Generate meta transaction nonces randomly

Instead of querying the TLProvider for the next nonce to use for an
identity, use the alternative approach of a random nonce. According to
the contract implementation such nonce must be generated within the
range of ]2^255, 2^256[.

Related to: #361

src/typings.ts

@@ -294,7 +294,7 @@ export interface RawTxObject {
   to?: string
   value?: number | string | BigNumber
   data?: string
-  nonce?: number
+  nonce?: number | string
   baseFee?: number | string | BigNumber
   gasLimit?: number | string | BigNumber
   gasPrice?: number | string | BigNumber

src/wallets/IdentityWallet.ts

@@ -32,6 +32,10 @@ import { AddressZero } from 'ethers/constants'
 const initcodeWithPadding =
   '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'
 
+const MIN_RANDOM_NONCE = new BigNumber(2).pow(255).plus(1)
+const MAX_RANDOM_NONCE = new BigNumber(2).pow(256)
+const RANDOM_NONCE_RANGE = MAX_RANDOM_NONCE.minus(MIN_RANDOM_NONCE)
+
 export class IdentityWallet implements TLWallet {
   public provider: TLProvider
 
@@ -333,7 +337,7 @@ export class IdentityWallet implements TLWallet {
   }
 
   public async prepareTransaction(rawTx: RawTxObject): Promise<TxObjectRaw> {
-    rawTx.nonce = await this.provider.getIdentityNonce(this.address)
+    rawTx.nonce = getRandomNonce()
 
     const metaTxFees = await this.getMetaTxFees(rawTx)
 
@@ -480,3 +484,20 @@ export function calculateIdentityAddress(
       .slice(2 + 2 * 12)
   return toChecksumAddress(address)
 }
+
+/**
+ * Generates a random nonce to use for meta transactions.
+ * The nonce fits into the range of ]2^255, 2^256[.
+ * This is an alternative to the up counting nonce (]0, 2^255[) without the need
+ * to query a [[TLProvider]].
+ */
+export function getRandomNonce(): string {
+  const exponentialMagnitute = MAX_RANDOM_NONCE.e + 1
+  const BigNumberForRandomNonces = BigNumber.clone({
+    EXPONENTIAL_AT: exponentialMagnitute,
+    ROUNDING_MODE: BigNumber.ROUND_DOWN
+  })
+  const random = BigNumberForRandomNonces.random(exponentialMagnitute)
+  const nonce = random.multipliedBy(RANDOM_NONCE_RANGE).plus(MIN_RANDOM_NONCE)
+  return nonce.integerValue().toString()
+}

tests/e2e/Identity.test.ts

@@ -81,6 +81,7 @@ describe('e2e', () => {
           identityWallet.address,
           0
         )
+        rawTx.nonce = firstNonce
         await trustlinesNetwork.payment.confirm(rawTx)
 
         const secondNonce = await relayProvider.getIdentityNonce(

tests/unit/IdentityWallet.test.ts

@@ -1,3 +1,4 @@
+import BigNumber from 'bignumber.js'
 import chai from 'chai'
 import chaiAsPromised from 'chai-as-promised'
 import 'mocha'
@@ -25,6 +26,7 @@ import {
 import { MetaTransaction } from '../../src/typings'
 import {
   calculateIdentityAddress,
+  getRandomNonce,
   IdentityWallet
 } from '../../src/wallets/IdentityWallet'
 
@@ -229,4 +231,24 @@ describe('unit', () => {
       })
     })
   })
+
+  describe('#getRandomNonce', () => {
+    const minNonce = new BigNumber(2).pow(255).plus(1)
+    const maxNonce = new BigNumber(2).pow(256)
+
+    it('should generate nonce in expected range', () => {
+      // TODO: how to test this properly? Define range as parameter?
+      for (let i = 0; i < 1000; i++) {
+        const nonce = getRandomNonce()
+        assert.isTrue(
+          new BigNumber(nonce).isGreaterThanOrEqualTo(minNonce),
+          'Random nonce is too small'
+        )
+        assert.isTrue(
+          new BigNumber(nonce).isLessThan(maxNonce),
+          'Random nonce is too big'
+        )
+      }
+    })
+  })
 })