feat: move RpoRandomCoin and define Rng trait

nits: minor

chore: update log and readme

CHANGELOG.md

@@ -3,6 +3,7 @@
 * Implemented the `PartialMmr` data structure (#195).
 * Updated Winterfell dependency to v0.7 (#200)
 * Implemented RPX hash function (#201).
+* Added `FeltRng` and `RpoRandomCoin` (#237).
 
 ## 0.7.1 (2023-10-10)
 

README.md

@@ -24,12 +24,18 @@ For performance benchmarks of these hash functions and their comparison to other
 The module also contains additional supporting components such as `NodeIndex`, `MerklePath`,  and `MerkleError`  to assist with tree indexation, opening proofs, and reporting inconsistent arguments/state.
 
 ## Signatures
-[DAS module](./src/dsa) provides a set of digital signature schemes supported by default in the Miden VM. Currently, these schemes are:
+[DSA module](./src/dsa) provides a set of digital signature schemes supported by default in the Miden VM. Currently, these schemes are:
 
 * `RPO Falcon512`: a variant of the [Falcon](https://falcon-sign.info/) signature scheme. This variant differs from the standard in that instead of using SHAKE256 hash function in the *hash-to-point* algorithm we use RPO256. This makes the signature more efficient to verify in Miden VM.
 
 For the above signatures, key generation and signing is available only in the `std` context (see [crate features](#crate-features) below), while signature verification is available in `no_std` context as well.
 
+## Pseudo-Random Element Generator
+[Pseudo random element generator module](./src/rand/) provides a set of traits and data structures that facilitate generating pseudo-random elements in the context of Miden VM and Miden rollup. The module currently includes:
+
+* `FeltRng`: a trait for generating random field elements and random 4 field elements.
+* `RpoRandomCoin`: a struct implementing `FeltRng` as well as the [`RandomCoin`](https://github.com/facebook/winterfell/blob/main/crypto/src/random/mod.rs) trait.
+    
 ## Crate features
 This crate can be compiled with the following features:
 

src/rand/mod.rs

@@ -1,3 +1,18 @@
 //! Pseudo-random element generation.
 
 pub use winter_crypto::{RandomCoin, RandomCoinError};
+
+use crate::{Felt, Word, ZERO};
+
+mod rpo;
+
+/// Pseudo-random element generator.
+///
+/// An instance can be used to draw, uniformly at random, basefield elements as well as `Word`s.
+pub trait FeltRng {
+    /// Draw, uniformly at random, a basefield element.
+    fn draw_element(&mut self) -> Felt;
+
+    /// Draw, uniformly at random, a `Word`.
+    fn draw_word(&mut self) -> Word;
+}

