tech13-1.c

#include <openssl/evp.h>
#include <openssl/sha.h>
#include <stdio.h>
#include <string.h>

enum { MAX_SIZE = 4096 };

int main(int argc, char* argv[])
{
    unsigned char data[MAX_SIZE] = "";
    int num_read = 0;

    num_read = fread(data, sizeof(data[0]), MAX_SIZE, stdin);

    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();

    unsigned char key[32], iv[32];

    unsigned char* password = (unsigned char*)argv[1];
    size_t pass_len = strlen(argv[1]);

    EVP_BytesToKey(
        EVP_aes_256_cbc(),
        EVP_sha256(),
        data + 8,
        password,
        pass_len,
        1,
        key,
        iv);

    EVP_DecryptInit(ctx, EVP_aes_256_cbc(), key, iv);

    unsigned char decrypted[MAX_SIZE] = "";
    int p_len = num_read;
    EVP_DecryptUpdate(ctx, decrypted, &p_len, data, num_read); 

    int num_decrypted = 0;
    EVP_DecryptFinal(ctx, decrypted + p_len, &num_decrypted);

    //fwrite(decrypted, sizeof(decrypted[0]), num_decrypted + p_len, stdout);
    printf("%s", (char*)decrypted);

    return 0;
}