fix: fields fromstring not working as intended (#7365)

Please read [contributing guidelines](CONTRIBUTING.md) and remove this
line.

noir-projects/noir-contracts/contracts/schnorr_hardcoded_account_contract/src/main.nr

@@ -9,8 +9,8 @@ contract SchnorrHardcodedAccount {
         auth_witness::get_auth_witness
     };
 
-    global public_key_x: Field = 0x0ede3d33c920df8fdf43f3e39ed38b0882c25b056620ef52fd016fe811aa2443;
-    global public_key_y: Field = 0x29155934ffaa105323695b5f91faadd84acc21f4a8bda2fad760f992d692bc7f;
+    global public_key_x: Field = 0x16b93f4afae55cab8507baeb8e7ab4de80f5ab1e9e1f5149bf8cd0d375451d90;
+    global public_key_y: Field = 0x208d44b36eb6e73b254921134d002da1a90b41131024e3b1d721259182106205;
 
     // Note: If you globally change the entrypoint signature don't forget to update default_entrypoint.ts
     #[aztec(private)]

yarn-project/foundation/src/fields/fields.test.ts

@@ -31,6 +31,18 @@ describe('GrumpkinScalar Serialization', () => {
 
     // Check if the deserialized instance is equal to the original
     expect(deserialized).toEqual(original);
+
+    // Note odd number of digits
+    const arbitraryString = '123';
+    const arbitraryHexString = '0x123';
+    const expectedBigInt = 291n;
+
+    expect(GrumpkinScalar.fromString(arbitraryString).toBigInt()).toEqual(expectedBigInt);
+    expect(GrumpkinScalar.fromString(arbitraryHexString).toBigInt()).toEqual(expectedBigInt);
+
+    const incorrectlyFormattedString = '12xx34xx45';
+
+    expect(() => GrumpkinScalar.fromString(incorrectlyFormattedString).toBigInt()).toThrow();
   });
 
   // Test case for GrumpkinScalar.toBuffer

yarn-project/foundation/src/fields/fields.ts

@@ -170,12 +170,13 @@ function random<T extends BaseField>(f: DerivedField<T>): T {
  */
 function fromHexString<T extends BaseField>(buf: string, f: DerivedField<T>) {
   const withoutPrefix = buf.replace(/^0x/i, '');
-  const buffer = Buffer.from(withoutPrefix, 'hex');
-
-  if (buffer.length === 0 && withoutPrefix.length > 0) {
+  const checked = withoutPrefix.match(/^[0-9A-F]+$/i)?.[0];
+  if (checked === undefined) {
     throw new Error(`Invalid hex-encoded string: "${buf}"`);
   }
 
+  const buffer = Buffer.from(checked.length % 2 === 1 ? '0' + checked : checked, 'hex');
+
   return new f(buffer);
 }
 
@@ -227,6 +228,11 @@ export class Fr extends BaseField {
     return fromBufferReduce(buffer, Fr);
   }
 
+  /**
+   * Creates a Fr instance from a hex string.
+   * @param buf - a hex encoded string.
+   * @returns the Fr instance
+   */
   static fromString(buf: string) {
     return fromHexString(buf, Fr);
   }
@@ -336,6 +342,11 @@ export class Fq extends BaseField {
     return fromBufferReduce(buffer, Fq);
   }
 
+  /**
+   * Creates a Fq instance from a hex string.
+   * @param buf - a hex encoded string.
+   * @returns the Fq instance
+   */
   static fromString(buf: string) {
     return fromHexString(buf, Fq);
   }