6_serialization.cpp

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT license.

#include "examples.h"

using namespace std;
using namespace seal;

/*
In this example we show how serialization works in Microsoft SEAL. Specifically,
we present important concepts that enable the user to optimize the data size when
communicating ciphertexts and keys for outsourced computation. Unlike the previous
examples, we organize this one in a client-server style for maximal clarity. The
server selects encryption parameters, the client generates keys, the server does
the encrypted computation, and the client decrypts.
*/
void example_serialization()
{
    print_example_banner("Example: Serialization");

    /*
    We require ZLIB or Zstandard support for this example to be available.
    */
#if (!defined(SEAL_USE_ZSTD) && !defined(SEAL_USE_ZLIB))
    cout << "Neither ZLIB nor Zstandard support is enabled; this example is not available." << endl;
    cout << endl;
    return;
#else
    /*
    We start by briefly discussing the Serializable<T> class template. This is
    a wrapper class that can wrap any serializable class, which include:

        - EncryptionParameters
        - Modulus
        - Plaintext and Ciphertext
        - SecretKey, PublicKey, RelinKeys, and GaloisKeys

    Serializable<T> provides minimal functionality needed to serialize the wrapped
    object by simply forwarding the calls to corresponding functions of the wrapped
    object of type T. The need for Serializable<T> comes from the fact that many
    Microsoft SEAL objects consist of two parts, one of which is pseudorandom data
    independent of the other part. Until the object is actually being used, the
    pseudorandom part can be instead stored as a seed. We will call objects with
    property `seedable'.

    For example, GaloisKeys can often be very large in size, but in reality half
    of the data is pseudorandom and can be stored as a seed. Since GaloisKeys are
    never used by the party that generates them, so it makes sense to expand the
    seed at the point deserialization. On the other hand, we cannot allow the user
    to accidentally try to use an unexpanded GaloisKeys object, which is prevented
    at by ensuring it is always wrapped in a Serializable<GaloisKeys> and can only
    be serialized.

    Only some Microsoft SEAL objects are seedable. Specifically, they are:

        - PublicKey, RelinKeys, and GaloisKeys
        - Ciphertext in secret-key mode (from Encryptor::encrypt_symmetric or
          Encryptor::encrypt_zero_symmetric)

    Importantly, ciphertexts in public-key mode are not seedable. Thus, it may
    be beneficial to use Microsoft SEAL in secret-key mode whenever the public
    key is not truly needed.

    There are a handful of functions that output Serializable<T> objects:

        - Encryptor::encrypt (and variants) output Serializable<Ciphertext>
        - KeyGenerator::create_... output Serializable<T> for different key types

    Note that Encryptor::encrypt is included in the above list, yet it produces
    ciphertexts in public-key mode that are not seedable. This is for the sake of
    consistency in the API for public-key and secret-key encryption. Functions
    that output Serializable<T> objects also have overloads that take a normal
    object of type T as a destination parameter, overwriting it. These overloads
    can be convenient for local testing where no serialization is needed and the
    object needs to be used at the point of construction. Such an object can no
    longer be transformed back to a seeded state.
    */

    /*
    To simulate client-server interaction, we set up a shared C++ stream. In real
    use-cases this can be a network buffer, a filestream, or any shared resource.

    It is critical to note that all data serialized by Microsoft SEAL is in binary
    form, so it is not meaningful to print the data as ASCII characters. Encodings
    such as Base64 would increase the data size, which is already a bottleneck in
    homomorphic encryption. Hence, serialization into text is not supported or
    recommended.

