OpenSSL ECC encrypt/decrypt

openssl-ecc.c

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;
}

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;
}