Added XYZZ coordinates and let z=0 represent infinity

crates/crypto/examples/consts_pedersen.rs

@@ -33,13 +33,13 @@ fn generate_consts(bits: u32) -> Result<String, std::fmt::Error> {
     write!(buf, "use crate::algebra::curve::AffinePoint;\n\n")?;
     write!(buf, "pub const CURVE_CONSTS_BITS: usize = {bits};\n\n")?;
 
-    push_points(&mut buf, "P1", &PEDERSEN_P1, 248, bits)?;
+    push_points(&mut buf, "P1", PEDERSEN_P1, 248, bits)?;
     buf.push_str("\n\n\n");
-    push_points(&mut buf, "P2", &PEDERSEN_P2, 4, bits)?;
+    push_points(&mut buf, "P2", PEDERSEN_P2, 4, bits)?;
     buf.push_str("\n\n\n");
-    push_points(&mut buf, "P3", &PEDERSEN_P3, 248, bits)?;
+    push_points(&mut buf, "P3", PEDERSEN_P3, 248, bits)?;
     buf.push_str("\n\n\n");
-    push_points(&mut buf, "P4", &PEDERSEN_P4, 4, bits)?;
+    push_points(&mut buf, "P4", PEDERSEN_P4, 4, bits)?;
 
     Ok(buf)
 }
@@ -48,12 +48,10 @@ fn generate_consts(bits: u32) -> Result<String, std::fmt::Error> {
 fn push_points(
     buf: &mut String,
     name: &str,
-    base: &ProjectivePoint,
+    base: AffinePoint,
     max_bits: u32,
     bits: u32,
 ) -> std::fmt::Result {
-    let base = AffinePoint::from(base);
-
     let full_chunks = max_bits / bits;
     let leftover_bits = max_bits % bits;
     let table_size_full = (1 << bits) - 1;

crates/crypto/src/algebra/curve/affine.rs

@@ -25,6 +25,20 @@ impl From<&ProjectivePoint> for AffinePoint {
     }
 }
 
+impl From<&XYZZPoint> for AffinePoint {
+    fn from(p: &XYZZPoint) -> Self {
+        let a = p.zzz.inverse().unwrap();
+        let b = (p.zz * a).square();
+        let x = p.x * b;
+        let y = p.y * a;
+        AffinePoint {
+            x,
+            y,
+            infinity: false,
+        }
+    }
+}
+
 impl AffinePoint {
     /// Create a point from (x,y) as raw u64's in Montgomery representation
     pub const fn from_raw(x: [u64; 4], y: [u64; 4]) -> Self {

crates/crypto/src/algebra/curve/mod.rs

@@ -4,9 +4,12 @@ mod gen_mul;
 mod params;
 mod projective;
 
+mod xyzz;
+
 pub use affine::AffinePoint;
 pub use params::{CURVE_A, CURVE_B, CURVE_G, CURVE_ORDER};
 pub use projective::ProjectivePoint;
+pub use xyzz::XYZZPoint;
 
 #[cfg(test)]
 mod tests;

crates/crypto/src/algebra/curve/projective.rs

@@ -4,26 +4,22 @@ use bitvec::slice::BitSlice;
 use crate::algebra::curve::*;
 use crate::algebra::field::*;
 
-/// A projective point on an elliptic curve over [MontFelt].
+/// A projective point on an elliptic curve over [MontFelt] satisfying:
+///   x = X / Z
+///   y = Y / Z
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct ProjectivePoint {
     pub x: MontFelt,
     pub y: MontFelt,
     pub z: MontFelt,
-    pub infinity: bool,
 }
 
 impl From<&AffinePoint> for ProjectivePoint {
     fn from(p: &AffinePoint) -> Self {
         let x = p.x;
         let y = p.y;
         let z = MontFelt::ONE;
-        ProjectivePoint {
-            x,
-            y,
-            z,
-            infinity: false,
-        }
+        ProjectivePoint { x, y, z }
     }
 }
 
@@ -36,7 +32,6 @@ impl ProjectivePoint {
             x,
             y,
             z: MontFelt::ONE,
-            infinity: false,
         }
     }
 
@@ -48,7 +43,6 @@ impl ProjectivePoint {
             x,
             y,
             z: MontFelt::ONE,
-            infinity: false,
         }
     }
 
