1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153

use core::marker::PhantomData;

use crate::{
    rand_core::{CryptoRng, RngCore, SeedableRng},
    typenum::{
        marker_traits::NonZero, type_operators::IsLessOrEqual, PartialDiv, Unsigned, U32, U64,
    },
    Sigma,
};
use digest::{BlockInput, FixedOutput, Update};
use generic_array::GenericArray;

/// A trait for a Fiat-Shamir proof transcript.
///
/// Really this is just a trait around a cryptographic hash that can produce a Fiat-Shamir challenge
/// from the statement and the announcement.
pub trait Transcript<S: Sigma>: Clone {
    /// Initializes the transcript. This must be called once when the prover is created. It
    /// shouldn't be called before each proof. Rather it should be called once and then the
    /// transcript should be cloned for each proof.
    fn initialize(&mut self, sigma: &S);

    /// Adds the prover's statement to the transcript. This must be called before [`get_challenge`].
    ///
    /// [`get_challenge`]: Self::get_challenge
    fn add_statement(&mut self, sigma: &S, statement: &S::Statement);

    /// Gets the verifier's synthetic challenge for the non-interactive proof.
    fn get_challenge(
        self,
        sigma: &S,
        announce: &S::Announcement,
    ) -> GenericArray<u8, S::ChallengeLength>;
}

/// A `Transcript` that can also generate a rng.
///
/// The prover needs an rng to generate it's [`AnnounceSecret`].
///
/// [`AnnounceSecret`]: crate::Sigma::AnnounceSecret
pub trait ProverTranscript<S: Sigma> {
    /// The type of Rng the transcript generates.
    type Rng: CryptoRng + RngCore;
    /// Generates an RNG from the transcript state and an input rng (`in_rng`) which should provide
    /// system randomness.
    fn gen_rng<R: CryptoRng + RngCore>(
        &self,
        sigma: &S,
        witness: &S::Witness,
        in_rng: Option<&mut R>,
    ) -> Self::Rng;
}

#[derive(Clone, Debug)]
/// A transcript which consists of a hash with fixed length output and a seedable RNG.
///
/// The [`SeedableRng`] specified must have the same seed length as the hash's output length.
/// `R` may be set to `()` but the it won't implement [`ProverTranscript`].
///
/// [`SeedableRng`]: rand_core::SeedableRng
pub struct HashTranscript<H, R = ()> {
    hash: H,
    rng: PhantomData<R>,
}

impl<H: Default, R> Default for HashTranscript<H, R> {
    fn default() -> Self {
        HashTranscript {
            hash: H::default(),
            rng: PhantomData,
        }
    }
}

#[derive(Clone)]
struct WriteHash<H>(H);

impl<H: Update> core::fmt::Write for WriteHash<H> {
    fn write_str(&mut self, s: &str) -> core::fmt::Result {
        self.0.update(s.as_bytes());
        Ok(())
    }
}

/// Implements a transcript for any hash that outputs 32 bytes but with a block size of 64 bytes (e.g. SHA256).
///
/// The implementation first tags the SHA256 instance with the Sigma protocol's name.
impl<H, S: Sigma, R: Clone> Transcript<S> for HashTranscript<H, R>
where
    S::ChallengeLength: IsLessOrEqual<U32>,
    <S::ChallengeLength as IsLessOrEqual<U32>>::Output: NonZero,
    H: BlockInput<BlockSize = U64> + FixedOutput<OutputSize = U32> + Update + Default + Clone,
{
    fn initialize(&mut self, sigma: &S) {
        let hashed_tag = {
            let mut hash = WriteHash(H::default());
            sigma
                .write_name(&mut hash)
                .expect("writing to hash won't fail");
            hash.0.finalize_fixed()
        };
        // I started doing this with plans to make this more generic than it is.
        // This loop will always run twice
        let fill_block =
            <<H::BlockSize as PartialDiv<H::OutputSize>>::Output as Unsigned>::to_usize();
        for _ in 0..fill_block {
            self.hash.update(&hashed_tag[..]);
        }
    }

    fn add_statement(&mut self, sigma: &S, statement: &S::Statement) {
        sigma.hash_statement(&mut self.hash, statement);
    }

    fn get_challenge(
        mut self,
        sigma: &S,
        announce: &S::Announcement,
    ) -> GenericArray<u8, S::ChallengeLength> {
        sigma.hash_announcement(&mut self.hash, announce);
        let challenge_bytes = self.hash.finalize_fixed();
        // truncate the hash output
        GenericArray::clone_from_slice(&challenge_bytes[..S::ChallengeLength::to_usize()])
    }
}

/// Implements a prover transcript for a 32-byte hash with a rng that takes a 32-byte seed.
impl<S, H, R> ProverTranscript<S> for HashTranscript<H, R>
where
    S: Sigma,
    H: Update + FixedOutput<OutputSize = U32> + Clone,
    R: SeedableRng + CryptoRng + RngCore + Clone,
    R::Seed: From<GenericArray<u8, U32>>,
{
    type Rng = R;

    fn gen_rng<SysRng: CryptoRng + RngCore>(
        &self,
        sigma: &S,
        witness: &S::Witness,
        in_rng: Option<&mut SysRng>,
    ) -> Self::Rng {
        let mut rng_hash = self.hash.clone();
        sigma.hash_witness(&mut rng_hash, witness);
        if let Some(rng) = in_rng {
            let mut randomness = [0u8; 32];
            rng.fill_bytes(&mut randomness);
            rng_hash.update(randomness);
        }
        let secret_seed = rng_hash.finalize_fixed();
        R::from_seed(secret_seed.into())
    }
}