Hashed provable type

CHANGELOG.md

@@ -15,12 +15,20 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
     _Security_ in case of vulnerabilities.
  -->
 
-## [Unreleased](https://github.com/o1-labs/o1js/compare/be748e42e...HEAD)
+## [Unreleased](https://github.com/o1-labs/o1js/compare/e5d1e0f...HEAD)
 
 ### Breaking changes
 
 - Reduce number of constraints of ECDSA verification by 5%, which breaks deployed contracts using ECDSA https://github.com/o1-labs/o1js/pull/1376
 
+### Added
+
+- Provable type `Packed<T>` to pack small field elements into fewer field elements https://github.com/o1-labs/o1js/pull/1376
+- Provable type `Hashed<T>` to represent provable types by their hash https://github.com/o1-labs/o1js/pull/1377
+  - This also exposes `Poseidon.hashPacked()` to efficiently hash an arbitrary type
+
+## [0.15.4](https://github.com/o1-labs/o1js/compare/be748e42e...e5d1e0f)
+
 ### Changed
 
 - Improve performance of Wasm Poseidon hashing by a factor of 13x https://github.com/o1-labs/o1js/pull/1378
@@ -33,7 +41,6 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Configurable `networkId` when declaring a Mina instance. https://github.com/o1-labs/o1js/pull/1387
   - Defaults to `"testnet"`, the other option is `"mainnet"`
   - The `networkId` parameter influences the algorithm used for signatures, and ensures that testnet transactions can't be replayed on mainnet
-- Provable type `Packed<T>` to pack small field elements into fewer field elements https://github.com/o1-labs/o1js/pull/1376
 
 ## [0.15.3](https://github.com/o1-labs/o1js/compare/1ad7333e9e...be748e42e)
 

src/examples/crypto/ecdsa/ecdsa.ts

@@ -4,7 +4,6 @@ import {
   createEcdsa,
   createForeignCurve,
   Bool,
-  Keccak,
   Bytes,
 } from 'o1js';
 

src/index.ts

@@ -36,7 +36,7 @@ export { Provable } from './lib/provable.js';
 export { Circuit, Keypair, public_, circuitMain } from './lib/circuit.js';
 export { UInt32, UInt64, Int64, Sign, UInt8 } from './lib/int.js';
 export { Bytes } from './lib/provable-types/provable-types.js';
-export { Packed } from './lib/provable-types/packed.js';
+export { Packed, Hashed } from './lib/provable-types/packed.js';
 export { Gadgets } from './lib/gadgets/gadgets.js';
 export { Types } from './bindings/mina-transaction/types.js';
 

src/lib/gadgets/ecdsa.unit-test.ts

@@ -129,7 +129,7 @@ let msgHash =
   );
 
 const ia = initialAggregator(Secp256k1);
-const config = { G: { windowSize: 4 }, P: { windowSize: 3 }, ia };
+const config = { G: { windowSize: 4 }, P: { windowSize: 4 }, ia };
 
 let program = ZkProgram({
   name: 'ecdsa',

src/lib/gadgets/elliptic-curve.ts

@@ -19,7 +19,7 @@ import { provable } from '../circuit_value.js';
 import { assertPositiveInteger } from '../../bindings/crypto/non-negative.js';
 import { arrayGet, assertBoolean } from './basic.js';
 import { sliceField3 } from './bit-slices.js';
-import { Packed } from '../provable-types/packed.js';
+import { Hashed } from '../provable-types/packed.js';
 
 // external API
 export { EllipticCurve, Point, Ecdsa };
@@ -228,7 +228,7 @@ function verifyEcdsa(
     G?: { windowSize: number; multiples?: Point[] };
     P?: { windowSize: number; multiples?: Point[] };
     ia?: point;
-  } = { G: { windowSize: 4 }, P: { windowSize: 3 } }
+  } = { G: { windowSize: 4 }, P: { windowSize: 4 } }
 ) {
   // constant case
   if (
@@ -414,12 +414,12 @@ function multiScalarMul(
     sliceField3(s, { maxBits, chunkSize: windowSizes[i] })
   );
 
-  // pack points to make array access more efficient
-  // a Point is 6 x 88-bit field elements, which are packed into 3 field elements
-  const PackedPoint = Packed.create(Point.provable);
+  // hash points to make array access more efficient
+  // a Point is 6 field elements, the hash is just 1 field element
+  const HashedPoint = Hashed.create(Point.provable);
 
-  let packedTables = tables.map((table) =>
-    table.map((point) => PackedPoint.pack(point))
+  let hashedTables = tables.map((table) =>
+    table.map((point) => HashedPoint.hash(point))
   );
 
   ia ??= initialAggregator(Curve);
@@ -436,10 +436,10 @@ function multiScalarMul(
           windowSize === 1
             ? points[j]
             : arrayGetGeneric(
-                PackedPoint.provable,
-                packedTables[j],
+                HashedPoint.provable,
+                hashedTables[j],
                 sj
-              ).unpack();
+              ).unhash();
 
         // ec addition
         let added = add(sum, sjP, Curve);

src/lib/hash.ts

@@ -14,6 +14,7 @@ export { Poseidon, TokenSymbol };
 
 // internal API
 export {
+  ProvableHashable,
   HashInput,
   HashHelpers,
   emptyHashWithPrefix,
@@ -24,6 +25,9 @@ export {
   hashConstant,
 };
 
+type Hashable<T> = { toInput: (x: T) => HashInput; empty: () => T };
+type ProvableHashable<T> = Provable<T> & Hashable<T>;
+
 class Sponge {
   #sponge: unknown;
 
