feat: derandomize RPO-STARK DSA (#358)

CHANGELOG.md

@@ -5,6 +5,7 @@
 - [BREAKING] Updated Winterfell dependency to v0.11 (#346).
 - Added RPO-STARK based DSA (#349).
 - Added benchmarks for DSA implementations (#354).
+- Implemented deterministic RPO-STARK based DSA (#358).
 
 ## 0.12.0 (2024-10-30)
 

src/dsa/rpo_stark/stark/mod.rs

@@ -1,11 +1,12 @@
+use alloc::vec::Vec;
 use core::marker::PhantomData;
 
 use prover::RpoSignatureProver;
-use rand::{distributions::Standard, prelude::Distribution, thread_rng, RngCore, SeedableRng};
 use rand_chacha::ChaCha20Rng;
-use winter_crypto::{ElementHasher, Hasher, SaltedMerkleTree};
+use winter_crypto::{ElementHasher, SaltedMerkleTree};
 use winter_math::fields::f64::BaseElement;
 use winter_prover::{Proof, ProofOptions, Prover};
+use winter_utils::Serializable;
 use winter_verifier::{verify, AcceptableOptions, VerifierError};
 
 use crate::{
@@ -24,10 +25,7 @@ pub struct RpoSignatureScheme<H: ElementHasher> {
     _h: PhantomData<H>,
 }
 
-impl<H: ElementHasher<BaseField = BaseElement> + Sync> RpoSignatureScheme<H>
+impl<H: ElementHasher<BaseField = BaseElement> + Sync> RpoSignatureScheme<H> {
-where
-    Standard: Distribution<<H as Hasher>::Digest>,
-{
     pub fn new(options: ProofOptions) -> Self {
         RpoSignatureScheme { options, _h: PhantomData }
     }
@@ -40,9 +38,7 @@ where
         let trace = prover.build_trace(sk);
 
         // generate the initial seed for the PRNG used for zero-knowledge
-        let mut seed = <ChaCha20Rng as SeedableRng>::Seed::default();
+        let seed: [u8; 32] = generate_seed(sk, msg);
-        let mut rng = thread_rng();
-        rng.fill_bytes(&mut seed);
 
         // generate the proof
         prover.prove(trace, Some(seed)).expect("failed to generate the signature")
@@ -67,3 +63,36 @@ where
         )
     }
 }
+
+/// Deterministically generates a seed for seeding the PRNG used for zero-knowledge.
+///
+/// This uses the argument described in [RFC 6979](https://datatracker.ietf.org/doc/html/rfc6979#section-3.5)
+/// § 3.5 where the concatenation of the private key and the hashed message, i.e., sk || H(m), is
+/// used in order to construct the initial seed of a PRNG.
+///
+/// Note that we hash in also a context string in order to domain separate between different
+/// instantiations of the signature scheme.
+#[inline]
+pub fn generate_seed(sk: [BaseElement; DIGEST_SIZE], msg: [BaseElement; DIGEST_SIZE]) -> [u8; 32] {
+    let context_bytes = "
+    Seed for PRNG used for Zero-knowledge in RPO-STARK signature scheme:
+        1. Version: Conjectured security
+        2. FRI queries: 30
+        3. Blowup factor: 8
+        4. Grinding bits: 12
+        5. Field extension degree: 2
+        6. FRI folding factor: 4
+        7. FRI remainder polynomial max degree: 7
+    "
+    .to_bytes();
+    let sk_bytes = sk.to_bytes();
+    let msg_bytes = msg.to_bytes();
+
+    let total_length = context_bytes.len() + sk_bytes.len() + msg_bytes.len();
+    let mut buffer = Vec::with_capacity(total_length);
+    buffer.extend_from_slice(&context_bytes);
+    buffer.extend_from_slice(&sk_bytes);
+    buffer.extend_from_slice(&msg_bytes);
+
+    blake3::hash(&buffer).into()
+}