Channel

stwo_cairo_verifier/.tool-versions

@@ -1 +1 @@
-scarb nightly-2024-06-01
+scarb nightly-2024-06-15

stwo_cairo_verifier/src/channel.cairo

@@ -0,0 +1,251 @@
+use core::array::SpanTrait;
+use core::poseidon::{poseidon_hash_span, hades_permutation};
+use core::traits::DivRem;
+
+use stwo_cairo_verifier::{BaseField, SecureField};
+use stwo_cairo_verifier::fields::qm31::QM31Trait;
+
+const M31_SHIFT: felt252 = 0x80000000; // 2**31.
+const M31_SHIFT_NZ_U256: NonZero<u256> = 0x80000000; // 2**31.
+pub const EXTENSION_FELTS_PER_HASH: usize = 2;
+pub const FELTS_PER_HASH: usize = 8;
+
+#[derive(Default, Drop)]
+pub struct ChannelTime {
+    n_challenges: usize,
+    n_sent: usize,
+}
+
+#[generate_trait]
+impl ChannelTimeImpl of ChannelTimeTrait {
+    fn inc_sent(ref self: ChannelTime) {
+        self.n_sent += 1;
+    }
+
+    fn inc_challenges(ref self: ChannelTime) {
+        self.n_challenges += 1;
+        self.n_sent = 0;
+    }
+}
+
+#[derive(Drop)]
+pub struct Channel {
+    digest: felt252,
+    channel_time: ChannelTime,
+}
+
+#[generate_trait]
+pub impl ChannelImpl of ChannelTrait {
+    fn new(digest: felt252) -> Channel {
+        Channel { digest, channel_time: Default::default(), }
+    }
+
+    fn get_digest(ref self: Channel) -> felt252 {
+        self.digest
+    }
+
+    fn draw_felt252(ref self: Channel) -> felt252 {
+        let (res, _, _) = hades_permutation(self.digest, self.channel_time.n_sent.into(), 2);
+        self.channel_time.inc_sent();
+        res
+    }
+
+    // TODO(spapini): Check that this is sound.
+    #[inline]
+    fn draw_base_felts(ref self: Channel) -> [BaseField; FELTS_PER_HASH] {
+        let mut cur = self.draw_felt252().into();
+        [
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+            extract_m31(ref cur),
+        ]
+    }
+
+    fn mix_digest(ref self: Channel, digest: felt252) {
+        let (s0, _, _) = hades_permutation(self.digest, digest, 2);
+        self.digest = s0;
+        self.channel_time.inc_challenges();
+    }
+
+    fn mix_felts(ref self: Channel, mut felts: Span<SecureField>) {
+        let mut res = array![self.digest];
+        loop {
+            match (felts.pop_front(), felts.pop_front()) {
+                (Option::None, _) => { break; },
+                (
+                    Option::Some(x), Option::None
+                ) => {
+                    let [x0, x1, x2, x3] = (*x).to_array();
+                    res
+                        .append(
+                            ((x0.into() * M31_SHIFT + x1.into()) * M31_SHIFT + x2.into())
+                                * M31_SHIFT
+                                + x3.into()
+                        );
+                    break;
+                },
+                (
+                    Option::Some(x), Option::Some(y)
+                ) => {
+                    let [x0, x1, x2, x3] = (*x).to_array();
+                    let [y0, y1, y2, y3] = (*y).to_array();
+                    res
+                        .append(
+                            ((x0.into() * M31_SHIFT + x1.into()) * M31_SHIFT + x2.into())
+                                * M31_SHIFT
+                                + x3.into()
+                        );
+                    res
+                        .append(
+                            ((y0.into() * M31_SHIFT + y1.into()) * M31_SHIFT + y2.into())
+                                * M31_SHIFT
+                                + y3.into()
+                        );
+                },
+            };
+        };
+
+        self.digest = poseidon_hash_span(res.span());
+
+        // TODO(spapini): do we need length padding?
+        self.channel_time.inc_challenges();
+    }
+
+    fn mix_nonce(ref self: Channel, nonce: u64) {
+        self.mix_digest(nonce.into())
+    }
+
+    fn draw_felt(ref self: Channel) -> SecureField {
+        let [a, b, c, d, _, _, _, _] = self.draw_base_felts();
+        QM31Trait::from_array([a, b, c, d])
+    }
+
+    fn draw_felts(ref self: Channel, mut n_felts: usize) -> Array<SecureField> {
+        let mut res: Array = Default::default();
+        loop {
+            if n_felts == 0 {
+                break;
+            }
+            let [r0, r1, r2, r3, r4, r5, r6, r7] = self.draw_base_felts();
+            res.append(QM31Trait::from_array([r0, r1, r2, r3]));
+            if n_felts == 1 {
+                break;
+            }
+            res.append(QM31Trait::from_array([r4, r5, r6, r7]));
+            n_felts -= 2;
+        };
+        res
+    }
+}
+
+#[inline]
+fn extract_m31<const N: usize>(ref num: u256) -> BaseField {
+    let (q, r) = DivRem::div_rem(num, M31_SHIFT_NZ_U256);
+    num = q;
+    let r: u32 = r.try_into().unwrap();
+    if r.into() == M31_SHIFT - 1 {
+        BaseField { inner: 0 }
+    } else {
+        BaseField { inner: r }
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::{Channel, ChannelTrait};
+    use stwo_cairo_verifier::fields::qm31::qm31;
+
+    #[test]
+    fn test_initialize_channel() {
+        let initial_digest = 0;
+        let channel = ChannelTrait::new(initial_digest);
+
+        // Assert that the channel is initialized correctly.
+        assert_eq!(channel.digest, initial_digest);
+        assert_eq!(channel.channel_time.n_challenges, 0);
+        assert_eq!(channel.channel_time.n_sent, 0);
+    }
+
+    #[test]
+    fn test_channel_time() {
+        let initial_digest = 0;
+        let mut channel = ChannelTrait::new(initial_digest);
+
+        assert_eq!(channel.channel_time.n_challenges, 0);
+        assert_eq!(channel.channel_time.n_sent, 0);
+
+        channel.draw_felt();
+        assert_eq!(channel.channel_time.n_challenges, 0);
+        assert_eq!(channel.channel_time.n_sent, 1);
+
+        channel.draw_felts(9);
+        assert_eq!(channel.channel_time.n_challenges, 0);
+        assert_eq!(channel.channel_time.n_sent, 6);
+
+        channel.mix_digest(0);
+        assert_eq!(channel.channel_time.n_challenges, 1);
+        assert_eq!(channel.channel_time.n_sent, 0);
+
+        channel.draw_felt();
+        assert_eq!(channel.channel_time.n_challenges, 1);
+        assert_eq!(channel.channel_time.n_sent, 1);
+        assert_ne!(channel.digest, initial_digest);
+    }
+
+
+    #[test]
+    pub fn test_draw_felt() {
+        let initial_digest = 0;
+        let mut channel = ChannelTrait::new(initial_digest);
+
+        let first_random_felt = channel.draw_felt();
+
+        // Assert that next random felt is different.
+        assert_ne!(first_random_felt, channel.draw_felt());
+    }
+
+    #[test]
+    pub fn test_draw_felts() {
+        let initial_digest = 0;
+        let mut channel = ChannelTrait::new(initial_digest);
+
+        let mut random_felts = channel.draw_felts(5);
+        random_felts.append_span(channel.draw_felts(4).span());
+
+        // Assert that all the random felts are unique.
+
+        assert_ne!(random_felts[0], random_felts[5]);
+    }
+
+    #[test]
+    pub fn test_mix_digest() {
+        let initial_digest = 0;
+        let mut channel = ChannelTrait::new(initial_digest);
+
+        let mut n: usize = 10;
+        while n > 0 {
+            n -= 1;
+            channel.draw_felt();
+        };
+
+        // Reseed channel and check the digest was changed.
+        channel.mix_digest(0);
+        assert_ne!(initial_digest, channel.digest);
+    }
+
+    #[test]
+    pub fn test_mix_felts() {
+        let initial_digest = 0;
+        let mut channel = ChannelTrait::new(initial_digest);
+
+        channel.mix_felts(array![qm31(1, 2, 3, 4), qm31(5, 6, 7, 8), qm31(9, 10, 11, 12)].span());
+
+        assert_ne!(initial_digest, channel.digest);
+    }
+}

