AA-166: Change #1 - document semi-abstracted nonce

EIPS/eip-4337.md

@@ -39,7 +39,7 @@ This proposal takes a different approach, avoiding any adjustments to the consen
 * **UserOperation** - a structure that describes a transaction to be sent on behalf of a user. To avoid confusion, it is not named "transaction".
   * Like a transaction, it contains "sender", "to", "calldata", "maxFeePerGas", "maxPriorityFee", "signature", "nonce"
   * unlike a transaction, it contains several other fields, described below
-  * also, the "nonce" and "signature" fields usage is not defined by the protocol, but by each account implementation
+  * also, the "signature" field usage is not defined by the protocol, but by each account implementation
 * **Sender** - the account contract sending a user operation.
 * **EntryPoint** - a singleton contract to execute bundles of UserOperations. Bundlers/Clients whitelist the supported entrypoint.
 * **Bundler** - a node (block builder) that bundles multiple UserOperations and create an EntryPoint.handleOps() transaction. Note that not all block-builders on the network are required to be bundlers
@@ -51,7 +51,7 @@ To avoid Ethereum consensus changes, we do not attempt to create new transaction
 | Field | Type | Description
 | - | - | - |
 | `sender` | `address` | The account making the operation |
-| `nonce` | `uint256` | Anti-replay parameter; also used as the salt for first-time account creation |
+| `nonce` | `uint256` | Anti-replay parameter (see "Semi-abstracted Nonce Support" ) |
 | `initCode` | `bytes` | The initCode of the account (needed if and only if the account is not yet on-chain and needs to be created) |
 | `callData` | `bytes` | The data to pass to the `sender` during the main execution call |
 | `callGasLimit` | `uint256` | The amount of gas to allocate the main execution call |
@@ -159,6 +159,59 @@ interface IAggregator {
 * **validateSignatures()** MUST validate the aggregated signature matches for all UserOperations in the array, and revert otherwise.
   This method is called on-chain by `handleOps()`
 
+#### Semi-abstracted Nonce Support
+
+In Ethereum protocol, the sequential transaction `nonce` value is used as a replay protection method as well as to
+determine the valid order of transaction being included in blocks.
+
+It also contributes to the transaction hash uniqueness, as a transaction by the same sender with the same
+nonce may not be included in the chain twice.
+
+However, requiring a single sequential `nonce` value is limiting the senders' ability to define their custom logic
+with regard to transaction ordering and replay protection.
+
+We are proposing to implement a nonce mechanism that uses a single `uint256` nonce value in the `UserOperation`,
+but treats it as two values:
+
+* 192-bit "key"
+* 64-bit "sequence"
+
+These values are represented on-chain as a member in the `EntryPoint` contract mapping:
+
+```solidity
+mapping(address => mapping(uint192 => uint256)) public nonces;
+```
+
+For each `key` the `sequence` is validated and incremented sequentially and monotonically by the `EntryPoint` for
+each UserOperation, however a new key can be introduced with an arbitrary value at any point.
+
+This approach maintains the guarantee of `UserOperation` hash uniqueness on-chain on the protocol level while allowing
+wallets to implement any custom logic they may need operating on a 192-bit "key" field, while fitting the 32 byte word.
+##### Usage examples
+
+1. Classic sequential nonce.
+
+   In order to require the wallet to have classic, sequential nonce, the validation function should perform:
+   ```solidity
+   require(userOp.nonce<type(uint64).max)
+   ```
+
+2. Ordered administrative events
+
+   In some cases, an account may need to have an "administrative" channel of operations running in parallel to normal
+   operations.
+
+   In this case, the account may use specific `key` when calling methods on the account itself:
+   ```solidity
+   bytes4 sig = bytes4(userOp.callData[0 : 4]);
+   uint key = userOp.nonce >> 64;
+   if (sig == ADMIN_METHODSIG) {
+       require(key == ADMIN_KEY, "wrong nonce-key for admin operation");
+   } else {
+       require(key == 0, "wrong nonce-key for normal operation");
+   }
+   ```
+
 #### Using signature aggregators
 
 An account signifies it uses signature aggregation returning its address from `validateUserOp`.