Nova/src/poseidon.rs
src/poseidon.rsRaw
1//! Poseidon Constants and Poseidon-based RO used in Nova
2use bellperson::{
3 gadgets::{
4 boolean::{AllocatedBit, Boolean},
5 num::AllocatedNum,
6 },
7 ConstraintSystem, SynthesisError,
8};
9use ff::{PrimeField, PrimeFieldBits};
10use generic_array::typenum::{U25, U27, U31};
11use neptune::{
12 circuit::poseidon_hash,
13 poseidon::{Poseidon, PoseidonConstants},
14};
15
16#[cfg(test)]
17use neptune::Strength;
18
19/// All Poseidon Constants that are used in Nova
20#[derive(Clone)]
21pub struct NovaPoseidonConstants<F>
22where
23 F: PrimeField,
24{
25 constants25: PoseidonConstants<F, U25>,
26 constants27: PoseidonConstants<F, U27>,
27 constants31: PoseidonConstants<F, U31>,
28}
29
30#[cfg(test)]
31impl<F> NovaPoseidonConstants<F>
32where
33 F: PrimeField,
34{
35 /// Generate Poseidon constants for the arities that Nova uses
36 pub fn new() -> Self {
37 let constants25 = PoseidonConstants::<F, U25>::new_with_strength(Strength::Strengthened);
38 let constants27 = PoseidonConstants::<F, U27>::new_with_strength(Strength::Strengthened);
39 let constants31 = PoseidonConstants::<F, U31>::new_with_strength(Strength::Strengthened);
40 Self {
41 constants25,
42 constants27,
43 constants31,
44 }
45 }
46}
47
48/// A Poseidon-based RO to use outside circuits
49pub struct PoseidonRO<Scalar>
50where
51 Scalar: PrimeField + PrimeFieldBits,
52{
53 // Internal State
54 state: Vec<Scalar>,
55 // Constants for Poseidon
56 constants: NovaPoseidonConstants<Scalar>,
57}
58
59impl<Scalar> PoseidonRO<Scalar>
60where
61 Scalar: PrimeField + PrimeFieldBits,
62{
63 #[allow(dead_code)]
64 pub fn new(constants: NovaPoseidonConstants<Scalar>) -> Self {
65 Self {
66 state: Vec::new(),
67 constants,
68 }
69 }
70
71 #[allow(dead_code)]
72 /// Flush the state of the RO
73 pub fn flush_state(&mut self) {
74 self.state = Vec::new();
75 }
76
77 /// Absorb a new number into the state of the oracle
78 #[allow(dead_code)]
79 pub fn absorb(&mut self, e: Scalar) {
80 self.state.push(e);
81 }
82
83 fn hash_inner(&mut self) -> Scalar {
84 match self.state.len() {
85 25 => {
86 Poseidon::<Scalar, U25>::new_with_preimage(&self.state, &self.constants.constants25).hash()
87 }
88 27 => {
89 Poseidon::<Scalar, U27>::new_with_preimage(&self.state, &self.constants.constants27).hash()
90 }
91 31 => {
92 Poseidon::<Scalar, U31>::new_with_preimage(&self.state, &self.constants.constants31).hash()
93 }
94 _ => {
95 panic!("Number of elements in the RO state does not match any of the arities used in Nova")
96 }
97 }
98 }
99
100 /// Compute a challenge by hashing the current state
101 #[allow(dead_code)]
102 pub fn get_challenge(&mut self) -> Scalar {
103 let hash = self.hash_inner();
104 // Only keep 128 bits
105 let bits = hash.to_le_bits();
106 let mut res = Scalar::zero();
107 let mut coeff = Scalar::one();
108 for bit in bits[0..128].into_iter() {
109 if *bit {
110 res += coeff;
111 }
112 coeff += coeff;
113 }
114 res
115 }
116
117 #[allow(dead_code)]
118 pub fn get_hash(&mut self) -> Scalar {
119 let hash = self.hash_inner();
120 // Only keep 250 bits
121 let bits = hash.to_le_bits();
122 let mut res = Scalar::zero();
123 let mut coeff = Scalar::one();
124 for bit in bits[0..250].into_iter() {
125 if *bit {
126 res += coeff;
127 }
128 coeff += coeff;
129 }
130 res
131 }
132}
133
134/// A Poseidon-based RO gadget to use inside the verifier circuit.