src/rand/rpo.rs

@@ -0,0 +1,188 @@
+pub use winter_crypto::{RandomCoin, RandomCoinError};
+use winter_math::{FieldElement, StarkField};
+
+use super::{Felt, FeltRng, Word, ZERO};
+use crate::hash::rpo::{Rpo256, RpoDigest};
+use crate::utils::collections::Vec;
+use crate::utils::vec;
+
+// CONSTANTS
+// ================================================================================================
+
+const STATE_WIDTH: usize = Rpo256::STATE_WIDTH;
+const RATE_START: usize = Rpo256::RATE_RANGE.start;
+const RATE_END: usize = Rpo256::RATE_RANGE.end;
+const HALF_RATE_WIDTH: usize = (Rpo256::RATE_RANGE.end - Rpo256::RATE_RANGE.start) / 2;
+
+// RPO RANDOM COIN
+// ================================================================================================
+/// A simplified version of the `SPONGE_PRG` reseedable pseudo-random number generator algorithm
+/// described in https://eprint.iacr.org/2011/499.pdf. The simplification is related to
+/// the following facts:
+/// 1. A call to the reseed method implies one and only one call to the permutation function.
+///  This is possible because in our case we never reseed with more than 4 field elements.
+/// 2. As a result of the previous point, we dont make use of an input buffer to accumulate seed
+///  material.
+pub struct RpoRandomCoin {
+    state: [Felt; STATE_WIDTH],
+    current: usize,
+}
+
+impl RpoRandomCoin {
+    fn draw_basefield(&mut self) -> Felt {
+        if self.current == RATE_END {
+            Rpo256::apply_permutation(&mut self.state);
+            self.current = RATE_START;
+        }
+
+        self.current += 1;
+        self.state[self.current - 1]
+    }
+}
+
+impl RandomCoin for RpoRandomCoin {
+    type BaseField = Felt;
+    type Hasher = Rpo256;
+
+    fn new(seed: &[Self::BaseField]) -> Self {
+        let mut state = [ZERO; STATE_WIDTH];
+        let digest: Word = Rpo256::hash_elements(seed).into();
+
+        for i in 0..HALF_RATE_WIDTH {
+            state[RATE_START + i] += digest[i];
+        }
+
+        // Absorb
+        Rpo256::apply_permutation(&mut state);
+
+        RpoRandomCoin { state, current: RATE_START }
+    }
+
+    fn reseed(&mut self, data: RpoDigest) {
+        // Reset buffer
+        self.current = RATE_START;
+
+        // Add the new seed material to the first half of the rate portion of the RPO state
+        let data: Word = data.into();
+
+        self.state[RATE_START] += data[0];
+        self.state[RATE_START + 1] += data[1];
+        self.state[RATE_START + 2] += data[2];
+        self.state[RATE_START + 3] += data[3];
+
+        // Absorb
+        Rpo256::apply_permutation(&mut self.state);
+    }
+
+    fn check_leading_zeros(&self, value: u64) -> u32 {
+        let value = Felt::new(value);
+        let mut state_tmp = self.state;
+
+        state_tmp[RATE_START] += value;
+
+        Rpo256::apply_permutation(&mut state_tmp);
+
+        let first_rate_element = state_tmp[RATE_START].as_int();
+        first_rate_element.trailing_zeros()
+    }
+
+    fn draw<E: FieldElement<BaseField = Felt>>(&mut self) -> Result<E, RandomCoinError> {
+        let ext_degree = E::EXTENSION_DEGREE;
+        let mut result = vec![ZERO; ext_degree];
+        for r in result.iter_mut().take(ext_degree) {
+            *r = self.draw_basefield();
+        }
+
+        let result = E::slice_from_base_elements(&result);
+        Ok(result[0])
+    }
+
+    fn draw_integers(
+        &mut self,
+        num_values: usize,
+        domain_size: usize,
+        nonce: u64,
+    ) -> Result<Vec<usize>, RandomCoinError> {
+        assert!(domain_size.is_power_of_two(), "domain size must be a power of two");
+        assert!(num_values < domain_size, "number of values must be smaller than domain size");
+
+        // absorb the nonce
+        let nonce = Felt::new(nonce);
+        self.state[RATE_START] += nonce;
+        Rpo256::apply_permutation(&mut self.state);
+
+        // reset the buffer
+        self.current = RATE_START;
+
+        // determine how many bits are needed to represent valid values in the domain
+        let v_mask = (domain_size - 1) as u64;
+
+        // draw values from PRNG until we get as many unique values as specified by num_queries
+        let mut values = Vec::new();
+        for _ in 0..1000 {
+            // get the next pseudo-random field element
+            let value = self.draw_basefield().as_int();
+
+            // use the mask to get a value within the range
+            let value = (value & v_mask) as usize;
+
+            values.push(value);
+            if values.len() == num_values {
+                break;
+            }
+        }
+
+        if values.len() < num_values {
+            return Err(RandomCoinError::FailedToDrawIntegers(num_values, values.len(), 1000));
+        }
+
+        Ok(values)
+    }
+}
+
+impl FeltRng for RpoRandomCoin {
+    fn draw_element(&mut self) -> Felt {
+        self.draw_basefield()
+    }
+
+    fn draw_word(&mut self) -> Word {
+        let mut output = [ZERO; 4];
+        for o in output.iter_mut() {
+            *o = self.draw_basefield();
+        }
+        output
+    }
+}
+
+// TESTS
+// ================================================================================================
+
+#[cfg(all(test, feature = "std"))]
+mod tests {
+    use super::{FeltRng, RandomCoin, RpoRandomCoin, ZERO};
+
+    #[test]
+    fn test_randfeltsgen_felt() {
+        let mut rpocoin = RpoRandomCoin::new(&[ZERO; 4]);
+        let output = rpocoin.draw_element();
+
+        let mut rpocoin = RpoRandomCoin::new(&[ZERO; 4]);
+        let expected = rpocoin.draw_basefield();
+
+        assert_eq!(output, expected);
+    }
+
+    #[test]
+    fn test_randfeltsgen_word() {
+        let mut rpocoin = RpoRandomCoin::new(&[ZERO; 4]);
+        let output = rpocoin.draw_word();
+
+        let mut rpocoin = RpoRandomCoin::new(&[ZERO; 4]);
+        let mut expected = [ZERO; 4];
+        for o in expected.iter_mut() {
+            *o = rpocoin.draw_basefield();
+        }
+
+        assert_eq!(output, expected);
+    }
+}