用c加密和解密文件

Question

Here I have a simple code for Encryption & Decryption a File in C as below,

#include <stdio.h>
int Encrypt(char * FILENAME, char * NEW_FILENAME)    
{
    FILE *inFile;   //Declare inFile
    FILE *outFile;  //Declare outFile
    char Byte;
    char newByte;
    int n;
    int i=0;

    inFile = fopen(FILENAME,"rb");
    outFile = fopen(NEW_FILENAME, "w");

    if(inFile == NULL || outfile == NULL){
        printf("Error in opening file");
            return 1;
    } else {
        printf("\nFile Opened, Encrypting");
        while(1){
                while ( !feof( inFile ) ){
                    Byte=fgetc(inFile);
                    newByte=Byte+25;
                    fputc(newByte,outFile);
                } 
            printf("End of File");
            break;
        }
        fclose(inFile);
        fclose(outFile);
    }
}

int Decrypt (char *FILENAME, char *NEW_FILENAME)
{
    FILE *inFile; //Declare inFile
    FILE *outFile; //Declare outFile

    char Byte;
    char newByte;
    int i=0;

    inFile = fopen(FILENAME,"rb");
    outFile = fopen(NEW_FILENAME, "w");

    if(inFile == NULL || outfile == NULL){
        printf("Error in opening file");
        return 1;
    } else {
        printf("File Opened, Decrypting");
        while(1){
            printf(".");
            while ( !feof( inFile ) ){
                Byte=fgetc(inFile);
                newByte=Byte-25;
                fputc(newByte,outFile);
            } 
            printf("End of File");
            break;
        }
        fclose(inFile);
        fclose(outFile);
    }
}

int main()
{
    char encFile[200];
    char newencFile[200];
    char decFile[200];
    char newdecFile[200];

    int choice;

    printf("Enter 1 to Encrypt  / 2 to Decrypt");
    scanf("%d",&choice);

    switch(choice)
    {
    case 1:
        printf("Enter the Source Filename:  ");
        scanf("%s",encFile);
        printf("Enter the Destination Filename:   ");
        scanf("%s",newencFile);
        Encrypt(encFile, newencFile);
        break;
    case 2:
        printf("Enter the Source Filename:   ");
        scanf("%s",decFile);
        printf("Enter the Destination Filename:   ");
        scanf("%s",newdecFile);
        Decrypt(decFile, newdecFile);
        break;
    }
    return 0;
}

This code works but at the end of file it also contain "ÿæ" such characters so it can't open in default text-editor in linux & becomes useless to store private details. I want to get same to same stuff back in the discript file. Please help for the same, Thanks in advance.

Answer 1

Your code is not correct. You probably expect that the encrypted file has the same size as the original file, but this is not the case. The faulty code is:

    while(1){
        printf(".");
        while ( !feof( inFile ) ){
            Byte=fgetc(inFile);
            newByte=Byte+25;
            fputc(newByte,outFile);
         } 
         printf("End of File");
         break;
    }

It must instead be:

while ((Byte = fgetc(inFile)) != EOF) {
  newByte = Byte + 25;
  if (fputc(newByte, outFile) == EOF) {
    /* ERROR */
  }
}
printf("End of File\n");

Note that the printf statement ends with a newline character ( \\n ). This is a style which you should follow. Otherwise the output may not show up immediately because of buffering.

Some more details: the feof function only checks if the end of file marker has been set . But when the fgetc function notices that it has reached the end of file, this marker has not been set yet. Instead, the fgetc function returns EOF in that case.

Oh, and please change the types of your variables.

char Byte; /* this is wrong. */
int Byte; /* this is correct. */

See any good introductory book on Programming in C for the details.

Answer 2

What about this new program that uses a different logic:

#include <stdio.h>
int main()
{
    char src[50], tgt[53], ch;
    printf ("Enter file name: ");
    gets (src);
    fs = fopen (src, "r");
    if (fs == NULL)
    {
        printf ("Error while opening file!\n");
        return 1;
    }
    sprintf (tgt, "%s.ed", src);
    ft = fopen (tgt, "w");
    while ((ch = fgetc (fs)) != EOF)
        fputc (~ch, ft);
    fclose (fs);
    fclose (ft);
    remove (src);
    rename (tgt, src);
    return 0;
}

There is no need to write separate functions/programs for both the purposes. Also, the same file will be encrypted by printing the one's complement of a file to another file and renaming the new one to the older one after deleting the older one.
Overall: This is very simple utility!

Answer 3

Before I explain the solution to the above code, Let me tell you these things first. Beaware of unneccessary defination of variables in your program. I see lots of memory wastage in your code.Everyone can write a program but the one makes it more simpler gets the crown.Remember that when you work on programming.

I see Unneccessary variable i,defination in your Encrypt and Decrypt functions. and There is no need to go for a new byte char defination You can sort off make it as

   byte=byte+25 \\ even that makes your work no need of Newbyte char variable.

Erase the int n,int i definations to make your code better.And the most important thing you really got to know is

    you cannot use char a;a=getc(Filepointer);

remember the EOF returns -1 when it reaches to the end of file and if your using a char .It doesnot work. Char data type can only handle positive data .Even if you assign -1 to char, it internally saves it as positive number you can check that thing on google.

      so never use char data type for checkin EOF use int datatype 

and as Ronald mentioned you make it as

       while((Byte=getc(Filepointer))!=EOF)

Thats the error you made.Make it as int datatype.And please check the previous questions.Becuase its actually a duplicate question.Some people can Treat as downvote.

The better style for encrypting and decrypting is using bitwise operations rather than addition

            Byte=Byte^200; To encrypt  \\ just a sample 
            Byte=Byte^200; To  decrypt \\ you can make more complicated

That how you gotta do. It gives you more security compared to adding 25 to your ACSII char.YOu can even encrypt it giving some predefined rules to it.Hope you got the answer and solved you problem.Thank you