stwo_cairo_verifier/src/fields.cairo

@@ -3,3 +3,4 @@ pub mod cm31;
 pub mod qm31;
 
 pub type BaseField = m31::M31;
+pub type SecureField = qm31::QM31;

stwo_cairo_verifier/src/fields/m31.cairo

@@ -90,6 +90,11 @@ pub impl M31Neg of Neg<M31> {
         }
     }
 }
+impl M31IntoFelt252 of Into<M31, felt252> {
+    fn into(self: M31) -> felt252 {
+        self.inner.into()
+    }
+}
 
 pub fn m31(val: u32) -> M31 {
     M31Impl::reduce_u32(val)

stwo_cairo_verifier/src/fields/qm31.cairo

@@ -12,7 +12,16 @@ pub struct QM31 {
 }
 
 #[generate_trait]
-impl QM31Impl of QM31Trait {
+pub impl QM31Impl of QM31Trait {
+    #[inline]
+    fn from_array(arr: [M31; 4]) -> QM31 {
+        let [a, b, c, d] = arr;
+        QM31 { a: CM31 { a: a, b: b }, b: CM31 { a: c, b: d } }
+    }
+    #[inline]
+    fn to_array(self: QM31) -> [M31; 4] {
+        [self.a.a, self.a.b, self.b.a, self.b.b]
+    }
     fn inverse(self: QM31) -> QM31 {
         assert_ne!(self, Zero::zero());
         let b2 = self.b * self.b;

stwo_cairo_verifier/src/lib.cairo

@@ -1,7 +1,8 @@
+mod channel;
 mod fields;
 mod utils;
 mod vcs;
 
-pub use fields::BaseField;
+pub use fields::{BaseField, SecureField};
 
 fn main() {}