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use crate::ciphertext::RadixCiphertext;
use crate::server_key::CheckError;
use crate::server_key::CheckError::CarryFull;
use crate::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 concrete_integer::gen_keys_radix;
/// use concrete_shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
///
/// // We have 4 * 2 = 8 bits of message
/// let num_blocks = 4;
/// let (cks, sks) = gen_keys_radix(&PARAM_MESSAGE_2_CARRY_2, num_blocks);
///
/// let msg = 40;
/// let scalar = 3;
///
/// let ct = cks.encrypt(msg);
///
/// // Compute homomorphically an addition:
/// let ct_res = sks.unchecked_scalar_sub(&ct, scalar);
///
/// // Decrypt:
/// let dec = cks.decrypt(&ct_res);
/// assert_eq!(msg - scalar, dec);
/// ```
pub fn unchecked_scalar_sub(&self, ct: &RadixCiphertext, scalar: u64) -> RadixCiphertext {
let mut result = ct.clone();
self.unchecked_scalar_sub_assign(&mut result, scalar);
result
}
pub fn unchecked_scalar_sub_assign(&self, ct: &mut RadixCiphertext, scalar: u64) {
//Bits of message put to 1
let mask = (self.key.message_modulus.0 - 1) as u64;
let modulus = self.key.message_modulus.0.pow(ct.blocks.len() as u32) as u64;
let neg_scalar = scalar.wrapping_neg() % modulus;
let mut power = 1_u64;
//Put each decomposition into a new ciphertext
for ct_i in ct.blocks.iter_mut() {
let mut decomp = neg_scalar & (mask * power);
decomp /= power;
self.key.unchecked_scalar_add_assign(ct_i, decomp as u8);
//modulus to the power i
power *= self.key.message_modulus.0 as u64;
}
}
/// Verifies if the subtraction of a ciphertext by scalar can be computed.
///
/// # Example
///
///```rust
/// use concrete_integer::gen_keys_radix;
/// use concrete_shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
///
/// // We have 4 * 2 = 8 bits of message
/// let num_blocks = 4;
/// let (cks, sks) = gen_keys_radix(&PARAM_MESSAGE_2_CARRY_2, num_blocks);
///
/// let msg = 40;
/// let scalar = 2;
///
/// let ct1 = cks.encrypt(msg);
///
/// // Check if we can perform an addition
/// let res = sks.is_scalar_sub_possible(&ct1, scalar);
///
/// assert_eq!(true, res);
/// ```
pub fn is_scalar_sub_possible(&self, ct: &RadixCiphertext, scalar: u64) -> bool {
//Bits of message put to 1
let mask = (self.key.message_modulus.0 - 1) as u64;
let modulus = self.key.message_modulus.0.pow(ct.blocks.len() as u32) as u64;
let neg_scalar = scalar.wrapping_neg() % modulus;
let mut power = 1_u64;
for ct_i in ct.blocks.iter() {
let mut decomp = neg_scalar & (mask * power);
decomp /= power;
if !self.key.is_scalar_add_possible(ct_i, decomp as u8) {
return false;
}
//modulus to the power i
power *= self.key.message_modulus.0 as u64;
}
true
}
/// 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 [CheckError::CarryFull] is returned.
///
/// # Example
///
/// ```rust
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// use concrete_integer::gen_keys_radix;
/// use concrete_shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
///
/// // We have 4 * 2 = 8 bits of message
/// let num_blocks = 4;
/// let (cks, sks) = gen_keys_radix(&PARAM_MESSAGE_2_CARRY_2, num_blocks);
///
/// let msg = 40;
/// let scalar = 4;
///
/// let ct = cks.encrypt(msg);
///
/// // Compute tne subtraction:
/// let ct_res = sks.checked_scalar_sub(&ct, scalar)?;
///
/// // Decrypt:
/// let dec = cks.decrypt(&ct_res);
/// assert_eq!(msg - scalar, dec);
/// # Ok(())
/// # }
/// ```
pub fn checked_scalar_sub(
&self,
ct: &RadixCiphertext,
scalar: u64,
) -> Result<RadixCiphertext, CheckError> {
if self.is_scalar_sub_possible(ct, scalar) {
Ok(self.unchecked_scalar_sub(ct, scalar))
} else {
Err(CarryFull)
}
}
/// 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 [CheckError::CarryFull] is returned.
///
/// # Example
///
/// ```rust
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// use concrete_integer::gen_keys_radix;
/// use concrete_shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
///
/// // We have 4 * 2 = 8 bits of message
/// let num_blocks = 4;
/// let (cks, sks) = gen_keys_radix(&PARAM_MESSAGE_2_CARRY_2, num_blocks);
///
/// let msg = 232;
/// let scalar = 83;
///
/// let mut ct = cks.encrypt(msg);
///
/// // Compute tne subtraction:
/// sks.checked_scalar_sub_assign(&mut ct, scalar)?;
///
/// // Decrypt:
/// let dec = cks.decrypt(&ct);
/// assert_eq!(msg - scalar, dec);
/// # Ok(())
/// # }
/// ```
pub fn checked_scalar_sub_assign(
&self,
ct: &mut RadixCiphertext,
scalar: u64,
) -> Result<(), CheckError> {
if self.is_scalar_sub_possible(ct, scalar) {
self.unchecked_scalar_sub_assign(ct, scalar);
Ok(())
} else {
Err(CarryFull)
}
}
/// Computes homomorphically a subtraction of a ciphertext by a scalar.
///
/// # Example
///
/// ```rust
/// use concrete_integer::gen_keys_radix;
/// use concrete_shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
///
/// // We have 4 * 2 = 8 bits of message
/// let num_blocks = 4;
/// let (cks, sks) = gen_keys_radix(&PARAM_MESSAGE_2_CARRY_2, num_blocks);
///
/// let msg = 165;
/// let scalar = 112;
///
/// let mut ct = cks.encrypt(msg);
///
/// // Compute homomorphically an addition:
/// let ct_res = sks.smart_scalar_sub(&mut ct, scalar);
///
/// // Decrypt:
/// let dec = cks.decrypt(&ct_res);
/// assert_eq!(msg - scalar, dec);
/// ```
pub fn smart_scalar_sub(&self, ct: &mut RadixCiphertext, scalar: u64) -> RadixCiphertext {
if !self.is_scalar_sub_possible(ct, scalar) {
self.full_propagate(ct);
}
self.unchecked_scalar_sub(ct, scalar)
}
pub fn smart_scalar_sub_assign(&self, ct: &mut RadixCiphertext, scalar: u64) {
if !self.is_scalar_sub_possible(ct, scalar) {
self.full_propagate(ct);
}
self.unchecked_scalar_sub_assign(ct, scalar);
}
}