server_key.unchecked_equal output error

[llllinyl]

Hi,

In the code below, I tested the equality comparison between the ciphertext of 96 and the sum of 96 ones, but I found that no matter how I adjusted it, the result was always false. This was true for both unchecked_equal and equal.

'''
#[test]
fn test_basic() {
let (client_key, server_key) = gen_keys(V1_4_PARAM_MESSAGE_7_CARRY_0_KS_PBS_GAUSSIAN_2M128);

let msg1 = 96u64;
let msg2 = 1u64;
let msg3 = 0u64;

let ct_1 = client_key.encrypt(msg1);
let ct_2 = client_key.encrypt(msg2);
let ct_3 = client_key.encrypt(msg3);
let mut ct_4 = ct_3.clone();

let add_start = Instant::now();
for _i in 0..96{
    server_key.unchecked_add_assign(&mut ct_4, &ct_2);
}
let add_time = add_start.elapsed();
println!("add time: {:?}", add_time);

let equal_start = Instant::now();
let ctbool = server_key.unchecked_equal(&ct_1, &ct_4);
let equal_time = equal_start.elapsed();
println!("equal time: {:?}", equal_time);

let output = client_key.decrypt(&ct_4);
println!("add expected {}, found {}", 96, output);
let result = client_key.decrypt(&ctbool);
println!("equal expected {}, found {}", true, result);

}
'''

[tmontaigu]

Hello,

The code is not going to work for multiple reasons:

• V1_4_PARAM_MESSAGE_7_CARRY_0_KS_PBS_GAUSSIAN_2M128 are 'weird' parameters as although they allow to
  encrypt messages in [0, 2^7[ as they have 7 bits of message, they have 0 bits of carry, meaning
  the only possible operation is a PBS (there is a max_noise_level set to 1 in the parameters), but you are doing 96 additions. Going above this max_noise_level, means the failure probability is worse than what the parameters advertise (2^-128), which leads to incorrect computations and/or decryptions

• unchecked_equal will actually use a bivariate PBS, this means that both encrypted messages are going to
  be concatenated into one encrypted block, however, doing this is also not legal with regards to V1_4_PARAM_MESSAGE_7_CARRY_0_KS_PBS_GAUSSIAN_2M128 max noise level, and this concatenation requires parameters that have at least as many carry bits as message bits since it's done via concatenated = msg_1 * 2^msg_bits + msg2

So I would recommend you to use the integer with PARAM_MESSAGE_2_CARRY_2_KS_PBS_GAUSSIAN_2M128/ high_level_api as it properly combines multiple blocks to
have all the functionalities of clear integers

[llllinyl]

Hello,

Thank you very much for your response. I will try some of the methods you mentioned or explore other approaches. I appreciate your patient explanation.