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use crate::integer::server_key::CheckError;
use crate::integer::{CrtCiphertext, ServerKey};
impl ServerKey {
/// Computes homomorphically a subtraction between a ciphertext and a scalar.
///
/// This function computes the operation without checking if it exceeds the capacity of the
/// ciphertext.
///
/// The result is returned as a new ciphertext.
///
/// # Example
///
///```rust
/// use tfhe::integer::gen_keys_crt;
/// use tfhe::shortint::parameters::PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128;
///
/// // Generate the client key and the server key:
/// let basis = vec![2, 3, 5];
/// let modulus: u64 = basis.iter().product();
/// let (cks, sks) = gen_keys_crt(PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128, basis);
///
/// let clear_1 = 14;
/// let clear_2 = 7;
/// // Encrypt two messages
/// let mut ctxt_1 = cks.encrypt(clear_1);
///
/// sks.unchecked_crt_scalar_sub_assign(&mut ctxt_1, clear_2);
///
/// // Decrypt
/// let res = cks.decrypt(&ctxt_1);
/// assert_eq!((clear_1 - clear_2) % modulus, res);
/// ```
pub fn unchecked_crt_scalar_sub(&self, ct: &CrtCiphertext, scalar: u64) -> CrtCiphertext {
let mut result = ct.clone();
self.unchecked_crt_scalar_sub_assign(&mut result, scalar);
result
}
pub fn unchecked_crt_scalar_sub_assign(&self, ct: &mut CrtCiphertext, scalar: u64) {
//Put each decomposition into a new ciphertext
for (ct_i, mod_i) in ct.blocks.iter_mut().zip(ct.moduli.iter()) {
let neg_scalar = (mod_i - scalar % mod_i) % mod_i;
self.key.unchecked_scalar_add_assign(ct_i, neg_scalar as u8);
}
}
/// Verifies if the subtraction of a ciphertext by scalar can be computed.
///
/// # Example
///
///```rust
/// use tfhe::integer::gen_keys_crt;
/// let basis = vec![2, 3, 5];
/// use tfhe::shortint::parameters::PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128;
///
/// // Generate the client key and the server key:
/// let (cks, sks) = gen_keys_crt(PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128, basis);
///
/// let clear_1 = 14;
/// let clear_2 = 7;
/// // Encrypt two messages
/// let ctxt_1 = cks.encrypt(clear_1);
///
/// sks.is_crt_scalar_sub_possible(&ctxt_1, clear_2).unwrap();
///
/// // Decrypt
/// let res = cks.decrypt(&ctxt_1);
/// ```
pub fn is_crt_scalar_sub_possible(
&self,
ct: &CrtCiphertext,
scalar: u64,
) -> Result<(), CheckError> {
for (ct_i, mod_i) in ct.blocks.iter().zip(ct.moduli.iter()) {
let neg_scalar = (mod_i - scalar % mod_i) % mod_i;
self.key
.is_scalar_add_possible(ct_i.noise_degree(), neg_scalar as u8)?;
}
Ok(())
}
/// Computes homomorphically a subtraction of a ciphertext by a scalar.
///
/// If the operation can be performed, the result is returned in a new ciphertext.
/// Otherwise a [CheckError] is returned.
///
/// # Example
///
/// ```rust
/// use tfhe::integer::gen_keys_crt;
/// use tfhe::shortint::parameters::PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128;
///
/// // Generate the client key and the server key:
/// let basis = vec![2, 3, 5];
/// let modulus: u64 = basis.iter().product();
/// let (cks, sks) = gen_keys_crt(PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128, basis);
///
/// let clear_1 = 14;
/// let clear_2 = 8;
///
/// let ctxt_1 = cks.encrypt(clear_1);
///
/// let ct_res = sks.checked_crt_scalar_sub(&ctxt_1, clear_2).unwrap();
///
/// // Decrypt:
/// let dec = cks.decrypt(&ct_res);
/// assert_eq!((clear_1 - clear_2) % modulus, dec);
/// ```
pub fn checked_crt_scalar_sub(
&self,
ct: &CrtCiphertext,
scalar: u64,
) -> Result<CrtCiphertext, CheckError> {
self.is_crt_scalar_sub_possible(ct, scalar)?;
Ok(self.unchecked_crt_scalar_sub(ct, scalar))
}
/// Computes homomorphically a subtraction of a ciphertext by a scalar.
///
/// If the operation can be performed, the result is returned in a new ciphertext.
/// Otherwise a [CheckError] is returned.
///
/// # Example
///
/// ```rust
/// use tfhe::integer::gen_keys_crt;
/// use tfhe::shortint::parameters::PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128;
///
/// // Generate the client key and the server key:
/// let basis = vec![2, 3, 5];
/// let modulus: u64 = basis.iter().product();
/// let (cks, sks) = gen_keys_crt(PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128, basis);
///
/// let clear_1 = 14;
/// let clear_2 = 7;
///
/// let mut ctxt_1 = cks.encrypt(clear_1);
///
/// sks.checked_crt_scalar_sub_assign(&mut ctxt_1, clear_2)
/// .unwrap();
///
/// // Decrypt:
/// let dec = cks.decrypt(&ctxt_1);
/// assert_eq!((clear_1 - clear_2) % modulus, dec);
/// ```
pub fn checked_crt_scalar_sub_assign(
&self,
ct: &mut CrtCiphertext,
scalar: u64,
) -> Result<(), CheckError> {
self.is_crt_scalar_sub_possible(ct, scalar)?;
self.unchecked_crt_scalar_sub_assign(ct, scalar);
Ok(())
}
/// Computes homomorphically a subtraction of a ciphertext by a scalar.
///
/// # Example
///
///```rust
/// use tfhe::integer::gen_keys_crt;
/// use tfhe::shortint::parameters::PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128;
///
/// // Generate the client key and the server key:
/// let basis = vec![2, 3, 5];
/// let modulus: u64 = basis.iter().product();
/// let (cks, sks) = gen_keys_crt(PARAM_MESSAGE_3_CARRY_3_KS_PBS_GAUSSIAN_2M128, basis);
///
/// let clear_1 = 14;
/// let clear_2 = 7;
/// // Encrypt two messages
/// let mut ctxt_1 = cks.encrypt(clear_1);
///
/// sks.smart_crt_scalar_sub_assign(&mut ctxt_1, clear_2);
///
/// // Decrypt
/// let res = cks.decrypt(&ctxt_1);
/// assert_eq!((clear_1 - clear_2) % modulus, res);
/// ```
pub fn smart_crt_scalar_sub(&self, ct: &mut CrtCiphertext, scalar: u64) -> CrtCiphertext {
if self.is_crt_scalar_sub_possible(ct, scalar).is_err() {
self.full_extract_message_assign(ct);
}
self.is_crt_scalar_sub_possible(ct, scalar).unwrap();
self.unchecked_crt_scalar_sub(ct, scalar)
}
pub fn smart_crt_scalar_sub_assign(&self, ct: &mut CrtCiphertext, scalar: u64) {
if self.is_crt_scalar_sub_possible(ct, scalar).is_err() {
self.full_extract_message_assign(ct);
}
self.is_crt_scalar_sub_possible(ct, scalar).unwrap();
self.unchecked_crt_scalar_sub_assign(ct, scalar);
}
}