Cryptage / décryptage des chaînes de texte à l'aide d'OpenSSL ECC

Car c'est tellement dur de trouver des exemples montrant comment utiliser ECC pour chiffrer les données, je pensais que je poste un peu de code à utiliser pour les autres. Pour la liste complète, consultez mon openssl-dev affichage:

http://www.mail-archive.com/[email protected]/msg28042.html

Fondamentalement, de son une rincée version utilisable de la façon d'utiliser ECDH pour fixer un bloc de données. ECDH est utilisé pour générer un secret partagé. Le secret partagé est ensuite hachée à l'aide de SHA-512. La résultante de 512 bits sont divisés, avec 256 servant de la clé de chiffrement symétrique (AES 256 dans mon exemple) et l'autre de 256 bits utilisée comme clé pour le HMAC. Mon application est vaguement basé sur le ECIES standard décrites par SECG groupe de travail.

Les fonctions clés sont ecies_encrypt() qui accepte la clé publique dans l'hex formulaire et renvoie les données chiffrées:

secure_t * ecies_encrypt(char *key, unsigned char *data, size_t length) {

void *body;
HMAC_CTX hmac;
int body_length;
secure_t *cryptex;
EVP_CIPHER_CTX cipher;
unsigned int mac_length;
EC_KEY *user, *ephemeral;
size_t envelope_length, block_length, key_length;
unsigned char envelope_key[SHA512_DIGEST_LENGTH], iv[EVP_MAX_IV_LENGTH], block[EVP_MAX_BLOCK_LENGTH];

//Simple sanity check.
if (!key || !data || !length) {
    printf("Invalid parameters passed in.\n");
    return NULL;
}

//Make sure we are generating enough key material for the symmetric ciphers.
if ((key_length = EVP_CIPHER_key_length(ECIES_CIPHER)) * 2 > SHA512_DIGEST_LENGTH) {
    printf("The key derivation method will not produce enough envelope key material for the chosen ciphers. {envelope = %i /required = %zu}", SHA512_DIGEST_LENGTH /8,
            (key_length * 2) /8);
    return NULL;
}

//Convert the user's public key from hex into a full EC_KEY structure.
if (!(user = ecies_key_create_public_hex(key))) {
    printf("Invalid public key provided.\n");
    return NULL;
}

//Create the ephemeral key used specifically for this block of data.
else if (!(ephemeral = ecies_key_create())) {
    printf("An error occurred while trying to generate the ephemeral key.\n");
    EC_KEY_free(user);
    return NULL;
}

//Use the intersection of the provided keys to generate the envelope data used by the ciphers below. The ecies_key_derivation() function uses
//SHA 512 to ensure we have a sufficient amount of envelope key material and that the material created is sufficiently secure.
else if (ECDH_compute_key(envelope_key, SHA512_DIGEST_LENGTH, EC_KEY_get0_public_key(user), ephemeral, ecies_key_derivation) != SHA512_DIGEST_LENGTH) {
    printf("An error occurred while trying to compute the envelope key. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EC_KEY_free(ephemeral);
    EC_KEY_free(user);
    return NULL;
}

//Determine the envelope and block lengths so we can allocate a buffer for the result.
else if ((block_length = EVP_CIPHER_block_size(ECIES_CIPHER)) == 0 || block_length > EVP_MAX_BLOCK_LENGTH || (envelope_length = EC_POINT_point2oct(EC_KEY_get0_group(
        ephemeral), EC_KEY_get0_public_key(ephemeral), POINT_CONVERSION_COMPRESSED, NULL, 0, NULL)) == 0) {
    printf("Invalid block or envelope length. {block = %zu /envelope = %zu}\n", block_length, envelope_length);
    EC_KEY_free(ephemeral);
    EC_KEY_free(user);
    return NULL;
}

//We use a conditional to pad the length if the input buffer is not evenly divisible by the block size.
else if (!(cryptex = secure_alloc(envelope_length, EVP_MD_size(ECIES_HASHER), length, length + (length % block_length ? (block_length - (length % block_length)) : 0)))) {
    printf("Unable to allocate a secure_t buffer to hold the encrypted result.\n");
    EC_KEY_free(ephemeral);
    EC_KEY_free(user);
    return NULL;
}

//Store the public key portion of the ephemeral key.
else if (EC_POINT_point2oct(EC_KEY_get0_group(ephemeral), EC_KEY_get0_public_key(ephemeral), POINT_CONVERSION_COMPRESSED, secure_key_data(cryptex), envelope_length,
        NULL) != envelope_length) {
    printf("An error occurred while trying to record the public portion of the envelope key. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EC_KEY_free(ephemeral);
    EC_KEY_free(user);
    secure_free(cryptex);
    return NULL;
}

//The envelope key has been stored so we no longer need to keep the keys around.
EC_KEY_free(ephemeral);
EC_KEY_free(user);

