加密/解密文件libgcrypt

Question

I've struggled to understand how to correctly use libgcrypt and have done my best to punch out some code. Nevertheless, I'm attempting to encrypt (AES 256) and decrypt a file utilizing the aforementioned library. The user will type the file name as a command-line argument. The encrypted file will be saved as a second file (output.txt). Then, I want to decrypt the file to ensure that my result was correct. Each time I run the program, the key that I generate has some foreign characters and the output file generated is blank. Am I using the derive function correctly? And am I just decrypting the file incorrectly? I apologize in advance for my terrible code.

#include <stdio.h>
#include <gcrypt.h>
#include <time.h>
#include <sys/stat.h>
//#define GCRY_CIPHER_AES256
//#define GCRY_KDF_PBKDF2
void intitializeLibrary();

int main(int argc, char **argv)
{
if(argc ==2){
    char *filename = argv[1];
    printf("The file you've requested to be encrypted is %s \n", filename);
    FILE *ifp, *ifpout;
    char *mode ="r";

    ifp = fopen(filename, mode);
    ifpout = fopen("output.txt", "w");
    if (ifp == NULL || ifpout == NULL){
        fprintf(stderr, "Can't open file! \n");
        exit(1);
    } else{
        intitializeLibrary();
        struct stat st;
        stat(filename, &st);
        int size = st.st_size;

        char *encBuffer = malloc(32);       
        char str[100];
        char keybuffer[32];
        char* salt = "CNT5410";
        char* iv = "assignment222222";
        printf("Enter password: ");
        gcry_cipher_hd_t handle;
        gets(str);

        gpg_error_t err;
        err = gcry_kdf_derive(str, 100, GCRY_KDF_PBKDF2, GCRY_CIPHER_AES256, salt, 7, 3, 32, keybuffer); 
        if(err){
            printf("Error in PBKDF2 \n");
            exit(1);
        }

        puts(keybuffer);

        err = gcry_cipher_open(&handle, GCRY_CIPHER_AES256, GCRY_CIPHER_MODE_ECB, 0);
        if(err){
            printf("Error in cipheropen\n");
            exit(1);
        }

        err = gcry_cipher_setkey(handle, keybuffer, 32);
        if(err){
            printf("Error in cipheropen\n");
            exit(1);
        }

        err = gcry_cipher_setiv(handle, iv, 16);
        int bytes;
        while(!feof(ifp)){
            bytes = fread(encBuffer, 1, size, ifp);
            if(!bytes){
                break;
            }
            while(bytes < 32){
                encBuffer[bytes++] = 0x0;
                gcry_cipher_encrypt(handle, encBuffer, size, NULL,0);
                bytes = fwrite(encBuffer, size, 16, ifpout);
            }
        }

        gcry_cipher_close(handle);
        fclose(ifp);
        fclose(ifpout);

        struct stat st2;
        stat(filename, &st2);
        int size2 = st2.st_size;

        err = gcry_cipher_open(&handle, GCRY_CIPHER_AES256, GCRY_CIPHER_MODE_ECB, 0);
        if(err){
            printf("Error in cipheropen\n");
            exit(1);
        }

        err = gcry_cipher_setkey(handle, keybuffer, 32);
        if(err){
            printf("Error in cipheropen\n");
            exit(1);
        }

        err = gcry_cipher_setiv(handle, iv, 16);
    ifp = fopen("output.txt", mode);
    ifpout = fopen(filename, "w");
        while(!feof(ifp)){
            bytes = fread(encBuffer, 1, size2, ifp);
            if(!bytes){
                break;
            }
            while(bytes < 32){
                encBuffer[bytes++] = 0x0;
                gcry_cipher_encrypt(handle, encBuffer, size2, NULL,0);
                bytes = fwrite(encBuffer, 1, size2, ifpout);
            }
        }

        free(encBuffer);
        encBuffer = NULL;


    } 
}else{
    puts("Please enter one file to encrypt");
}
return 0;
}

void intitializeLibrary(){

    /* Version check should be the very first call because it
            makes sure that important subsystems are intialized. */
            if (!gcry_check_version (GCRYPT_VERSION)){
            fputs ("libgcrypt version mismatch\n", stderr);
            exit (2);
            }

              /* Disable secure memory.  */
              gcry_control (GCRYCTL_DISABLE_SECMEM, 0);

              /* ... If required, other initialization goes here.  */

              /* Tell Libgcrypt that initialization has completed. */
              gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);

              /* Disable secure memory.  */
              gcry_control (GCRYCTL_DISABLE_SECMEM, 0);

              /* ... If required, other initialization goes here.  */

              /* Tell Libgcrypt that initialization has completed. */
              gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);
}

Answer 1

You are assuming that the key consists of printable characters. It does not, it consists of bytes ( char in C/C++) that may have any value (0..255 or, in your case, if unsigned, -128 to 127). If you print them to standard output then the standard output will decode these bytes using the currently configured character encoding and show the result. So that basically shows any kind of garbage. So this is no problem; print out the key in hexadecimals to get an idea of the key value.

As for the error, you are opening your files in text mode instead of binary mode. Ciphertext, just like the key, contains of random bytes. You should open your ciphertext files in binary mode. Alternatively you can convert the ciphertext bytes to hexadecimals or base 64.

From the fopen manual pages:

In order to open a file as a binary file, a "b" character has to be included in the mode string. This additional "b" character can either be appended at the end of the string (thus making the following compound modes: "rb", "wb", "ab", "r+b", "w+b", "a+b") or be inserted between the letter and the "+" sign for the mixed modes ("rb+", "wb+", "ab+").

Try and learn about encoding such as hexadecimals and base 64 as well as character encoding such as ASCII and UTF-8. You need that if you want to perform any kind of encryption/decryption.