validate key using the serialize function

common/src/keypair.ts

@@ -1,37 +1,34 @@
 import { keccak256, isBytesLike, concat, toBeArray } from 'ethers'
 import { Keypair as MaciKeypair, PrivKey, PubKey } from 'maci-domainobjs'
 
-const SNARK_FIELD_SIZE = BigInt(
-  '21888242871839275222246405745257275088548364400416034343698204186575808495617'
-)
-
 /**
- * Returns a BabyJub-compatible value. This function is modified from
- * the MACI's genRandomBabyJubValue(). Instead of returning random value
- * for the private key, it derives the private key from the users
- * signature hash
+ * Derives the MACI private key from the users signature hash
  * @param hash - user's signature hash
+ * @return The MACI private key
  */
 function genPrivKey(hash: string): PrivKey {
-  // Prevent modulo bias
-  //const lim = BigInt('0x10000000000000000000000000000000000000000000000000000000000000000')
-  //const min = (lim - SNARK_FIELD_SIZE) % SNARK_FIELD_SIZE
-  const min = BigInt(
-    '6350874878119819312338956282401532410528162663560392320966563075034087161851'
-  )
-
   if (!isBytesLike(hash)) {
-    throw new Error(`Hash must be a hex string: ${hash}`)
+    throw new Error(`genPrivKey() error. Hash must be a hex string: ${hash}`)
   }
 
-  let hashBN = BigInt(hash)
-  // don't think we'll enter the for loop below, but, just in case
-  for (let counter = 1; hashBN < min; counter++) {
-    const data = concat([toBeArray(hashBN), toBeArray(counter)])
-    hashBN = BigInt(keccak256(data))
+  let rawPrivKey = BigInt(hash)
+  let pubKey: PubKey | null = null
+
+  for (let counter = 1; pubKey === null; counter++) {
+    try {
+      const privKey = new PrivKey(rawPrivKey)
+      const keypair = new Keypair(privKey)
+
+      // this will throw 'Invalid public key' if key is not on the Baby Jubjub elliptic curve
+      keypair.pubKey.serialize()
+
+      pubKey = keypair.pubKey
+    } catch {
+      const data = concat([toBeArray(rawPrivKey), toBeArray(counter)])
+      rawPrivKey = BigInt(keccak256(data))
+    }
   }
 
-  const rawPrivKey = hashBN % SNARK_FIELD_SIZE
   return new PrivKey(rawPrivKey)
 }