    We feel it is important to remind users that filestream serialization will
    always require the ios::binary flag to signal that the serialized data is
    binary data and not text. For example, an appropriate output filestream could
    be set up as:

        ofstream ofs("filename", ios::binary);

    In this example we use an std::stringstream, where the ios::binary flag is
    not needed. Note that the default constructor of std::stringstream opens the
    stream with ios::in | ios::out so both reading and writing will be possible.
    */
    stringstream parms_stream;
    stringstream data_stream;
    stringstream sk_stream;

    /*
    The server first determines the computation and sets encryption parameters
    accordingly.
    */
    {
        EncryptionParameters parms(scheme_type::ckks);
        size_t poly_modulus_degree = 8192;
        parms.set_poly_modulus_degree(poly_modulus_degree);
        parms.set_coeff_modulus(CoeffModulus::Create(poly_modulus_degree, { 50, 20, 50 }));

        /*
        Serialization of the encryption parameters to our shared stream is very
        simple with the EncryptionParameters::save function.
        */
        auto size = parms.save(parms_stream);

        /*
        The return value of this function is the actual byte count of data written
        to the stream.
        */
        print_line(__LINE__);
        cout << "EncryptionParameters: wrote " << size << " bytes" << endl;

        /*
        Before moving on, we will take some time to discuss further options in
        serialization. These will become particularly important when the user
        needs to optimize communication and storage sizes.

        It is possible to enable or disable compression for serialization by
        providing EncryptionParameters::save with the desired compression mode as
        in the following examples:

            auto size = parms.save(shared_stream, compr_mode_type::none);
            auto size = parms.save(shared_stream, compr_mode_type::zlib);
            auto size = parms.save(shared_stream, compr_mode_type::zstd);

        If Microsoft SEAL is compiled with Zstandard or ZLIB support, the default
        is to use one of them. If available, Zstandard is preferred over ZLIB due
        to its speed.

        Compression can have a substantial impact on the serialized data size,
        because ciphertext and key data consists of many uniformly random integers
        modulo the coeff_modulus primes. Especially when using CKKS, the primes in
        coeff_modulus can be relatively small compared to the 64-bit words used to
        store the ciphertext and key data internally. Serialization writes full
        64-bit words to the destination buffer or stream, possibly leaving in many
        zero bytes corresponding to the high-order bytes of the 64-bit words. One
        convenient way to get rid of these zeros is to apply a general-purpose
        compression algorithm on the encrypted data. The compression rate can be
        significant (up to 50-60%) when using CKKS with small primes.
        */

        /*
        It is also possible to serialize data directly to a buffer. For this, one
        needs to know an upper bound for the required buffer size, which can be
        obtained using the EncryptionParameters::save_size function. This function
        also accepts the desired compression mode, or uses the default option
        otherwise.

        In more detail, the output of EncryptionParameters::save_size is as follows:

            - Exact buffer size required for compr_mode_type::none;
            - Upper bound on the size required for compr_mode_type::zlib or
              compr_mode_type::zstd.

        As we can see from the print-out, the sizes returned by these functions
        are significantly larger than the compressed size written into the shared
        stream in the beginning. This is normal: compression yielded a significant
        improvement in the data size, however, it is impossible to know ahead of
        time the exact size of the compressed data. If compression is not used,
        then the size is exactly determined by the encryption parameters.
        */
        print_line(__LINE__);
        cout << "EncryptionParameters: data size upper bound (compr_mode_type::none): "
             << parms.save_size(compr_mode_type::none) << endl;
        cout << "             "
             << "EncryptionParameters: data size upper bound (compression): "
             << parms.save_size(/* Serialization::compr_mode_default */) << endl;

        /*
        As an example, we now serialize the encryption parameters to a fixed size
        buffer.
        */
        vector<seal_byte> byte_buffer(static_cast<size_t>(parms.save_size()));
        parms.save(reinterpret_cast<seal_byte *>(byte_buffer.data()), byte_buffer.size());

        /*
        To illustrate deserialization, we load back the encryption parameters
        from our buffer into another instance of EncryptionParameters. Note how
        EncryptionParameters::load in this case requires the size of the buffer,
        which is larger than the actual data size of the compressed parameters.
        The serialization format includes the true size of the data and the size
        of the buffer is only used for a sanity check.
        */
        EncryptionParameters parms2;
        parms2.load(reinterpret_cast<const seal_byte *>(byte_buffer.data()), byte_buffer.size());

        /*
        We can check that the saved and loaded encryption parameters indeed match.
        */
        print_line(__LINE__);
        cout << "EncryptionParameters: parms == parms2: " << boolalpha << (parms == parms2) << endl;