//For now we use an empty initialization vector.
memset(iv, 0, EVP_MAX_IV_LENGTH);

//Setup the cipher context, the body length, and store a pointer to the body buffer location.
EVP_CIPHER_CTX_init(&cipher);
body = secure_body_data(cryptex);
body_length = secure_body_length(cryptex);

//Initialize the cipher with the envelope key.
if (EVP_EncryptInit_ex(&cipher, ECIES_CIPHER, NULL, envelope_key, iv) != 1 || EVP_CIPHER_CTX_set_padding(&cipher, 0) != 1 || EVP_EncryptUpdate(&cipher, body,
        &body_length, data, length - (length % block_length)) != 1) {
    printf("An error occurred while trying to secure the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EVP_CIPHER_CTX_cleanup(&cipher);
    secure_free(cryptex);
    return NULL;
}

//Check whether all of the data was encrypted. If they don't match up, we either have a partial block remaining, or an error occurred.
else if (body_length != length) {

    //Make sure all that remains is a partial block, and their wasn't an error.
    if (length - body_length >= block_length) {
        printf("Unable to secure the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(&cipher);
        secure_free(cryptex);
        return NULL;
    }

    //Copy the remaining data into our partial block buffer. The memset() call ensures any extra bytes will be zero'ed out.
    memset(block, 0, EVP_MAX_BLOCK_LENGTH);
    memcpy(block, data + body_length, length - body_length);

    //Advance the body pointer to the location of the remaining space, and calculate just how much room is still available.
    body += body_length;
    if ((body_length = secure_body_length(cryptex) - body_length) < 0) {
        printf("The symmetric cipher overflowed!\n");
        EVP_CIPHER_CTX_cleanup(&cipher);
        secure_free(cryptex);
        return NULL;
    }

    //Pass the final partially filled data block into the cipher as a complete block. The padding will be removed during the decryption process.
    else if (EVP_EncryptUpdate(&cipher, body, &body_length, block, block_length) != 1) {
        printf("Unable to secure the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
        EVP_CIPHER_CTX_cleanup(&cipher);
        secure_free(cryptex);
        return NULL;
    }
}

//Advance the pointer, then use pointer arithmetic to calculate how much of the body buffer has been used. The complex logic is needed so that we get
//the correct status regardless of whether there was a partial data block.
body += body_length;
if ((body_length = secure_body_length(cryptex) - (body - secure_body_data(cryptex))) < 0) {
    printf("The symmetric cipher overflowed!\n");
    EVP_CIPHER_CTX_cleanup(&cipher);
    secure_free(cryptex);
    return NULL;
}

else if (EVP_EncryptFinal_ex(&cipher, body, &body_length) != 1) {
    printf("Unable to secure the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EVP_CIPHER_CTX_cleanup(&cipher);
    secure_free(cryptex);
    return NULL;
}

EVP_CIPHER_CTX_cleanup(&cipher);

//Generate an authenticated hash which can be used to validate the data during decryption.
HMAC_CTX_init(&hmac);
mac_length = secure_mac_length(cryptex);

//At the moment we are generating the hash using encrypted data. At some point we may want to validate the original text instead.
if (HMAC_Init_ex(&hmac, envelope_key + key_length, key_length, ECIES_HASHER, NULL) != 1 || HMAC_Update(&hmac, secure_body_data(cryptex), secure_body_length(cryptex))
        != 1 || HMAC_Final(&hmac, secure_mac_data(cryptex), &mac_length) != 1) {
    printf("Unable to generate a data authentication code. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    HMAC_CTX_cleanup(&hmac);
    secure_free(cryptex);
    return NULL;
}

HMAC_CTX_cleanup(&hmac);

return cryptex;
}

Et ecies_decrypt (), qui prend la clé privée, de nouveau en forme hexagonale, et décrypte les préalablement assuré de la mémoire tampon:

unsigned char * ecies_decrypt(char *key, secure_t *cryptex, size_t *length) {

HMAC_CTX hmac;
size_t key_length;
int output_length;
EVP_CIPHER_CTX cipher;
EC_KEY *user, *ephemeral;
unsigned int mac_length = EVP_MAX_MD_SIZE;
unsigned char envelope_key[SHA512_DIGEST_LENGTH], iv[EVP_MAX_IV_LENGTH], md[EVP_MAX_MD_SIZE], *block, *output;

//Simple sanity check.
if (!key || !cryptex || !length) {
    printf("Invalid parameters passed in.\n");
    return NULL;
}

//Make sure we are generating enough key material for the symmetric ciphers.
else if ((key_length = EVP_CIPHER_key_length(ECIES_CIPHER)) * 2 > SHA512_DIGEST_LENGTH) {
    printf("The key derivation method will not produce enough envelope key material for the chosen ciphers. {envelope = %i /required = %zu}", SHA512_DIGEST_LENGTH /8,
            (key_length * 2) /8);
    return NULL;
}

