Support BigUint plaintext moduli in BFV #358
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- Refactored `BfvParameters` to support arbitrary precision plaintext moduli using `BigUint`. - Updated `Plaintext` struct to use an enum `PlaintextValues` (`Small(u64)` or `Large(BigUint)`) to maintain performance for standard `u64` moduli while enabling large moduli. - Updated `SecretKey` decryption to correctly handle `BigUint` plaintexts by reconstructing the value from RNS representation. - Updated `FheEncoder` and `FheDecoder` implementations to support `BigUint` values. - Added comprehensive tests for encryption, decryption, and homomorphic addition with `BigUint` plaintext moduli. Co-authored-by: tlepoint <1345502+tlepoint@users.noreply.github.com>
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crates/fhe/src/bfv/plaintext_vec.rs
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| let value_enum = match par.plaintext { | ||
| crate::bfv::PlaintextModulus::Small(_) => PlaintextValues::Small(v.iter().map(|x| x.to_u64().unwrap_or(0)).collect::<Vec<_>>().into_boxed_slice()), | ||
| crate::bfv::PlaintextModulus::Large(_) => PlaintextValues::Large(v.iter().map(|x| BigUint::from(x.clone())).collect::<Vec<_>>().into_boxed_slice()), |
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When the plaintext modulus is Small, BigUint inputs that exceed u64::MAX are silently converted to 0 via to_u64().unwrap_or(0). If callers use the BigUint encoder generically (e.g., a large literal or a value already reduced mod t but still > u64::MAX), the stored PlaintextValues::Small will not match the intended message, and Plaintext::to_poly() will encrypt the wrong value. This should return an error or explicitly reduce modulo the plaintext modulus instead of zeroing on overflow.
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- Added `PlaintextModulus` enum to `BfvParameters` to support both `Small` (u64) and `Large` (BigUint) moduli. - Updated `Plaintext` to store values in `PlaintextValues` enum (`Small` or `Large`). - Updated `PlaintextVec` encoders/decoders to handle both variants. - Updated `SecretKey` decryption to correctly lift RNS residues to `BigUint` for large moduli. - Added new tests in `crates/fhe/tests/biguint_support.rs`. - Updated `fhe.proto` to include optional `plaintext_big` field. Co-authored-by: tlepoint <1345502+tlepoint@users.noreply.github.com>
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This PR adds support for using
BigUintas the plaintext modulus in the BFV scheme. This allows for plaintext moduli larger than 62 bits.Key changes:
BfvParametersnow stores the plaintext modulus asBigUintinternally, exposingplaintext()foru64(panics if too large) andplaintext_big()forBigUint.Plaintextstruct now holds values inPlaintextValues, which is an enumSmall(Box<[u64]>)orLarge(Box<[BigUint]>). This optimization ensures that operations with small moduli remain performant.SecretKey::try_decryptwas updated to properly reconstruct large integer coefficients from the ciphertext polynomial's RNS representation when the plaintext modulus is large.PlaintextVecand related encoders/decoders were updated to handleBigUintinputs and the newPlaintextValuesstructure.tests/biguint_support.rsverify the functionality with a 128-bit Mersenne prime modulus.PR created automatically by Jules for task 4494186955965380558 started by @tlepoint