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计算器模式（Counter (CTR)） 计算器模式不常见，在CTR模式中， 有一个自增的算子，这个算子用密钥加密之后的输出和明文异或的结果得到密文，相当于一次一密。这种加密方式简单快速，安全可靠，而且可以并行加密，但是 在计算器不能维持很长的情况下，密钥只能使用一次 。
#include
#include
#include
#include
#include
#include
#include
#include
// 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,int bytes_read)
{
//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,int bytes_read)
{
//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);
int len = strlen(text);
cipher = TextEncrypt(key, text,len);
printf("Chiper text: %s\n", cipher);
decrypted = TextDecrypt(key, cipher,len);
printf("Decrypted text: %s\n", decrypted);
getc(stdin);
return 0;
}
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