decrypt_file_stepbystep.cpp

Voici un exemple de programme permettant de déchiffrer un fichier pas à pas (algorithme symétrique). Ce code est générique et permet d'utiliser tous les algorithmes présent dans la librairie. Les éléments fixés ne le sont qu'à titre d'exemple.

/* -----------------------------------------------------------------------
 *
 * Copyright (c) 2002-2005 Mr Ludovic FLAMENT <ludovic.flament@free.fr>, LIEVIN, FRANCE
 *
 * Termes :
 *
 * L'utilisation et/ou la redistribution de ce code (avec ou sans modification),
 * ainsi que de tous les composants du produit (librairies, documentation, exemples, ...)
 * est soumise aux termes de la licence qui vous est attribuée par son auteur.
 *
 * -----------------------------------------------------------------------
 */

#include <stdlib.h>
#include <stdio.h>

#include <include/ecctk-prototype.h>
#include <include/ecctk-cipher.h>

/* example is only for RC4
 */
s2byte decrypt_file_stepbystep()
{
    /* set input/output file name
     */
    s1byte  infile[]    = "protected_file",
            outfile[]   = "unprotected_file";

    /* set a static 192 bits key
     */
    u1byte  key[] = {0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
                     0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff,
                     0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77};

    /* tmp buffer to read input file by block
     * and buffer to write encrypted data
     */
    u1byte  rbuff[8192],
            out[8192]; /* no padding with RC4 */

    s4byte  rsize = 0,
            outsize = 0;

    FILE    *file_in,
            *file_out;

    /* RC4 context
     */
    RC4_CTX *ctx;

    /* initialize the context
     */
    ctx = RC4_Init(ECCTK_CIPHER_DECRYPT, key, sizeof(key));
    if (ctx == NULL)
    {
        fprintf(stderr, "Initialize RC4 context failed\n");
        return EXIT_FAILURE;
    }

    /* open input file
     */
    file_in = fopen(infile, "rb");
    if (file_in == NULL)
    {
        fprintf(stderr, "Open %s failed\n", infile);
        return EXIT_FAILURE;
    }
    
    /* open output file
     */
    file_out = fopen(outfile, "wb");
    if (file_out == NULL)
    {
        fprintf(stderr, "Open %s failed\n", outfile);

        fclose(file_in);
        return EXIT_FAILURE;
    }

    /* read input file by block and update decrypt
     */
    while (!feof(file_in))
    {
        if (outsize)
        {
            /* write encrypted data
             */
            if (fwrite(out, sizeof(u1byte), outsize, file_out) <= 0)
            {
                fprintf(stderr, "Write on %s failed\n", outfile);

                fclose(file_in);
                fclose(file_out);
                RC4_Free(ctx);
                return EXIT_FAILURE;
            }
        }

        /* read plain data
         */
        rsize = fread(rbuff, sizeof(u1byte), sizeof(rbuff), file_in);
        if (rsize <= 0)
        {
            fprintf(stderr, "Read on %s failed\n", infile);

            fclose(file_in);
            fclose(file_out);
            RC4_Free(ctx);
            return EXIT_FAILURE;
        }

        /* update the read block
         */
        outsize = RC4_Update(ctx, rbuff, out, rsize);
        if (outsize < ECCTK_CIPHER_NO_ERROR)
        {
            fprintf(stderr, "Update RC4 context failed (Error : %ld)\n", outsize);

            fclose(file_in);
            fclose(file_out);
            RC4_Free(ctx);
            return EXIT_FAILURE;
        }
    }

    /* finalize last block
     */
    outsize = RC4_Final(ctx, out);
    if (outsize < ECCTK_CIPHER_NO_ERROR)
    {
        fprintf(stderr, "Finalize RC4 context failed (Error : %ld)\n", outsize);

        fclose(file_in);
        fclose(file_out);
        RC4_Free(ctx);
        return EXIT_FAILURE;
    }

    if (outsize)
    {
        /* write encrypted data
         */
        if (fwrite(out, sizeof(u1byte), outsize, file_out) <= 0)
        {
            fprintf(stderr, "Write on %s failed\n", outfile);

            fclose(file_in);
            fclose(file_out);
            RC4_Free(ctx);
            return EXIT_FAILURE;
        }
    }

    fprintf(stdout, "Decrypt %s success, %s generated\n", infile, outfile);

    /* close file, free context
     */
    fclose(file_in);
    fclose(file_out);
    RC4_Free(ctx);

    return EXIT_SUCCESS;
}

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