go-ethereum/docs/_clef/Rules.md

---
title: Rules
---

# Rules

The `signer` binary contains a ruleset engine, implemented with [OttoVM](https://github.com/robertkrimen/otto)

It enables usecases like the following:

* I want to auto-approve transactions with contract `CasinoDapp`, with up to `0.05 ether` in value to maximum `1 ether` per 24h period
* I want to auto-approve transaction to contract `EthAlarmClock` with `data`=`0xdeadbeef`, if `value=0`, `gas < 44k` and `gasPrice < 40Gwei`

The two main features that are required for this to work well are;

1. Rule Implementation: how to create, manage and interpret rules in a flexible but secure manner
2. Credential managements and credentials; how to provide auto-unlock without exposing keys unnecessarily.

The section below deals with both of them

## Rule Implementation

A ruleset file is implemented as a `js` file. Under the hood, the ruleset-engine is a `SignerUI`, implementing the same methods as the `json-rpc` methods
defined in the UI protocol. Example:

```javascript

function asBig(str){
    if(str.slice(0,2) == "0x"){ return new BigNumber(str.slice(2),16)}
    return new BigNumber(str)
}

// Approve transactions to a certain contract if value is below a certain limit
function ApproveTx(req){

    var limit = big.Newint("0xb1a2bc2ec50000")
	var value = asBig(req.transaction.value);

	if(req.transaction.to.toLowerCase()=="0xae967917c465db8578ca9024c205720b1a3651a9")
	    && value.lt(limit) ){
	    return "Approve"
	 }
    // If we return "Reject", it will be rejected.
    // By not returning anything, it will be passed to the next UI, for manual processing
}

//Approve listings if request made from IPC
function ApproveListing(req){
    if (req.metadata.scheme == "ipc"){ return "Approve"}
}

```

Whenever the external API is called (and the ruleset is enabled), the `signer` calls the UI, which is an instance of a ruleset-engine. The ruleset-engine
invokes the corresponding method. In doing so, there are three possible outcomes:

1. JS returns "Approve"
  * Auto-approve request
2. JS returns "Reject"
  * Auto-reject request
3. Error occurs, or something else is returned
  * Pass on to `next` ui: the regular UI channel.

A more advanced example can be found below, "Example 1: ruleset for a rate-limited window", using `storage` to `Put` and `Get` `string`s by key.

* At the time of writing, storage only exists as an ephemeral unencrypted implementation, to be used during testing.

### Things to note

The Otto vm has a few [caveats](https://github.com/robertkrimen/otto):

* "use strict" will parse, but does nothing.
* The regular expression engine (re2/regexp) is not fully compatible with the ECMA5 specification.
* Otto targets ES5. ES6 features (eg: Typed Arrays) are not supported.

Additionally, a few more have been added

* The rule execution cannot load external javascript files.
* The only preloaded libary is [`bignumber.js`](https://github.com/MikeMcl/bignumber.js) version `2.0.3`. This one is fairly old, and is not aligned with the documentation at the github repository.
* Each invocation is made in a fresh virtual machine. This means that you cannot store data in global variables between invocations. This is a deliberate choice -- if you want to store data, use the disk-backed `storage`, since rules should not rely on ephemeral data.
* Javascript API parameters are _always_ an object. This is also a design choice, to ensure that parameters are accessed by _key_ and not by order. This is to prevent mistakes due to missing parameters or parameter changes.
* The JS engine has access to `storage` and `console`.

#### Security considerations

##### Security of ruleset

Some security precautions can be made, such as:

* Never load `ruleset.js` unless the file is `readonly` (`r-??-??-?`). If the user wishes to modify the ruleset, he must make it writeable and then set back to readonly.
  * This is to prevent attacks where files are dropped on the users disk.
* Since we're going to have to have some form of secure storage (not defined in this section), we could also store the `sha3` of the `ruleset.js` file in there.
  * If the user wishes to modify the ruleset, he'd then have to perform e.g. `signer --attest /path/to/ruleset --credential <creds>`

##### Security of implementation

The drawbacks of this very flexible solution is that the `signer` needs to contain a javascript engine. This is pretty simple to implement, since it's already
implemented for `geth`. There are no known security vulnerabilities in, nor have we had any security-problems with it so far.

