AES_ctr128_encrypt string and vice versa ANSI C

Question

I need to insert a 128 hex key to cipher by blocks (16) a string content, and then I want to decrypyt that string(crypted) to a plain text.

Sometimes I have the same output, but not totaly correct! I am doing this right?!

My code now:

#include <openssl/aes.h>
#include <openssl/rand.h> 
#include <openssl/hmac.h>
#include <openssl/buffer.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>

// Code example uses partail code from: http://stackoverflow.com/questions/3141860/aes-ctr-256-encryption-mode-of-operation-on-openssl
// Mostly in the ctr_ state, and init_ctr functions. 

struct ctr_state 
{ 
    unsigned char ivec[AES_BLOCK_SIZE];  
    unsigned int num; 
    unsigned char ecount[AES_BLOCK_SIZE]; 
}; 


AES_KEY key; 

int bytes_read, bytes_written;   
unsigned char indata[AES_BLOCK_SIZE]; 
unsigned char outdata[AES_BLOCK_SIZE];
unsigned char iv[AES_BLOCK_SIZE]; //16?
struct ctr_state state;


int init_ctr(struct ctr_state *state, const unsigned char iv[16])
{        
    /*O aes_ctr128_encrypt exige um 'num' e um 'ecount' definidos a zero na primeira chamada. */
    state->num = 0;
    memset(state->ecount, 0, AES_BLOCK_SIZE); //16?

    /* Inicilaização do contador no 'ivec' a 0 */
    memset(state->ecount, 0, 16); //16?

    /* Copia o IV para o 'ivec' */
    memcpy(state->ivec, iv, 16); //16?
}

char * TextEncrypt(const unsigned char* enc_key, char * text)
{ 
    //Cria vector com valores aleatórios
    if(!RAND_bytes(iv, AES_BLOCK_SIZE))
    {
        printf("Erro: Não foi possivel criar bytes aleatorios.\n");
        exit(1);    
    }

    //Inicializa a chave de encriptação
    if (AES_set_encrypt_key(enc_key, 128, &key) < 0)
    {
        fprintf(stderr, "Nao foi possível definir chave de encriptacao.");
        exit(1);
    }

    init_ctr(&state, iv); //Chamada do contador

    bytes_read = strlen(text);

    AES_set_encrypt_key(enc_key, 128, &key);    

    //Encripta em blocos de 16 bytes e guarda o texto cifrado numa string -> outdata
    AES_ctr128_encrypt(text, outdata, bytes_read, &key, state.ivec, state.ecount, &state.num);

    fflush(stdin);
    return outdata;
}

char * TextDecrypt(const unsigned char* enc_key, unsigned char* cypherText)
{       

    //Inicialização da Chave de encriptação 
    if (AES_set_encrypt_key(enc_key, 128, &key) < 0)
    {
        fprintf(stderr, "Nao foi possível definir chave de decodificacao.");
        exit(1);
    }

    init_ctr(&state, iv);//Chamada do contador

    //Encripta em blocos de 16 bytes e escreve o ficheiro output.txt cifrado         
    bytes_read = strlen(cypherText);    

    AES_set_encrypt_key(enc_key, 128, &key);

    AES_ctr128_encrypt(cypherText, outdata, bytes_read, &key, state.ivec, state.ecount, &state.num);

    fflush(stdin);
    return outdata;
}

int main(int argc, char *argv[])
{
    char text [] = "Text test to my program";

    //Receive hexadecimal 128 bits key 
    unsigned const char * key = "1234567812345678";
    //unsigned const char * key = "9EF4BCDE";   
    char * cipher, * decrypted;

    printf("Clean text: %s\n", text);

    cipher = TextEncrypt(key, text);
    printf("Chiper text: %s\n", cipher);

    decrypted = TextDecrypt(key, cipher);
    printf("Decrypted text: %s\n", decrypted);

    getc(stdin);
    return 0;
}

Can anybody help, please?! Thanks.

Answer 1

You are using strlen() on your CipherText, which may contain null bytes. This will cause bytes_read to be less than the actual length of the data you want to decrypt.