Efficient scalar mul and other Scalar improvements

src/lib/provable/gadgets/common.ts

@@ -4,9 +4,19 @@ import { Tuple } from '../../util/types.js';
 import type { Bool } from '../bool.js';
 import { fieldVar } from '../gates.js';
 import { existsOne } from '../core/exists.js';
-import { createField } from '../core/field-constructor.js';
+import { createField, isBool } from '../core/field-constructor.js';
 
-export { toVars, toVar, isVar, assert, bitSlice, divideWithRemainder };
+export {
+  toVars,
+  toVar,
+  isVar,
+  assert,
+  bitSlice,
+  bit,
+  divideWithRemainder,
+  packBits,
+  isConstant,
+};
 
 /**
  * Given a Field, collapse its AST to a pure Var. See {@link FieldVar}.
@@ -56,8 +66,33 @@ function bitSlice(x: bigint, start: number, length: number) {
   return (x >> BigInt(start)) & ((1n << BigInt(length)) - 1n);
 }
 
+function bit(x: bigint, i: number) {
+  return (x >> BigInt(i)) & 1n;
+}
+
 function divideWithRemainder(numerator: bigint, denominator: bigint) {
   const quotient = numerator / denominator;
   const remainder = numerator - denominator * quotient;
   return { quotient, remainder };
 }
+
+// pack bools into a single field element
+
+/**
+ * Helper function to pack bits into a single field element.
+ * Just returns the sum without any boolean checks.
+ */
+function packBits(bits: (Field | Bool)[]): Field {
+  let n = bits.length;
+  let sum = createField(0n);
+  for (let i = 0; i < n; i++) {
+    let bit = bits[i];
+    if (isBool(bit)) bit = bit.toField();
+    sum = sum.add(bit.mul(1n << BigInt(i)));
+  }
+  return sum.seal();
+}
+
+function isConstant(...args: (Field | Bool)[]): boolean {
+  return args.every((x) => x.isConstant());
+}

src/lib/provable/gadgets/comparison.ts

@@ -9,21 +9,27 @@ import { Field3, ForeignField } from './foreign-field.js';
 import { l, l2, multiRangeCheck } from './range-check.js';
 import { witness } from '../types/witness.js';
 
+// external API
 export {
   // generic comparison gadgets for inputs in a narrower range < p/2
   assertLessThanGeneric,
   assertLessThanOrEqualGeneric,
   lessThanGeneric,
   lessThanOrEqualGeneric,
+
   // comparison gadgets for full range inputs
   assertLessThanFull,
   assertLessThanOrEqualFull,
   lessThanFull,
   lessThanOrEqualFull,
+
   // legacy, unused
   compareCompatible,
 };
 
+// internal API
+export { fieldToField3 };
+
 /**
  * Prove x <= y assuming 0 <= x, y < c.
  * The upper bound c must satisfy 2c <= p, where p is the field order.

