.');
    if(!dot || dot == filename) return "";
    return dot + 1;
}

unsigned int encryptDecrypt(unsigned int c, unsigned int ed){
    if( ed == 'e' ){
        printf("%d before operated on.\n", c);
        c &= 134;
        printf("%d after &134.\n", c);
        c ^= 6;
        printf("%d after ^6. \n", c);
        c <<= 3; 
        printf("%d after <<3\n", c);
    }
    else {
        c >>= 3;
        c ^= 6;
        c &= 134;       
    }   
    return c;
}

输出:

ZacBook:bitoperations $ cat input1.txt
Please encrypt this message.
ZacBook:bitoperations $ ./encrypt.o input1.txt output.txt
Enter e to encrypt, d to decrypt: e
80 before operated on.
0 after &134.
6 after ^6. 
48 after <<3
108 before operated on.
4 after &134.
2 after ^6. 
16 after <<3
[...Many more of these debug lines]
2 after &134.
4 after ^6. 
32 after <<3
Encrypted data written.
ZacBook:bitoperations $ cat output.txt
00 0  00000 0  0 

如您所见，unsigned int 正在成功运行。我认为问题出在 putc() 上，但我已尝试将 c 的类型更改为 char 和 int，但均无效。

Answer 1

你的主要问题是&=是一个有损转换:也就是说你丢失了数据。

同上<<=和 >>= ，因为两者都会导致极端 1 位丢失。

坚持异或你会更幸运；起初至少。那是因为 x ^ y ^ y是x .

你可以消除putc ＆C。通过将加密/解密过程与数据采集阶段隔离开来，并在让事情正常运行的同时对输入进行硬编码。