        /*
        The functions presented and used here exist for all Microsoft SEAL objects
        that are meaningful to serialize. However, it is important to understand
        more advanced techniques that can be used for further compressing the data
        size. We will present these techniques below.
        */
    }

    /*
    Client starts by loading the encryption parameters, sets up the SEALContext,
    and creates the required keys.
    */
    {
        EncryptionParameters parms;
        parms.load(parms_stream);

        /*
        Seek the parms_stream get head back to beginning of the stream because we
        will use the same stream to read the parameters repeatedly.
        */
        parms_stream.seekg(0, parms_stream.beg);

        SEALContext context(parms);

        KeyGenerator keygen(context);
        auto sk = keygen.secret_key();
        PublicKey pk;
        keygen.create_public_key(pk);

        /*
        We need to save the secret key so we can decrypt later.
        */
        sk.save(sk_stream);

        /*
        As in previous examples, in this example we will encrypt in public-key
        mode. If we want to send a public key over the network, we should instead
        have created it as a seeded object as follows:

            Serializable<PublicKey> pk = keygen.create_public_key();

        In this example we will also use relinearization keys. These we will
        absolutely want to create as seeded objects to minimize communication
        cost, unlike in prior examples.
        */
        Serializable<RelinKeys> rlk = keygen.create_relin_keys();

        /*
        To demonstrate the significant space saving from this method, we will
        create another set of relinearization keys, this time fully expanded.
        */
        RelinKeys rlk_big;
        keygen.create_relin_keys(rlk_big);

        /*
        We serialize both relinearization keys to demonstrate the concrete size
        difference. If compressed serialization is used, the compression rate
        will be the same in both cases. We omit specifying the compression mode
        to use the default, as determined by the Microsoft SEAL build system.
        */
        auto size_rlk = rlk.save(data_stream);
        auto size_rlk_big = rlk_big.save(data_stream);

        print_line(__LINE__);
        cout << "Serializable<RelinKeys>: wrote " << size_rlk << " bytes" << endl;
        cout << "             "
             << "RelinKeys wrote " << size_rlk_big << " bytes" << endl;

        /*
        Seek back in data_stream to where rlk data ended, i.e., size_rlk_big
        bytes backwards from current position.
        */
        data_stream.seekp(-size_rlk_big, data_stream.cur);

        /*
        Next set up the CKKSEncoder and Encryptor, and encrypt some numbers.
        */
        double scale = pow(2.0, 20);
        CKKSEncoder encoder(context);
        Plaintext plain1, plain2;
        encoder.encode(2.3, scale, plain1);
        encoder.encode(4.5, scale, plain2);

        Encryptor encryptor(context, pk);

        /*
        The client will not compute on ciphertexts that it creates, so it can
        just as well create Serializable<Ciphertext> objects. In fact, we do
        not even need to name those objects and instead immediately call
        Serializable<Ciphertext>::save.
        */
        auto size_encrypted1 = encryptor.encrypt(plain1).save(data_stream);

        /*
        As we discussed in the beginning of this example, ciphertexts can be
        created in a seeded state in secret-key mode, providing a huge reduction
        in the data size upon serialization. To do this, we need to provide the
        Encryptor with the secret key in its constructor, or at a later point
        with the Encryptor::set_secret_key function, and use the
        Encryptor::encrypt_symmetric function to encrypt.
        */
        encryptor.set_secret_key(sk);
        auto size_sym_encrypted2 = encryptor.encrypt_symmetric(plain2).save(data_stream);

        /*
        The size reduction is substantial.
        */
        print_line(__LINE__);
        cout << "Serializable<Ciphertext> (public-key): wrote " << size_encrypted1 << " bytes" << endl;
        cout << "             "
             << "Serializable<Ciphertext> (seeded secret-key): wrote " << size_sym_encrypted2 << " bytes" << endl;

        /*
        We have seen how creating seeded objects can result in huge space
        savings compared to creating unseeded objects. This is particularly
        important when creating Galois keys, which can be very large. We have
        seen how secret-key encryption can be used to achieve much smaller
        ciphertext sizes when the public-key functionality is not needed.