src/lib/provable/gadgets/scalar.ts

@@ -0,0 +1,114 @@
+import type { Field } from '../field.js';
+import type { Bool } from '../bool.js';
+import { Fq } from '../../../bindings/crypto/finite-field.js';
+import { PallasAffine } from '../../../bindings/crypto/elliptic-curve.js';
+import { fieldToField3 } from './comparison.js';
+import { Field3, ForeignField } from './foreign-field.js';
+import { exists, existsOne } from '../core/exists.js';
+import { bit, isConstant, packBits } from './common.js';
+import { TupleN } from '../../util/types.js';
+import { l } from './range-check.js';
+import { createField } from '../core/field-constructor.js';
+import { Snarky } from '../../../snarky.js';
+import { Provable } from '../provable.js';
+import { Group } from '../group.js';
+
+export { scale, scaleShiftedSplit5 };
+
+/**
+ * Gadget to scale a point by a scalar, where the scalar is represented as a _native_ Field.
+ */
+function scale(P: { x: Field; y: Field }, s: Field): Group {
+  // constant case
+  let { x, y } = P;
+  if (x.isConstant() && y.isConstant() && s.isConstant()) {
+    let sP = PallasAffine.scale(
+      PallasAffine.fromNonzero({ x: x.toBigInt(), y: y.toBigInt() }),
+      s.toBigInt()
+    );
+    return new Group({ x: createField(sP.x), y: createField(sP.y) });
+  }
+
+  // compute t = s - 2^254 mod q using foreign field subtraction
+  let sBig = fieldToField3(s);
+  let twoTo254 = Field3.from(1n << 254n);
+  let [t0, t1, t2] = ForeignField.sub(sBig, twoTo254, Fq.modulus);
+
+  // split t into 250 high bits and 5 low bits
+  // => split t0 into [5, 83]
+  let tLo = exists(5, () => {
+    let t = t0.toBigInt();
+    return [bit(t, 0), bit(t, 1), bit(t, 2), bit(t, 3), bit(t, 4)];
+  });
+  let tLoBools = TupleN.map(tLo, (x) => x.assertBool());
+  let tHi0 = existsOne(() => t0.toBigInt() >> 5n);
+
+  // prove split
+  // since we know that t0 < 2^88, this proves that t0High < 2^83
+  packBits(tLo)
+    .add(tHi0.mul(1n << 5n))
+    .assertEquals(t0);
+
+  // pack tHi
+  let tHi = tHi0
+    .add(t1.mul(1n << (l - 5n)))
+    .add(t2.mul(1n << (2n * l - 5n)))
+    .seal();
+
+  // return (t + 2^254)*P = (s - 2^254 + 2^254)*P = s*P
+  return scaleShiftedSplit5(P, tHi, tLoBools);
+}
+
+/**
+ * Internal helper to compute `(t + 2^254)*P`.
+ * `t` is expected to be split into 250 high bits (t >> 5) and 5 low bits (t & 0xf1).
+ *
+ * The gadget proves that `tHi` is in [0, 2^250) but assumes that `tLo` consists of bits.
+ */
+function scaleShiftedSplit5(
+  { x, y }: { x: Field; y: Field },
+  tHi: Field,
+  tLo: TupleN<Bool, 5>
+): Group {
+  // constant case
+  if (isConstant(x, y, tHi, ...tLo)) {
+    let sP = PallasAffine.scale(
+      PallasAffine.fromNonzero({ x: x.toBigInt(), y: y.toBigInt() }),
+      Fq.add(packBits(tLo).toBigInt() + (tHi.toBigInt() << 5n), 1n << 254n)
+    );
+    return new Group({ x: createField(sP.x), y: createField(sP.y) });
+  }
+
+  // R = (2*(t >> 5) + 1 + 2^250)P
+  let [, RMl] = Snarky.group.scaleFastUnpack(
+    [0, x.value, y.value],
+    [0, tHi.value],
+    250
+  );
+  let P = new Group({ x, y });
+  let R = new Group({ x: RMl[0], y: RMl[1] });
+  let [t0, t1, t2, t3, t4] = tLo;
+
+  // TODO: use faster group ops which don't allow zero inputs
+
+  // R = t4 ? R : R - P = ((t >> 4) + 2^250)P
+  R = Provable.if(t4, R, R.sub(P));
+
+  // R = ((t >> 3) + 2^251)P
+  R = R.add(R);
+  R = Provable.if(t3, R.add(P), R);
+
+  // R = ((t >> 2) + 2^252)P
+  R = R.add(R);
+  R = Provable.if(t2, R.add(P), R);
+
+  // R = ((t >> 1) + 2^253)P
+  R = R.add(R);
+  R = Provable.if(t1, R.add(P), R);
+
+  // R = (t + 2^254)P
+  R = R.add(R);
+  R = Provable.if(t0, R.add(P), R);
+
+  return R;
+}