Comparing ciphertexts and scalars

Zama TFHE-rs
1

Dear Zama,

I just wanted to ask how would someone compare a ciphertext and a scalar in your library? If for example an integer was encrypted using the client key and at the server we want to compare the encrypted integer to a non-encrypted one knowing that we don’t have access to the client key and the server key cannot be used for encryption

Thank you in advance.

Sincerely,
Ali

2

There are different ways to compare, depending on what your ciphertext is, i.e. is it a RadixCiphertext of Ciphertext.

If you want to compare a RadixCiphertext(where secret key is used to encrypt an integer) you can use

let res = sk.smart_scalar_eq_parallelized(&mut cipher, scalar)

where sk is Radix Server Key

If you want to compare a Ciphertext (where secret key is used to encrypt a shortint) you can use

let equal = |x:u64|
    if x == scalar {
        1
    }else {
        0
    }
;
let acc = shortint_sk.generate_accumulator(&equal);
let res = shortint_sk.apply_lookup_table(&cipher, &acc);

where shortint_sk is Shortint Server Key

1 Like
3

hello @Professor_Ali @divyesh answer is good however for shortint you also have scalar comparison operations like ServerKey in tfhe::shortint::server_key - Rust

Also you can use so called “default operations” which are less error prone to use with respect to performance ServerKey in tfhe::shortint::server_key - Rust

Cheers

1 Like