diff --git a/src/chaum_pedersen_signature.rs b/src/chaum_pedersen_signature.rs index ab4709a..e217953 100644 --- a/src/chaum_pedersen_signature.rs +++ b/src/chaum_pedersen_signature.rs @@ -13,7 +13,7 @@ use crate::dual_scalar_mul::DualScalarMultiplication; use crate::engine::EngineBLS; use crate::nugget::{NuggetBLS, NuggetPublicKey, NuggetSignature}; use crate::serialize::SerializableToBytes; -use crate::{Message, SecretKeyVT}; +use crate::{Message, SecretKey, SecretKeyVT}; pub type DLEQProof = (::Scalar, ::Scalar); pub type ChaumPedersenSignature = NuggetSignature; @@ -322,10 +322,12 @@ where &self, message_point_as_bytes: &Vec, ) -> <::PublicKeyGroup as PrimeGroup>::ScalarField { - let secret_key_as_bytes = self.to_bytes(); - let mut secret_key_hasher = H::default(); + + let mut secret_key_as_bytes = self.to_bytes(); secret_key_hasher.update(secret_key_as_bytes.as_slice()); + ::zeroize::Zeroize::zeroize(secret_key_as_bytes.as_mut_slice()); + let hashed_secret_key = secret_key_hasher.finalize_fixed_reset().to_vec(); let hasher = as HashToField< @@ -336,6 +338,53 @@ where } } +/// Side-channel-protected variant: BLS signing goes through +/// `SecretKey::seeded_sign` (resplits the key with deterministic +/// randomness), while DLEQ proof generation and witness derivation +/// delegate to the vartime form, which is acceptable because they +/// operate over auxiliary scalars rather than the long-term key. +impl + ChaumPedersenSigner for SecretKey +where + S: PrimeGroup + SerializableToBytes, +{ + fn generate_cp_signature(&mut self, message: &Message) -> ChaumPedersenSignature { + let bls_signature = self.seeded_sign(message); + let dleq = >::generate_dleq_proof( + self, + message, + bls_signature.0, + ); + NuggetSignature(bls_signature.0, dleq) + } + + fn generate_dleq_proof( + &mut self, + message: &Message, + bls_signature: E::SignatureGroup, + ) -> DLEQProof { + as ChaumPedersenSigner>::generate_dleq_proof( + &mut self.into_vartime(), + message, + bls_signature, + ) + } + + fn generate_witness_scaler( + &self, + _message_point_as_bytes: &Vec, + ) -> <::PublicKeyGroup as PrimeGroup>::ScalarField { + // Unreachable: `generate_dleq_proof` for `SecretKey` delegates to + // the `SecretKeyVT` impl, which calls its own `generate_witness_scaler`. + // Trait coherence forces us to provide a body here, but no caller + // ever lands on it. + unimplemented!( + "SecretKey::generate_witness_scaler is never called; \ + dleq generation delegates to SecretKeyVT" + ) + } +} + #[cfg(all(test, feature = "std"))] mod tests { use rand::thread_rng; diff --git a/src/engine.rs b/src/engine.rs index 0c7dfa2..7079bcd 100644 --- a/src/engine.rs +++ b/src/engine.rs @@ -32,7 +32,7 @@ use ark_ec::{ }; use ark_ff::field_hashers::{DefaultFieldHasher, HashToField}; use ark_ff::{Field, PrimeField, UniformRand, Zero}; -use ark_serialize::CanonicalSerialize; +use ark_serialize::{CanonicalSerialize, Valid}; use rand::Rng; use rand_core::RngCore; @@ -198,8 +198,7 @@ pub trait EngineBLS { signature, )]; Self::final_exponentiation(Self::miller_loop(inputs.into_iter().map(|t| t).chain(&lhs))) - .unwrap() - == (PairingOutput::::zero()) //zero is the target_field::one !! + == Some(PairingOutput::::zero()) //zero is the target_field::one !! } /// Prepared negative of the generator of the public key curve. @@ -219,6 +218,17 @@ pub trait EngineBLS { Self::PublicKeyPrepared::from(g_affine) } + /// Reject public keys that are the identity element or not in the + /// prime-order subgroup. Defends verification against inputs that + /// bypass the deserialization-time check — e.g. direct tuple-struct + /// construction, or aggregates that sum to the identity. + /// + /// Implements `KeyValidate` from + /// . + fn validate_public_key(g: &Self::PublicKeyGroupAffine) -> bool { + !g.is_zero() && g.check().is_ok() + } + /// Process the signature to be use in pairing. This has to be /// implemented by the type of BLS system implementing the engine /// by calling either prepare_g1 or prepare_g2 based on which group diff --git a/src/experimental/bit.rs b/src/experimental/bit.rs index 27c91b5..1f04db7 100644 --- a/src/experimental/bit.rs +++ b/src/experimental/bit.rs @@ -632,7 +632,9 @@ mod tests { .collect::>(); let mut bitsig1 = BitSignedMessage::::new(pop.clone(), &msg1); - assert!(bitsig1.verify()); // verifiers::verify_with_distinct_messages(&dms,true) + // Empty aggregate: the summed public key is the identity element, + // which `validate_public_key` rejects per IETF KeyValidate. + assert!(!bitsig1.verify()); for (i, sig) in sigs1.iter().enumerate().take(2) { assert!(bitsig1.add(sig).is_ok() == (i < 4)); assert!(bitsig1.verify()); // verifiers::verify_with_distinct_messages(&dms,true) @@ -687,7 +689,9 @@ mod tests { let mut countsig = CountSignedMessage::::new(pop.clone(), msg1); assert!(countsig.signers.len() == 1); - assert!(countsig.verify()); // verifiers::verify_with_distinct_messages(&dms,true) + // Empty aggregate: the summed public key is the identity element, + // which `validate_public_key` rejects per IETF KeyValidate. + assert!(!countsig.verify()); assert!(countsig.add_bitsig(&bitsig1).is_ok()); assert!(bitsig1.signature == countsig.signature); assert!(countsig.signers.len() == 1); diff --git a/src/experimental/schnorr_pop.rs b/src/experimental/schnorr_pop.rs index e06371d..b486f8f 100644 --- a/src/experimental/schnorr_pop.rs +++ b/src/experimental/schnorr_pop.rs @@ -41,11 +41,13 @@ impl BLSSchnorrPo //The pseudo random witness is generated similar to eddsa witness //hash(secret_key|publick_key) fn witness_scalar(&self) -> <::PublicKeyGroup as PrimeGroup>::ScalarField { - let secret_key_as_bytes = self.secret.to_bytes(); + let mut secret_key_as_bytes = self.secret.to_bytes(); let public_key_as_bytes = ::public_key_point_to_byte(&self.public.0); let mut secret_key_hasher = H::default(); secret_key_hasher.update(secret_key_as_bytes.as_slice()); + ::zeroize::Zeroize::zeroize(secret_key_as_bytes.as_mut_slice()); + let hashed_secret_key = secret_key_hasher.finalize_fixed_reset().to_vec(); let hasher = as HashToField< diff --git a/src/lib.rs b/src/lib.rs index c97a2e6..d839f4f 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -215,7 +215,8 @@ impl Message { fn cipher_suite(&self) -> Vec { let id = match self.2 { MessageType::ProofOfPossession => PROOF_OF_POSSESSION_ID, - _ => NORMAL_MESSAGE_SIGNATURE_ID, + MessageType::NormalAssumingPoP => NORMAL_MESSAGE_SIGNATURE_ID, + MessageType::NormalBasic => NORMAL_MESSAGE_SIGNATURE_ID, }; let h2c_suite_id = [ @@ -228,7 +229,7 @@ impl Message { let sc_tag = match self.2 { MessageType::ProofOfPossession => POP_MESSAGE, MessageType::NormalAssumingPoP => NORMAL_MESSAGE_SIGNATURE_ASSUMING_POP, - _ => NORMAL_MESSAGE_SIGNATURE_BASIC, + MessageType::NormalBasic => NORMAL_MESSAGE_SIGNATURE_BASIC, }; [id, &h2c_suite_id[..], sc_tag].concat() diff --git a/src/nugget.rs b/src/nugget.rs index 0c7df81..734f36f 100644 --- a/src/nugget.rs +++ b/src/nugget.rs @@ -25,7 +25,7 @@ use crate::chaum_pedersen_signature::{ChaumPedersenSigner, ChaumPedersenVerifier use crate::dual_scalar_mul::DualScalarMultiplication; use crate::chaum_pedersen_signature::DLEQProof; use crate::serialize::SerializableToBytes; -use crate::single::{Keypair, KeypairVT, PublicKey, SecretKeyVT, Signature}; +use crate::single::{Keypair, KeypairVT, PublicKey, SecretKey, SecretKeyVT, Signature}; use crate::{EngineBLS, Message, Signed}; /// Wrapper for a point in the signature group which is supposed to @@ -89,6 +89,26 @@ where } } +/// Side-channel-protected variant: signing goes through the +/// `ChaumPedersenSigner` impl for `SecretKey`, so the resplit happens +/// on the split key (no `into_vartime` conversion is done here). +impl NuggetBLS for SecretKey +where + S: PrimeGroup + SerializableToBytes, +{ + fn into_public_key_in_signature_group(&self) -> PublicKeyInSignatureGroup { + NuggetBLS::::into_public_key_in_signature_group(&self.into_vartime()) + } + + fn into_public_key_in_sister_group(&self) -> PublicKeyInSisterGroup { + self.into_vartime().into_public_key_in_sister_group() + } + + fn sign(&mut self, message: &Message) -> NuggetSignature { + ChaumPedersenSigner::::generate_cp_signature(self, &message) + } +} + impl NuggetBLS for KeypairVT where S: PrimeGroup + SerializableToBytes, @@ -112,16 +132,16 @@ where S: PrimeGroup + SerializableToBytes, { fn into_public_key_in_signature_group(&self) -> PublicKeyInSignatureGroup { - NuggetBLS::::into_public_key_in_signature_group(&self.into_vartime()) + NuggetBLS::::into_public_key_in_signature_group(&self.secret) } fn into_public_key_in_sister_group(&self) -> PublicKeyInSisterGroup { - self.into_vartime().into_public_key_in_sister_group() + NuggetBLS::::into_public_key_in_sister_group(&self.secret) } /// Sign a message using a Seedabale RNG created from a seed derived from the message and key fn sign(&mut self, message: &Message) -> NuggetSignature { - NuggetBLS::::sign(&mut self.into_vartime(), message) + NuggetBLS::::sign(&mut self.secret, message) } } diff --git a/src/nugget_pop.rs b/src/nugget_pop.rs index c05f2f3..9ec7126 100644 --- a/src/nugget_pop.rs +++ b/src/nugget_pop.rs @@ -92,7 +92,11 @@ impl let mut randomized_pub_in_g1 = public_key_in_signature_group; randomized_pub_in_g1 *= randomization_coefficient; let signature = E::prepare_signature(self.0 + randomized_pub_in_g1); - let prepared_public_key = E::prepare_public_key(public_key_of_prover.1); + let Some(prepared_public_key) = + crate::verifiers::validate_and_prepare_public_key::(public_key_of_prover.1) + else { + return false; + }; let prepared = [ ( prepared_public_key.clone(), @@ -162,12 +166,13 @@ where #[cfg(all(test, feature = "std"))] mod tests { use crate::double_nugget::DoubleNuggetBLS; - use crate::engine::TinyBLS381; + use crate::engine::{EngineBLS, TinyBLS381}; use crate::serialize::SerializableToBytes; use crate::single::Keypair; use crate::{nugget_pop::NuggetBLSPoP, NuggetDoublePublicKey}; use crate::{ProofOfPossession, ProofOfPossessionGenerator}; + use ark_ff::Zero; use rand::thread_rng; use sha2::Sha256; @@ -324,4 +329,35 @@ mod tests { NuggetBLSnCPPoP, >(); } + + /// A keypair with secret scalar zero and identity public key. + /// `SecretKeyVT::sign` returns `sk * H(msg) = identity` for such + /// a keypair, so the PoP produced by `generate_pok` is itself + /// identity. The pairing equation in `NuggetBLSPoP::verify` then + /// reduces to `identity == identity`, so without `validate_public_key` + /// the verifier would accept — rejection here is attributable to + /// the public key check. + #[test] + fn nugget_bls_pop_rejects_identity_pk() { + use crate::single::{PublicKey, SecretKeyVT}; + let mut keypair = Keypair:: { + public: PublicKey::( + ::PublicKeyGroup::zero(), + ), + secret: SecretKeyVT::(::Scalar::zero()) + .into_split(thread_rng()), + }; + let pop = as ProofOfPossessionGenerator< + TinyBLS381, + Sha256, + NuggetDoublePublicKey, + NuggetBLSPoP, + >>::generate_pok(&mut keypair); + let double_pk = DoubleNuggetBLS::into_nugget_double_public_key(&keypair); + assert!(! as ProofOfPossession< + TinyBLS381, + Sha256, + NuggetDoublePublicKey, + >>::verify(&pop, &double_pk)); + } } diff --git a/src/single.rs b/src/single.rs index 7b826b3..6eeb967 100644 --- a/src/single.rs +++ b/src/single.rs @@ -45,6 +45,7 @@ use sha3::{ }; use digest::Digest; +use zeroize::{Zeroize, ZeroizeOnDrop}; use core::iter::once; @@ -54,8 +55,8 @@ use crate::{EngineBLS, Message, Signed}; /// Secret signing key lacking the side channel protections from /// key splitting. Avoid using directly in production. -#[derive(CanonicalSerialize, CanonicalDeserialize)] -pub struct SecretKeyVT(pub E::Scalar); +#[derive(CanonicalSerialize, CanonicalDeserialize, Zeroize, ZeroizeOnDrop)] +pub struct SecretKeyVT(#[zeroize] pub E::Scalar); impl Clone for SecretKeyVT { fn clone(&self) -> Self { @@ -163,10 +164,11 @@ impl SecretKeyVT { /// Secret signing key including the side channel protections from /// key splitting. +#[derive(ZeroizeOnDrop)] pub struct SecretKey { - key: [E::Scalar; 2], - old_unsigned: E::SignatureGroup, - old_signed: E::SignatureGroup, + #[zeroize] key: [E::Scalar; 2], + #[zeroize] old_unsigned: E::SignatureGroup, + #[zeroize] old_signed: E::SignatureGroup, } impl Clone for SecretKey { @@ -269,6 +271,30 @@ impl SecretKey { self.sign_once(message) } + /// Sign deterministically, reseeding the resplit RNG from a hash + /// of both key halves and the message. + pub fn seeded_sign(&mut self, message: &Message) -> Signature { + let mut serialized_part1 = [0u8; 32]; + let mut serialized_part2 = [0u8; 32]; + self.key[0] + .serialize_compressed(&mut serialized_part1[..]) + .unwrap(); + self.key[1] + .serialize_compressed(&mut serialized_part2[..]) + .unwrap(); + + let seed_digest = Sha256::new() + .chain_update(serialized_part1) + .chain_update(serialized_part2) + .chain_update(message.0); + + ::zeroize::Zeroize::zeroize(&mut serialized_part1); + ::zeroize::Zeroize::zeroize(&mut serialized_part2); + + let seed: [u8; 32] = seed_digest.finalize().into(); + self.sign(message, StdRng::from_seed(seed)) + } + /// Derive our public key from our secret key /// /// We do not resplit for side channel protections here since @@ -458,7 +484,11 @@ impl Signature { /// Verify a single BLS signature pub fn verify(&self, message: &Message, publickey: &PublicKey) -> bool { - let publickey = E::prepare_public_key(publickey.0); + let Some(publickey) = + crate::verifiers::validate_and_prepare_public_key::(publickey.0) + else { + return false; + }; // TODO: Bentchmark these two variants // Variant 1. Do not batch any normalizations let message = E::prepare_signature(message.hash_to_signature_curve::()); @@ -601,23 +631,7 @@ impl Keypair { /// Sign a message using a Seedabale RNG created from a seed derived from the message and key pub fn sign(&mut self, message: &Message) -> Signature { - let mut serialized_part1 = [0u8; 32]; - let mut serialized_part2 = [0u8; 32]; - self.secret.key[0] - .serialize_compressed(&mut serialized_part1[..]) - .unwrap(); - self.secret.key[1] - .serialize_compressed(&mut serialized_part2[..]) - .unwrap(); - - let seed_digest = Sha256::new() - .chain_update(serialized_part1) - .chain_update(serialized_part2) - .chain_update(message.0); - - let seed: [u8; 32] = seed_digest.finalize().into(); - - self.sign_with_rng::(message, SeedableRng::from_seed(seed)) + self.secret.seeded_sign(message) } #[cfg(feature = "std")] @@ -992,4 +1006,25 @@ mod tests { random_seed.as_slice(), ); } + + /// Keypair with secret=0, public=identity produces an identity + /// signature, so the pairing equation reduces to `identity == identity` + /// — an unprotected `Signature::verify` would accept. Rejection here + /// is attributable to `validate_public_key`. + #[test] + fn signature_verify_rejects_identity_pk() { + use ark_ff::Zero; + use rand::{rngs::StdRng, SeedableRng}; + + type EB = UsualBLS; + + let mut keypair = Keypair:: { + public: PublicKey::(::PublicKeyGroup::zero()), + secret: SecretKeyVT::(::Scalar::zero()) + .into_split(StdRng::from_seed([0u8; 32])), + }; + let s = keypair.signed_message(&Message::new(b"ctx", b"test message")); + assert!(!s.signature.verify(&s.message, &s.publickey)); + assert!(!Signed::verify(&s)); + } } diff --git a/src/verifiers.rs b/src/verifiers.rs index 20fbb05..927a410 100644 --- a/src/verifiers.rs +++ b/src/verifiers.rs @@ -37,6 +37,22 @@ pub type SignatureAffine = <::SignatureGroup as CurveGroup>:: // ── Shared helpers ────────────────────────────────────────────────── +/// Validate a public key with `EngineBLS::validate_public_key` and, on +/// success, return the prepared form. Returns `None` if the public key +/// is the identity element or not in the prime-order subgroup. All +/// verifier paths should go through this wrapper so the check cannot +/// be skipped by callers constructing `PublicKey` directly or by +/// aggregates that sum to the identity. +pub fn validate_and_prepare_public_key( + g: impl Into>, +) -> Option { + let g_affine: PublicKeyAffine = g.into(); + if !E::validate_public_key(&g_affine) { + return None; + } + Some(E::prepare_public_key(g_affine)) +} + /// Verify from fully normalized (affine) inputs. /// All public keys, messages, and the signature must already be in affine form. /// This prepares the pairing inputs and calls `verify_prepared`. @@ -46,11 +62,13 @@ fn verify_normalized( affine_signature: SignatureAffine, ) -> bool { let prepared_sig = E::prepare_signature(affine_signature); - let prepared = affine_publickeys - .iter() - .zip(affine_messages) - .map(|(pk, m)| (E::prepare_public_key(*pk), E::prepare_signature(*m))) - .collect::>(); + let mut prepared = Vec::with_capacity(affine_publickeys.len()); + for (pk, m) in affine_publickeys.iter().zip(affine_messages) { + let Some(prepared_pk) = validate_and_prepare_public_key::(*pk) else { + return false; + }; + prepared.push((prepared_pk, E::prepare_signature(*m))); + } E::verify_prepared(prepared_sig, prepared.iter()) } @@ -143,15 +161,18 @@ fn normalize_messages_and_signature( /// Simple unoptimized BLS signature verification. Useful for testing. pub fn verify_unoptimized(s: S) -> bool { let signature = S::E::prepare_signature(s.signature().0); - let prepared = s - .messages_and_publickeys() - .map(|(message, public_key)| { - ( - S::E::prepare_public_key(public_key.borrow().0), - S::E::prepare_signature(message.borrow().hash_to_signature_curve::()), - ) - }) - .collect::>(); + let mut prepared = Vec::new(); + for (message, public_key) in s.messages_and_publickeys() { + let Some(prepared_pk) = + validate_and_prepare_public_key::(public_key.borrow().0) + else { + return false; + }; + prepared.push(( + prepared_pk, + S::E::prepare_signature(message.borrow().hash_to_signature_curve::()), + )); + } S::E::verify_prepared(signature, prepared.iter()) } @@ -365,13 +386,47 @@ fn verify_with_gaussian_elimination(s: S) -> bool { #[cfg(test)] mod tests { use super::*; - use crate::{Keypair, Message, UsualBLS}; - use ark_bls12_381::Bls12_381; + use crate::single::{SecretKeyVT, SignedMessage}; + use crate::{Keypair, Message, PublicKey, UsualBLS}; + use ark_bls12_381::{Bls12_381, Fq, G1Affine}; + use ark_ec::AffineRepr; + use ark_ff::{UniformRand, Zero}; + use ark_serialize::Valid; use rand::rngs::StdRng; use rand::SeedableRng; type EB = UsualBLS; + /// Keypair with secret scalar zero and identity public key. The + /// produced signature is also identity, so every pairing equation + /// reduces to `identity == identity` and an unprotected verifier + /// would accept. Any rejection observed in the verifier tests + /// below is therefore attributable to `validate_public_key`. + fn identity_keypair() -> Keypair { + Keypair { + public: PublicKey::(::PublicKeyGroup::zero()), + secret: SecretKeyVT::(::Scalar::zero()) + .into_split(StdRng::from_seed([0u8; 32])), + } + } + + fn identity_signed() -> SignedMessage { + identity_keypair().signed_message(&Message::new(b"ctx", b"test message")) + } + + /// A G1 point on the curve but outside the prime-order subgroup. + fn non_subgroup_g1() -> G1Affine { + let mut rng = StdRng::from_seed([42u8; 32]); + loop { + let x = Fq::rand(&mut rng); + if let Some(point) = G1Affine::get_point_from_x_unchecked(x, false) { + if point.check().is_err() { + return point; + } + } + } + } + #[test] fn verify_simple_single_signature() { let good = Message::new(b"ctx", b"test message"); @@ -401,4 +456,30 @@ mod tests { let signed = keypair.signed_message(&good); assert!(verify_unoptimized(&signed)); } + + #[test] + fn verify_simple_rejects_identity_pk() { + assert!(!verify_simple(&identity_signed())); + } + + #[test] + fn verify_unoptimized_rejects_identity_pk() { + assert!(!verify_unoptimized(&identity_signed())); + } + + #[test] + fn verify_with_distinct_messages_rejects_identity_pk() { + assert!(!verify_with_distinct_messages(&identity_signed(), true)); + } + + #[test] + fn validate_and_prepare_public_key_rejects_identity() { + let identity = PublicKeyAffine::::zero(); + assert!(validate_and_prepare_public_key::(identity).is_none()); + } + + #[test] + fn validate_and_prepare_public_key_rejects_non_subgroup() { + assert!(validate_and_prepare_public_key::(non_subgroup_g1()).is_none()); + } }