@@ -96,6 +100,24 @@ const Poseidon = {
     return { x, y: { x0, x1 } };
   },
 
+  /**
+   * Hashes a provable type efficiently.
+   *
+   * ```ts
+   * let skHash = Poseidon.hashPacked(PrivateKey, secretKey);
+   * ```
+   *
+   * Note: Instead of just doing `Poseidon.hash(value.toFields())`, this
+   * uses the `toInput()` method on the provable type to pack the input into as few
+   * field elements as possible. This saves constraints because packing has a much
+   * lower per-field element cost than hashing.
+   */
+  hashPacked<T>(type: Hashable<T>, value: T) {
+    let input = type.toInput(value);
+    let packed = packToFields(input);
+    return Poseidon.hash(packed);
+  },
+
   initialState(): [Field, Field, Field] {
     return [Field(0), Field(0), Field(0)];
   },

src/lib/provable-types/packed.ts

@@ -6,11 +6,11 @@ import {
 } from '../circuit_value.js';
 import { Field } from '../field.js';
 import { assert } from '../gadgets/common.js';
-import { Poseidon, packToFields } from '../hash.js';
+import { Poseidon, ProvableHashable, packToFields } from '../hash.js';
 import { Provable } from '../provable.js';
-import { fields } from './fields.js';
+import { fields, modifiedField } from './fields.js';
 
-export { Packed };
+export { Packed, Hashed };
 
 /**
  * Packed<T> is a "packed" representation of any type T.
@@ -63,6 +63,10 @@ class Packed<T> {
         packed: fields(packedSize),
         value: Unconstrained.provable,
       }) as ProvableHashable<Packed<T>>;
+
+      static empty(): Packed<T> {
+        return Packed_.pack(type.empty());
+      }
     };
   }
 
@@ -120,8 +124,6 @@ class Packed<T> {
   }
 }
 
-type ProvableHashable<T> = Provable<T> & { toInput: (x: T) => HashInput };
-
 function countFields(input: HashInput) {
   let n = input.fields?.length ?? 0;
   let pendingBits = 0;
@@ -137,3 +139,114 @@ function countFields(input: HashInput) {
 
   return n;
 }
+
+/**
+ * Hashed<T> represents a type T by its hash.
+ *
+ * Since a hash is only a single field element, this can be more efficient in provable code
+ * where the number of constraints depends on the number of field elements per value.
+ *
+ * For example, `Provable.if(bool, x, y)` takes O(n) constraints, where n is the number of field
+ * elements in x and y. With Hashed, this is reduced to O(1).
+ *
+ * The downside is that you will pay the overhead of hashing your values, so it helps to experiment
+ * in which parts of your code a hashed representation is beneficial.
+ *
+ * Usage:
+ *
+ * ```ts
+ * // define a hashed type from a type
+ * let HashedType = Hashed.create(MyType);
+ *
+ * // hash a value
+ * let hashed = HashedType.hash(value);
+ *
+ * // ... operations on hashes, more efficient than on plain values ...
+ *
+ * // unhash to get the original value
+ * let value = hashed.unhash();
+ * ```
+ */
+class Hashed<T> {
+  hash: Field;
+  value: Unconstrained<T>;
+
+  /**
+   * Create a hashed representation of `type`. You can then use `HashedType.hash(x)` to wrap a value in a `Hashed`.
+   */
+  static create<T>(
+    type: ProvableHashable<T>,
+    hash?: (t: T) => Field
+  ): typeof Hashed<T> {
+    let _hash = hash ?? ((t: T) => Poseidon.hashPacked(type, t));
+
+    let dummyHash = _hash(type.empty());
+
+    return class Hashed_ extends Hashed<T> {
+      static _innerProvable = type;
+      static _provable = provableFromClass(Hashed_, {
+        hash: modifiedField({ empty: () => dummyHash }),
+        value: Unconstrained.provable,
+      }) as ProvableHashable<Hashed<T>>;
+
+      static _hash = _hash satisfies (t: T) => Field;
+
+      static empty(): Hashed<T> {
+        return new this(dummyHash, Unconstrained.from(type.empty()));
+      }
+    };
+  }
+
+  constructor(hash: Field, value: Unconstrained<T>) {
+    this.hash = hash;
+    this.value = value;
+  }
+
+  static _hash(_: any): Field {
+    assert(false, 'Hashed not initialized');
+  }
+
+  /**
+   * Wrap a value, and represent it by its hash in provable code.
+   */
+  static hash<T>(value: T): Hashed<T> {
+    let hash = this._hash(value);
+    return new this(hash, Unconstrained.from(value));
+  }
+
+  /**
+   * Unwrap a value from its hashed variant.
+   */
+  unhash(): T {
+    let value = Provable.witness(this.Constructor.innerProvable, () =>
+      this.value.get()
+    );
+
+    // prove that the value hashes to the hash
+    let hash = this.Constructor._hash(value);
+    this.hash.assertEquals(hash);
+
+    return value;
+  }
+
+  toFields(): Field[] {
+    return [this.hash];
+  }
+
+  // dynamic subclassing infra
+  static _provable: ProvableHashable<Hashed<any>> | undefined;
+  static _innerProvable: ProvableHashable<any> | undefined;
+
+  get Constructor(): typeof Hashed {
+    return this.constructor as typeof Hashed;
+  }
+
+  static get provable(): ProvableHashable<Hashed<any>> {
+    assert(this._provable !== undefined, 'Hashed not initialized');
+    return this._provable;
+  }
+  static get innerProvable(): ProvableHashable<any> {
+    assert(this._innerProvable !== undefined, 'Hashed not initialized');
+    return this._innerProvable;
+  }
+}