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feat: centralize encoding in shortint
The plaintext encoding in shortint was duplicated all over the code This commit centralize the encoding used for shortint, so that if an encoding fix is needed there should be one place to do it.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,120 @@ | ||
| use crate::core_crypto::entities::{Cleartext, Plaintext}; | ||
| use crate::core_crypto::prelude::CiphertextModulusKind; | ||
| use crate::shortint::{CarryModulus, CiphertextModulus, MessageModulus, ShortintParameterSet}; | ||
|
|
||
| #[derive(Copy, Clone, Debug, PartialEq, Eq)] | ||
| pub(crate) enum PaddingBit { | ||
| No = 0, | ||
| Yes = 1, | ||
| } | ||
|
|
||
| fn compute_delta( | ||
| ciphertext_modulus: CiphertextModulus, | ||
| message_modulus: MessageModulus, | ||
| carry_modulus: CarryModulus, | ||
| padding_bit: PaddingBit, | ||
| ) -> u64 { | ||
| match ciphertext_modulus.kind() { | ||
| CiphertextModulusKind::Native => { | ||
| (1u64 << (u64::BITS - 1 - padding_bit as u32)) / (carry_modulus.0 * message_modulus.0) | ||
| * 2 | ||
| } | ||
| CiphertextModulusKind::Other | CiphertextModulusKind::NonNativePowerOfTwo => { | ||
| ciphertext_modulus.get_custom_modulus() as u64 | ||
| / (carry_modulus.0 * message_modulus.0) | ||
| / if padding_bit == PaddingBit::Yes { 2 } else { 1 } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| pub(crate) struct ShortintEncoding { | ||
| pub(crate) ciphertext_modulus: CiphertextModulus, | ||
| pub(crate) message_modulus: MessageModulus, | ||
| pub(crate) carry_modulus: CarryModulus, | ||
| pub(crate) padding_bit: PaddingBit, | ||
| } | ||
|
|
||
| impl ShortintEncoding { | ||
| pub(crate) fn delta(&self) -> u64 { | ||
| compute_delta( | ||
| self.ciphertext_modulus, | ||
| self.message_modulus, | ||
| self.carry_modulus, | ||
| self.padding_bit, | ||
| ) | ||
| } | ||
| } | ||
|
|
||
| impl ShortintEncoding { | ||
| fn plaintext_space(&self) -> u64 { | ||
| self.message_modulus.0 | ||
| * self.carry_modulus.0 | ||
| * if self.padding_bit == PaddingBit::No { | ||
| 1 | ||
| } else { | ||
| 2 | ||
| } | ||
| } | ||
| pub(crate) fn from_parameters( | ||
| params: impl Into<ShortintParameterSet>, | ||
| padding_bit: PaddingBit, | ||
| ) -> Self { | ||
| let params = params.into(); | ||
| Self { | ||
| ciphertext_modulus: params.ciphertext_modulus(), | ||
| message_modulus: params.message_modulus(), | ||
| carry_modulus: params.carry_modulus(), | ||
| padding_bit, | ||
| } | ||
| } | ||
|
|
||
| pub(crate) fn encode(&self, value: Cleartext<u64>) -> Plaintext<u64> { | ||
| let delta = compute_delta( | ||
| self.ciphertext_modulus, | ||
| self.message_modulus, | ||
| self.carry_modulus, | ||
| self.padding_bit, | ||
| ); | ||
|
|
||
| Plaintext(value.0.wrapping_mul(delta)) | ||
| } | ||
|
|
||
| pub(crate) fn decode(&self, value: Plaintext<u64>) -> Cleartext<u64> { | ||
| assert!(self.ciphertext_modulus.is_compatible_with_native_modulus()); | ||
| let delta = self.delta(); | ||
|
|
||
| // The bit before the message | ||
| let rounding_bit = delta >> 1; | ||
|
|
||
| // Compute the rounding bit | ||
| let rounding = (value.0 & rounding_bit) << 1; | ||
|
|
||
| // Force the decoded value to be in the correct range | ||
| Cleartext((value.0.wrapping_add(rounding) / delta) % (self.plaintext_space())) | ||
| } | ||
| } | ||
|
|
||
| #[test] | ||
| fn test_pow_2_encoding_ci_run_filter() { | ||
| use crate::shortint::parameters::V0_10_PARAM_MESSAGE_2_CARRY_2_KS_PBS_GAUSSIAN_2M64; | ||
| const CIPHERTEXT_MODULUS: u64 = 1u64 << 62; | ||
|
|
||
| let mut params = V0_10_PARAM_MESSAGE_2_CARRY_2_KS_PBS_GAUSSIAN_2M64; | ||
| params.carry_modulus = CarryModulus(1); | ||
| params.ciphertext_modulus = CiphertextModulus::new(CIPHERTEXT_MODULUS as u128); | ||
|
|
||
| let encoding = ShortintEncoding::from_parameters(params, PaddingBit::Yes); | ||
| let (cks, _sks) = crate::shortint::gen_keys(params); | ||
| for m in 0..params.message_modulus.0 { | ||
| let encoded = encoding.encode(Cleartext(m)); | ||
| assert!( | ||
| encoded.0 < (CIPHERTEXT_MODULUS / 2), | ||
| "encoded message goes beyond its allowed space" | ||
| ); | ||
|
|
||
| let ct = cks.encrypt(m); | ||
|
|
||
| let decrypted = cks.decrypt(&ct); | ||
| assert_eq!(decrypted, m); | ||
| } | ||
| } |
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