//Convert the user's public key from hex into a full EC_KEY structure.
else if (!(user = ecies_key_create_private_hex(key))) {
    printf("Invalid private key provided.\n");
    return NULL;
}

//Create the ephemeral key used specifically for this block of data.
else if (!(ephemeral = ecies_key_create_public_octets(secure_key_data(cryptex), secure_key_length(cryptex)))) {
    printf("An error occurred while trying to recreate the ephemeral key.\n");
    EC_KEY_free(user);
    return NULL;
}

//Use the intersection of the provided keys to generate the envelope data used by the ciphers below. The ecies_key_derivation() function uses
//SHA 512 to ensure we have a sufficient amount of envelope key material and that the material created is sufficiently secure.
else if (ECDH_compute_key(envelope_key, SHA512_DIGEST_LENGTH, EC_KEY_get0_public_key(ephemeral), user, ecies_key_derivation) != SHA512_DIGEST_LENGTH) {
    printf("An error occurred while trying to compute the envelope key. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EC_KEY_free(ephemeral);
    EC_KEY_free(user);
    return NULL;
}

//The envelope key material has been extracted, so we no longer need the user and ephemeral keys.
EC_KEY_free(ephemeral);
EC_KEY_free(user);

//Use the authenticated hash of the ciphered data to ensure it was not modified after being encrypted.
HMAC_CTX_init(&hmac);

//At the moment we are generating the hash using encrypted data. At some point we may want to validate the original text instead.
if (HMAC_Init_ex(&hmac, envelope_key + key_length, key_length, ECIES_HASHER, NULL) != 1 || HMAC_Update(&hmac, secure_body_data(cryptex), secure_body_length(cryptex))
        != 1 || HMAC_Final(&hmac, md, &mac_length) != 1) {
    printf("Unable to generate the authentication code needed for validation. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    HMAC_CTX_cleanup(&hmac);
    return NULL;
}

HMAC_CTX_cleanup(&hmac);

//We can use the generated hash to ensure the encrypted data was not altered after being encrypted.
if (mac_length != secure_mac_length(cryptex) || memcmp(md, secure_mac_data(cryptex), mac_length)) {
    printf("The authentication code was invalid! The ciphered data has been corrupted!\n");
    return NULL;
}

//Create a buffer to hold the result.
output_length = secure_body_length(cryptex);
if (!(block = output = malloc(output_length + 1))) {
    printf("An error occurred while trying to allocate memory for the decrypted data.\n");
    return NULL;
}

//For now we use an empty initialization vector. We also clear out the result buffer just to be on the safe side.
memset(iv, 0, EVP_MAX_IV_LENGTH);
memset(output, 0, output_length + 1);

EVP_CIPHER_CTX_init(&cipher);

//Decrypt the data using the chosen symmetric cipher.
if (EVP_DecryptInit_ex(&cipher, ECIES_CIPHER, NULL, envelope_key, iv) != 1 || EVP_CIPHER_CTX_set_padding(&cipher, 0) != 1 || EVP_DecryptUpdate(&cipher, block,
        &output_length, secure_body_data(cryptex), secure_body_length(cryptex)) != 1) {
    printf("Unable to decrypt the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EVP_CIPHER_CTX_cleanup(&cipher);
    free(output);
    return NULL;
}

block += output_length;
if ((output_length = secure_body_length(cryptex) - output_length) != 0) {
    printf("The symmetric cipher failed to properly decrypt the correct amount of data!\n");
    EVP_CIPHER_CTX_cleanup(&cipher);
    free(output);
    return NULL;
}

if (EVP_DecryptFinal_ex(&cipher, block, &output_length) != 1) {
    printf("Unable to decrypt the data using the chosen symmetric cipher. {error = %s}\n", ERR_error_string(ERR_get_error(), NULL));
    EVP_CIPHER_CTX_cleanup(&cipher);
    free(output);
    return NULL;
}

EVP_CIPHER_CTX_cleanup(&cipher);

*length = secure_orig_length(cryptex);
return output;
}

Je poste ceci car personnellement, je ne pouvais pas trouver d'autres exemples de la façon de sécuriser les fichiers à l'aide de l'ECC et la bibliothèque OpenSSL. Qui a dit que sa vaut la peine de mentionner des solutions de rechange qui ne sont pas utiliser OpenSSL. On est seccure qui suit un modèle similaire à mon exemple, il s'appuie libgcrypt. Depuis libgcrypt ne fournit pas tous de la sous-ECC fonctions nécessaires, le seccure programme comble les lacunes et met en œuvre l'ECC logique manquant de libgcrypt.

Un autre programme intéressant de regarder est SKS, qui utilise un semblable ECC chiffrement à base de processus l'exemple ci-dessus, mais n'ont pas de dépendances externes (donc tout le code ECC est là pour vous de regarder).