135pub struct PoseidonROGadget<Scalar>
136where
137 Scalar: PrimeField + PrimeFieldBits,
138{
139 // Internal state
140 state: Vec<AllocatedNum<Scalar>>,
141 constants: NovaPoseidonConstants<Scalar>,
142}
143
144impl<Scalar> PoseidonROGadget<Scalar>
145where
146 Scalar: PrimeField + PrimeFieldBits,
147{
148 /// Initialize the internal state and set the poseidon constants
149 #[allow(dead_code)]
150 pub fn new(constants: NovaPoseidonConstants<Scalar>) -> Self {
151 Self {
152 state: Vec::new(),
153 constants,
154 }
155 }
156
157 /// Flush the state of the RO
158 pub fn flush_state(&mut self) {
159 self.state = Vec::new();
160 }
161
162 /// Absorb a new number into the state of the oracle
163 #[allow(dead_code)]
164 pub fn absorb(&mut self, e: AllocatedNum<Scalar>) {
165 self.state.push(e);
166 }
167
168 fn hash_inner<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
169 where
170 CS: ConstraintSystem<Scalar>,
171 {
172 let out = match self.state.len() {
173 25 => poseidon_hash(
174 cs.namespace(|| "Poseidon hash"),
175 self.state.clone(),
176 &self.constants.constants25,
177 )?,
178 27 => poseidon_hash(
179 cs.namespace(|| "Poseidon hash"),
180 self.state.clone(),
181 &self.constants.constants27,
182 )?,
183 31 => poseidon_hash(
184 cs.namespace(|| "Poseidon hash"),
185 self.state.clone(),
186 &self.constants.constants31,
187 )?,
188 _ => {
189 panic!("Number of elements in the RO state does not match any of the arities used in Nova")
190 }
191 };
192
193 // return the hash as a vector of bits
194 Ok(
195 out
196 .to_bits_le_strict(cs.namespace(|| "poseidon hash to boolean"))?
197 .iter()
198 .map(|boolean| match boolean {
199 Boolean::Is(ref x) => x.clone(),
200 _ => panic!("Wrong type of input. We should have never reached there"),
201 })
202 .collect(),
203 )
204 }
205
206 /// Compute a challenge by hashing the current state
207 #[allow(dead_code)]
208 pub fn get_challenge<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
209 where
210 CS: ConstraintSystem<Scalar>,
211 {
212 let bits = self.hash_inner(cs.namespace(|| "hash"))?;
213 // Only keep 128 bits
214 Ok(bits[..128].into())
215 }
216
217 #[allow(dead_code)]
218 pub fn get_hash<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
219 where
220 CS: ConstraintSystem<Scalar>,
221 {
222 let bits = self.hash_inner(cs.namespace(|| "hash"))?;
223 // Only keep 250 bits
224 Ok(bits[..250].into())
225 }
226}
227
228#[cfg(test)]
229mod tests {
230 use super::*;
231 type S = pasta_curves::pallas::Scalar;
232 type G = pasta_curves::pallas::Point;
233 use crate::{bellperson::solver::SatisfyingAssignment, gadgets::utils::le_bits_to_num};
234 use ff::Field;
235 use rand::rngs::OsRng;
236
237 #[test]
238 fn test_poseidon_ro() {
239 // Check that the number computed inside the circuit is equal to the number computed outside the circuit
240 let mut csprng: OsRng = OsRng;
241 let constants = NovaPoseidonConstants::new();
242 let mut ro: PoseidonRO<S> = PoseidonRO::new(constants.clone());
243 let mut ro_gadget: PoseidonROGadget<S> = PoseidonROGadget::new(constants);
244 let mut cs: SatisfyingAssignment<G> = SatisfyingAssignment::new();
245 for i in 0..31 {
246 let num = S::random(&mut csprng);
247 ro.absorb(num);
248 let num_gadget =
249 AllocatedNum::alloc(cs.namespace(|| format!("data {}", i)), || Ok(num)).unwrap();
250 let _ = num_gadget
251 .inputize(&mut cs.namespace(|| format!("input {}", i)))
252 .unwrap();
253 ro_gadget.absorb(num_gadget);
254 }
255 let num = ro.get_challenge();
256 let num2_bits = ro_gadget.get_challenge(&mut cs).unwrap();
257 let num2 = le_bits_to_num(&mut cs, num2_bits).unwrap();
258 assert_eq!(num, num2.get_value().unwrap());
259 }
260}