        We would also like to draw attention to the fact there we could easily
        serialize multiple Microsoft SEAL objects sequentially in a stream. Each
        object writes its own size into the stream, so deserialization knows
        exactly how many bytes to read. We will see this working below.
        */
    }

    /*
    The server can now compute on the encrypted data. We will recreate the
    SEALContext and set up an Evaluator here.
    */
    {
        EncryptionParameters parms;
        parms.load(parms_stream);
        parms_stream.seekg(0, parms_stream.beg);
        SEALContext context(parms);

        Evaluator evaluator(context);

        /*
        Next we need to load relinearization keys and the ciphertexts from our
        data_stream.
        */
        RelinKeys rlk;
        Ciphertext encrypted1, encrypted2;

        /*
        Deserialization is as easy as serialization.
        */
        rlk.load(context, data_stream);
        encrypted1.load(context, data_stream);
        encrypted2.load(context, data_stream);

        /*
        Compute the product, rescale, and relinearize.
        */
        Ciphertext encrypted_prod;
        evaluator.multiply(encrypted1, encrypted2, encrypted_prod);
        evaluator.relinearize_inplace(encrypted_prod, rlk);
        evaluator.rescale_to_next_inplace(encrypted_prod);

        /*
        we use data_stream to communicate encrypted_prod back to the client.
        there is no way to save the encrypted_prod as a seeded object: only
        freshly encrypted secret-key ciphertexts can be seeded. Note how the
        size of the result ciphertext is smaller than the size of a fresh
        ciphertext because it is at a lower level due to the rescale operation.
        */
        data_stream.seekp(0, parms_stream.beg);
        data_stream.seekg(0, parms_stream.beg);
        auto size_encrypted_prod = encrypted_prod.save(data_stream);

        print_line(__LINE__);
        cout << "Ciphertext (secret-key): wrote " << size_encrypted_prod << " bytes" << endl;
    }

    /*
    In the final step the client decrypts the result.
    */
    {
        EncryptionParameters parms;
        parms.load(parms_stream);
        parms_stream.seekg(0, parms_stream.beg);
        SEALContext context(parms);

        /*
        Load back the secret key from sk_stream.
        */
        SecretKey sk;
        sk.load(context, sk_stream);
        Decryptor decryptor(context, sk);
        CKKSEncoder encoder(context);

        Ciphertext encrypted_result;
        encrypted_result.load(context, data_stream);

        Plaintext plain_result;
        decryptor.decrypt(encrypted_result, plain_result);
        vector<double> result;
        encoder.decode(plain_result, result);

        print_line(__LINE__);
        cout << "Result: " << endl;
        print_vector(result, 3, 7);
    }

    /*
    Finally, we give a little bit more explanation of the structure of data
    serialized by Microsoft SEAL. Serialized data always starts with a 16-byte
    SEALHeader struct, as defined in native/src/seal/serialization.h, and is
    followed by the possibly compressed data for the object.

    A SEALHeader contains the following data:

        [offset 0] 2-byte magic number 0xA15E (Serialization::seal_magic)
        [offset 2] 1-byte indicating the header size in bytes (always 16)
        [offset 3] 1-byte indicating the Microsoft SEAL major version number
        [offset 4] 1-byte indicating the Microsoft SEAL minor version number
        [offset 5] 1-byte indicating the compression mode type
        [offset 6] 2-byte reserved field (unused)
        [offset 8] 8-byte size in bytes of the serialized data, including the header

    Currently Microsoft SEAL supports only little-endian systems.

    As an example, we demonstrate the SEALHeader created by saving a plaintext.
    Note that the SEALHeader is never compressed, so there is no need to specify
    the compression mode.
    */
    Plaintext pt("1x^2 + 3");
    stringstream stream;
    auto data_size = pt.save(stream);

    /*
    We can now load just the SEALHeader back from the stream as follows.
    */
    Serialization::SEALHeader header;
    Serialization::LoadHeader(stream, header);

    /*
    Now confirm that the size of data written to stream matches with what is
    indicated by the SEALHeader.
    */
    print_line(__LINE__);
    cout << "Size written to stream: " << data_size << " bytes" << endl;
    cout << "             "
         << "Size indicated in SEALHeader: " << header.size << " bytes" << endl;
    cout << endl;
#endif
}