@@ -63,8 +57,7 @@ impl ProjectivePoint {
         Self {
             x: MontFelt::ZERO,
             y: MontFelt::ZERO,
-            z: MontFelt::ONE,
-            infinity: true,
+            z: MontFelt::ZERO,
         }
     }
 
@@ -73,9 +66,14 @@ impl ProjectivePoint {
         self.y = -self.y;
     }
 
+    /// Check if the point is the point of infinity
+    pub fn is_infinity(&self) -> bool {
+        self.z.is_zero()
+    }
+
     /// Double a point
     pub fn double(&mut self) {
-        if self.infinity {
+        if self.is_infinity() {
             return;
         }
 
@@ -99,14 +97,13 @@ impl ProjectivePoint {
 
     /// Add a point to this point
     pub fn add(&mut self, other: &ProjectivePoint) {
-        if other.infinity {
+        if other.is_infinity() {
             return;
         }
-        if self.infinity {
+        if self.is_infinity() {
             self.x = other.x;
             self.y = other.y;
             self.z = other.z;
-            self.infinity = other.infinity;
             return;
         }
         let u0 = self.x * other.z;
@@ -115,7 +112,7 @@ impl ProjectivePoint {
         let t1 = other.y * self.z;
         if u0 == u1 {
             if t0 != t1 {
-                self.infinity = true;
+                self.z = MontFelt::ZERO;
             } else {
                 self.double();
             }
@@ -144,11 +141,15 @@ impl ProjectivePoint {
         if other.infinity {
             return;
         }
-        if self.infinity {
+        if self.is_infinity() {
             self.x = other.x;
             self.y = other.y;
-            self.z = MontFelt::ONE;
-            self.infinity = other.infinity;
+            self.z = if other.infinity {
+                MontFelt::ZERO
+            } else {
+                MontFelt::ONE
+            };
+
             return;
         }
         let u0 = self.x;
@@ -157,7 +158,7 @@ impl ProjectivePoint {
         let t1 = other.y * self.z;
         if u0 == u1 {
             if t0 != t1 {
-                self.infinity = true;
+                self.z = MontFelt::ZERO;
                 return;
             } else {
                 self.double();

crates/crypto/src/algebra/curve/xyzz.rs

@@ -0,0 +1,72 @@
+use bitvec::order::Lsb0;
+use bitvec::slice::BitSlice;
+
+use crate::algebra::curve::*;
+use crate::algebra::field::*;
+
+/// A XYZZ point on an elliptic curve over [MontFelt] satisfying:
+///   x = X / ZZ
+///   y = Y / ZZ
+///   ZZ^3 = ZZZ^2
+///
+/// This point representation is used for fast table-based scalar multiplication
+/// and only include add_affine and add_affine_unchecked operations.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct XYZZPoint {
+    pub x: MontFelt,
+    pub y: MontFelt,
+    pub zz: MontFelt,
+    pub zzz: MontFelt,
+}
+
+impl From<&AffinePoint> for XYZZPoint {
+    fn from(p: &AffinePoint) -> Self {
+        let x = p.x;
+        let y = p.y;
+        let zz = MontFelt::ONE;
+        let zzz = MontFelt::ONE;
+        XYZZPoint { x, y, zz, zzz }
+    }
+}
+
+impl XYZZPoint {
+    /// Check if the point is the point of infinity
+    pub fn is_infinity(&self) -> bool {
+        self.zz.is_zero()
+    }
+
+    /// Add an affine point to this point
+    pub fn add_affine(&mut self, other: &AffinePoint) {
+        if other.infinity {
+            return;
+        }
+        if self.is_infinity() {
+            self.x = other.x;
+            self.y = other.y;
+            let z = if other.infinity {
+                MontFelt::ZERO
+            } else {
+                MontFelt::ONE
+            };
+            self.zz = z;
+            self.zzz = z;
+
+            return;
+        }
+        self.add_affine_unchecked(other);
+    }
+
+    /// Add an affine point to this point, neither must be the point of infinity
+    pub fn add_affine_unchecked(&mut self, other: &AffinePoint) {
+        // See https://www.hyperelliptic.org/EFD/g1p/auto-shortw-xyzz.html#addition-madd-2008-s
+        let p = other.x * self.zz - self.x;
+        let r = other.y * self.zzz - self.y;
+        let pp = p.square();
+        let ppp = p * pp;
+        let q = self.x * pp;
+        self.x = r.square() - ppp - q.double();
+        self.y = r * (q - self.x) - self.y * ppp;
+        self.zz *= pp;
+        self.zzz *= ppp;
+    }
+}

crates/crypto/src/hash/pedersen/gens.rs

@@ -1,34 +1,34 @@
 //! Generators for the Pedersen hash function.
 //!
 //! See <https://docs.starkware.co/starkex/crypto/pedersen-hash-function.html>
-use crate::algebra::curve::ProjectivePoint;
+use crate::algebra::curve::AffinePoint;
 
