J'essaie d'écrire un exemple de programme pour faire du cryptage AES en utilisant Openssl.
Cette réponse est assez populaire, je vais donc proposer quelque chose de plus à jour depuis qu'OpenSSL a ajouté certains modes de fonctionnement qui vous seront probablement utiles.
D'abord, n'utilisez pas AES_encrypt y AES_decrypt . Ils sont de bas niveau et plus difficiles à utiliser. De plus, il s'agit d'une routine uniquement logicielle, et elle sera jamais utiliser l'accélération matérielle, comme AES-NI. Enfin, il est sujet à des problèmes d'endianess sur certaines plateformes obscures.
Au lieu de cela, utilisez le EVP_* interfaces. Le site EVP_* Les fonctions utilisent l'accélération matérielle, comme AES-NI, si elle est disponible. Et il ne souffre pas de problèmes d'endianess sur les plateformes obscures.
Deuxièmement, vous pouvez utiliser un mode comme CBC, mais le texte chiffré manquera de garanties d'intégrité et d'authenticité. Il faut donc généralement utiliser un mode comme EAX, CCM ou GCM. (Ou vous devez appliquer manuellement un HMAC après le chiffrement sous une clé distincte).
Troisièmement, OpenSSL possède une page wiki qui vous intéressera probablement : Cryptage et décryptage authentifiés par EVP . Il utilise le mode GCM.
Enfin, voici le programme pour crypter en utilisant AES/GCM. L'exemple du wiki OpenSSL est basé sur celui-ci.
#include <openssl/evp.h>
#include <openssl/aes.h>
#include <openssl/err.h>
#include <string.h>
int main(int arc, char *argv[])
{
OpenSSL_add_all_algorithms();
ERR_load_crypto_strings();
/* Set up the key and iv. Do I need to say to not hard code these in a real application? :-) */
/* A 256 bit key */
static const unsigned char key[] = "01234567890123456789012345678901";
/* A 128 bit IV */
static const unsigned char iv[] = "0123456789012345";
/* Message to be encrypted */
unsigned char plaintext[] = "The quick brown fox jumps over the lazy dog";
/* Some additional data to be authenticated */
static const unsigned char aad[] = "Some AAD data";
/* Buffer for ciphertext. Ensure the buffer is long enough for the
* ciphertext which may be longer than the plaintext, dependant on the
* algorithm and mode
*/
unsigned char ciphertext[128];
/* Buffer for the decrypted text */
unsigned char decryptedtext[128];
/* Buffer for the tag */
unsigned char tag[16];
int decryptedtext_len = 0, ciphertext_len = 0;
/* Encrypt the plaintext */
ciphertext_len = encrypt(plaintext, strlen(plaintext), aad, strlen(aad), key, iv, ciphertext, tag);
/* Do something useful with the ciphertext here */
printf("Ciphertext is:\n");
BIO_dump_fp(stdout, ciphertext, ciphertext_len);
printf("Tag is:\n");
BIO_dump_fp(stdout, tag, 14);
/* Mess with stuff */
/* ciphertext[0] ^= 1; */
/* tag[0] ^= 1; */
/* Decrypt the ciphertext */
decryptedtext_len = decrypt(ciphertext, ciphertext_len, aad, strlen(aad), tag, key, iv, decryptedtext);
if(decryptedtext_len < 0)
{
/* Verify error */
printf("Decrypted text failed to verify\n");
}
else
{
/* Add a NULL terminator. We are expecting printable text */
decryptedtext[decryptedtext_len] = '\0';
/* Show the decrypted text */
printf("Decrypted text is:\n");
printf("%s\n", decryptedtext);
}
/* Remove error strings */
ERR_free_strings();
return 0;
}
void handleErrors(void)
{
unsigned long errCode;
printf("An error occurred\n");
while(errCode = ERR_get_error())
{
char *err = ERR_error_string(errCode, NULL);
printf("%s\n", err);
}
abort();
}
int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *aad,
int aad_len, unsigned char *key, unsigned char *iv,
unsigned char *ciphertext, unsigned char *tag)
{
EVP_CIPHER_CTX *ctx = NULL;
int len = 0, ciphertext_len = 0;
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();
/* Initialise the encryption operation. */
if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
handleErrors();
/* Set IV length if default 12 bytes (96 bits) is not appropriate */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
handleErrors();
/* Initialise key and IV */
if(1 != EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();
/* Provide any AAD data. This can be called zero or more times as
* required
*/
if(aad && aad_len > 0)
{
if(1 != EVP_EncryptUpdate(ctx, NULL, &len, aad, aad_len))
handleErrors();
}
/* Provide the message to be encrypted, and obtain the encrypted output.
* EVP_EncryptUpdate can be called multiple times if necessary
*/
if(plaintext)
{
if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
handleErrors();
ciphertext_len = len;
}
/* Finalise the encryption. Normally ciphertext bytes may be written at
* this stage, but this does not occur in GCM mode
*/
if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) handleErrors();
ciphertext_len += len;
/* Get the tag */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag))
handleErrors();
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
return ciphertext_len;
}
int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *aad,
int aad_len, unsigned char *tag, unsigned char *key, unsigned char *iv,
unsigned char *plaintext)
{
EVP_CIPHER_CTX *ctx = NULL;
int len = 0, plaintext_len = 0, ret;
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();
/* Initialise the decryption operation. */
if(!EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
handleErrors();
/* Set IV length. Not necessary if this is 12 bytes (96 bits) */
if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
handleErrors();
/* Initialise key and IV */
if(!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();
/* Provide any AAD data. This can be called zero or more times as
* required
*/
if(aad && aad_len > 0)
{
if(!EVP_DecryptUpdate(ctx, NULL, &len, aad, aad_len))
handleErrors();
}
/* Provide the message to be decrypted, and obtain the plaintext output.
* EVP_DecryptUpdate can be called multiple times if necessary
*/
if(ciphertext)
{
if(!EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
handleErrors();
plaintext_len = len;
}
/* Set expected tag value. Works in OpenSSL 1.0.1d and later */
if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, tag))
handleErrors();
/* Finalise the decryption. A positive return value indicates success,
* anything else is a failure - the plaintext is not trustworthy.
*/
ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
if(ret > 0)
{
/* Success */
plaintext_len += len;
return plaintext_len;
}
else
{
/* Verify failed */
return -1;
}
}
