5_rotation

(使用BatchEncoder的)BFV方案和CKKS方案都支持加密数字上的自然向量运算。除了计算slot之外,还可以旋转加密向量。

1.BFV

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print_example_banner("Example: Rotation / Rotation in BFV");

EncryptionParameters parms(scheme_type::bfv);

size_t poly_modulus_degree = 8192;
parms.set_poly_modulus_degree(poly_modulus_degree);
parms.set_coeff_modulus(CoeffModulus::BFVDefault(poly_modulus_degree));
parms.set_plain_modulus(PlainModulus::Batching(poly_modulus_degree, 20));

SEALContext context(parms);
print_parameters(context);
cout << endl;

KeyGenerator keygen(context);
SecretKey secret_key = keygen.secret_key();
PublicKey public_key;
keygen.create_public_key(public_key);
RelinKeys relin_keys;
keygen.create_relin_keys(relin_keys);
Encryptor encryptor(context, public_key);
Evaluator evaluator(context);
Decryptor decryptor(context, secret_key);

BatchEncoder batch_encoder(context);
size_t slot_count = batch_encoder.slot_count();
size_t row_size = slot_count / 2;
cout << "Plaintext matrix row size: " << row_size << endl;

vector<uint64_t> pod_matrix(slot_count, 0ULL);
pod_matrix[0] = 0ULL;
pod_matrix[1] = 1ULL;
pod_matrix[2] = 2ULL;
pod_matrix[3] = 3ULL;
pod_matrix[row_size] = 4ULL;
pod_matrix[row_size + 1] = 5ULL;
pod_matrix[row_size + 2] = 6ULL;
pod_matrix[row_size + 3] = 7ULL;

cout << "Input plaintext matrix:" << endl;
print_matrix(pod_matrix, row_size);

首先使用BatchEncoder将矩阵编码为明文并加密。

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Plaintext plain_matrix;
print_line(__LINE__);
cout << "Encode and encrypt." << endl;
batch_encoder.encode(pod_matrix, plain_matrix);
Ciphertext encrypted_matrix;
encryptor.encrypt(plain_matrix, encrypted_matrix);
cout << " + Noise budget in fresh encryption: " << decryptor.invariant_noise_budget(encrypted_matrix) << " bits"
<< endl;
cout << endl;

旋转需要另一种特殊密钥"Galois key",可以使用KeyGenerator创建。

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GaloisKeys galois_keys;
keygen.create_galois_keys(galois_keys);

将矩阵的行向左旋转3步,解密、解码并打印出。

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print_line(__LINE__);
cout << "Rotate rows 3 steps left." << endl;
evaluator.rotate_rows_inplace(encrypted_matrix, 3, galois_keys);
Plaintext plain_result;
cout << " + Noise budget after rotation: " << decryptor.invariant_noise_budget(encrypted_matrix) << " bits"
<< endl;
cout << " + Decrypt and decode ...... Correct." << endl;
decryptor.decrypt(encrypted_matrix, plain_result);
batch_encoder.decode(plain_result, pod_matrix);
print_matrix(pod_matrix, row_size);

也可以旋转列

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print_line(__LINE__);
cout << "Rotate columns." << endl;
evaluator.rotate_columns_inplace(encrypted_matrix, galois_keys);
cout << " + Noise budget after rotation: " << decryptor.invariant_noise_budget(encrypted_matrix) << " bits"
<< endl;
cout << " + Decrypt and decode ...... Correct." << endl;
decryptor.decrypt(encrypted_matrix, plain_result);
batch_encoder.decode(plain_result, pod_matrix);
print_matrix(pod_matrix, row_size);

最后将行向右旋转4步,解密、解码并打印出

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print_line(__LINE__);
cout << "Rotate rows 4 steps right." << endl;
evaluator.rotate_rows_inplace(encrypted_matrix, -4, galois_keys);
cout << " + Noise budget after rotation: " << decryptor.invariant_noise_budget(encrypted_matrix) << " bits"
<< endl;
cout << " + Decrypt and decode ...... Correct." << endl;
decryptor.decrypt(encrypted_matrix, plain_result);
batch_encoder.decode(plain_result, pod_matrix);
print_matrix(pod_matrix, row_size);

旋转操作不消耗任何噪声预算。但是这要求特殊素数至少和其他素数一样大。SEAL没有对特殊素数的大小进行要求,因此这需要用户自行保证。

2.CKKS

CKKS中的旋转操作与BFV中类似

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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, { 40, 40, 40, 40, 40 }));

SEALContext context(parms);
print_parameters(context);
cout << endl;

KeyGenerator keygen(context);
SecretKey secret_key = keygen.secret_key();
PublicKey public_key;
keygen.create_public_key(public_key);
RelinKeys relin_keys;
keygen.create_relin_keys(relin_keys);
GaloisKeys galois_keys;
keygen.create_galois_keys(galois_keys);
Encryptor encryptor(context, public_key);
Evaluator evaluator(context);
Decryptor decryptor(context, secret_key);

CKKSEncoder ckks_encoder(context);

size_t slot_count = ckks_encoder.slot_count();
cout << "Number of slots: " << slot_count << endl;
vector<double> input;
input.reserve(slot_count);
double curr_point = 0;
double step_size = 1.0 / (static_cast<double>(slot_count) - 1);
for (size_t i = 0; i < slot_count; i++, curr_point += step_size)
{
input.push_back(curr_point);
}
cout << "Input vector:" << endl;
print_vector(input, 3, 7);

auto scale = pow(2.0, 50);

print_line(__LINE__);
cout << "Encode and encrypt." << endl;
Plaintext plain;
ckks_encoder.encode(input, scale, plain);
Ciphertext encrypted;
encryptor.encrypt(plain, encrypted);

Ciphertext rotated;
print_line(__LINE__);
cout << "Rotate 2 steps left." << endl;
evaluator.rotate_vector(encrypted, 2, galois_keys, rotated);
cout << " + Decrypt and decode ...... Correct." << endl;
decryptor.decrypt(rotated, plain);
vector<double> result;
ckks_encoder.decode(plain, result);
print_vector(result, 3, 7);

在CKKS方案中,可以使用Evaluator::complex_conjugate对加密复数向量求共轭。这实际上也是一种旋转操作,需要用到Galois key。


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