 /// Montgomery representation of the Stark curve constant P0.
-pub const PEDERSEN_P0: ProjectivePoint = ProjectivePoint::from_hex(
+pub const PEDERSEN_P0: AffinePoint = AffinePoint::from_hex(
     "49EE3EBA8C1600700EE1B87EB599F16716B0B1022947733551FDE4050CA6804",
     "3CA0CFE4B3BC6DDF346D49D06EA0ED34E621062C0E056C1D0405D266E10268A",
 );
 
 /// Montgomery representation of the Stark curve constant P1.
-pub const PEDERSEN_P1: ProjectivePoint = ProjectivePoint::from_hex(
+pub const PEDERSEN_P1: AffinePoint = AffinePoint::from_hex(
     "234287DCBAFFE7F969C748655FCA9E58FA8120B6D56EB0C1080D17957EBE47B",
     "3B056F100F96FB21E889527D41F4E39940135DD7A6C94CC6ED0268EE89E5615",
 );
 
 /// Montgomery representation of the Stark curve constant P2.
-pub const PEDERSEN_P2: ProjectivePoint = ProjectivePoint::from_hex(
+pub const PEDERSEN_P2: AffinePoint = AffinePoint::from_hex(
     "4FA56F376C83DB33F9DAB2656558F3399099EC1DE5E3018B7A6932DBA8AA378",
     "3FA0984C931C9E38113E0C0E47E4401562761F92A7A23B45168F4E80FF5B54D",
 );
 
 /// Montgomery representation of the Stark curve constant P3.
-pub const PEDERSEN_P3: ProjectivePoint = ProjectivePoint::from_hex(
+pub const PEDERSEN_P3: AffinePoint = AffinePoint::from_hex(
     "4BA4CC166BE8DEC764910F75B45F74B40C690C74709E90F3AA372F0BD2D6997",
     "40301CF5C1751F4B971E46C4EDE85FCAC5C59A5CE5AE7C48151F27B24B219C",
 );
 
 /// Montgomery representation of the Stark curve constant P4.
-pub const PEDERSEN_P4: ProjectivePoint = ProjectivePoint::from_hex(
+pub const PEDERSEN_P4: AffinePoint = AffinePoint::from_hex(
     "54302DCB0E6CC1C6E44CCA8F61A63BB2CA65048D53FB325D36FF12C49A58202",
     "1B77B3E37D13504B348046268D8AE25CE98AD783C25561A879DCC77E99C2426",
 );

crates/crypto/src/hash/pedersen/hash.rs

@@ -19,20 +19,22 @@ pub fn pedersen_hash(a: Felt, b: Felt) -> Felt {
 
     // Preprocessed material is lookup-tables for each chunk of bits
     let table_size = (1 << CURVE_CONSTS_BITS) - 1;
-    let add_points = |acc: &mut ProjectivePoint, bits: &BitSlice<u8, _>, prep: &[AffinePoint]| {
+    let add_points = |acc: &mut XYZZPoint, bits: &BitSlice<u8, _>, prep: &[AffinePoint]| {
         bits.chunks(CURVE_CONSTS_BITS)
             .enumerate()
             .for_each(|(i, v)| {
                 let offset: usize = v.load_le();
                 if offset > 0 {
                     // Table lookup at 'offset-1' in table for chunk 'i'
-                    acc.add_affine(&prep[i * table_size + offset - 1]);
+                    // We can add unchecked, since the accumulator and prep. point cannot be infinity.
+                    acc.add_affine_unchecked(&prep[i * table_size + offset - 1]);
                 }
             });
     };
 
     // Compute hash
-    let mut acc = PEDERSEN_P0;
+    let mut acc = XYZZPoint::from(&PEDERSEN_P0);
+
     add_points(&mut acc, &a_bits[..248], &CURVE_CONSTS_P1); // Add a_low * P1
     add_points(&mut acc, &a_bits[248..252], &CURVE_CONSTS_P2); // Add a_high * P2
     add_points(&mut acc, &b_bits[..248], &CURVE_CONSTS_P3); // Add b_low * P3