The javascript engine would be an added attack surface; but if the validation of `rulesets` is made good (with hash-based attestation), the actual javascript cannot be considered
an attack surface -- if an attacker can control the ruleset, a much simpler attack would be to implement an "always-approve" rule instead of exploiting the js vm. The only benefit
to be gained from attacking the actual `signer` process from the `js` side would be if it could somehow extract cryptographic keys from memory.

##### Security in usability

Javascript is flexible, but also easy to get wrong, especially when users assume that `js` can handle large integers natively. Typical errors
include trying to multiply `gasCost` with `gas` without using `bigint`:s.

It's unclear whether any other DSL could be more secure; since there's always the possibility of erroneously implementing a rule.


## Credential management

The ability to auto-approve transaction means that the signer needs to have necessary credentials to decrypt keyfiles. These passwords are hereafter called `ksp` (keystore pass).

### Example implementation

Upon startup of the signer, the signer is given a switch: `--seed <path/to/masterseed>`
The `seed` contains a blob of bytes, which is the master seed for the `signer`.

The `signer` uses the `seed` to:

* Generate the `path` where the settings are stored.
  * `./settings/1df094eb-c2b1-4689-90dd-790046d38025/vault.dat`
  * `./settings/1df094eb-c2b1-4689-90dd-790046d38025/rules.js`
* Generate the encryption password for `vault.dat`.

The `vault.dat` would be an encrypted container storing the following information:

* `ksp` entries
* `sha256` hash of `rules.js`
* Information about pair:ed callers (not yet specified)

### Security considerations

This would leave it up to the user to ensure that the `path/to/masterseed` is handled in a secure way. It's difficult to get around this, although one could
imagine leveraging OS-level keychains where supported. The setup is however in general similar to how ssh-keys are  stored in `.ssh/`.


# Implementation status

This is now implemented (with ephemeral non-encrypted storage for now, so not yet enabled).

## Example 1: ruleset for a rate-limited window


```javascript

	function big(str){
		if(str.slice(0,2) == "0x"){ return new BigNumber(str.slice(2),16)}
		return new BigNumber(str)
	}

	// Time window: 1 week
	var window = 1000* 3600*24*7;

	// Limit : 1 ether
	var limit = new BigNumber("1e18");

	function isLimitOk(transaction){
		var value = big(transaction.value)
		// Start of our window function
		var windowstart = new Date().getTime() - window;

		var txs = [];
		var stored = storage.Get('txs');

		if(stored != ""){
			txs = JSON.parse(stored)
		}
		// First, remove all that have passed out of the time-window
		var newtxs = txs.filter(function(tx){return tx.tstamp > windowstart});
		console.log(txs, newtxs.length);

		// Secondly, aggregate the current sum
		sum = new BigNumber(0)

		sum = newtxs.reduce(function(agg, tx){ return big(tx.value).plus(agg)}, sum);
		console.log("ApproveTx > Sum so far", sum);
		console.log("ApproveTx > Requested", value.toNumber());

		// Would we exceed weekly limit ?
		return sum.plus(value).lt(limit)

	}
	function ApproveTx(r){
		if (isLimitOk(r.transaction)){
			return "Approve"
		}
		return "Nope"
	}

	/**
	* OnApprovedTx(str) is called when a transaction has been approved and signed. The parameter
 	* 'response_str' contains the return value that will be sent to the external caller.
	* The return value from this method is ignore - the reason for having this callback is to allow the
	* ruleset to keep track of approved transactions.
	*
	* When implementing rate-limited rules, this callback should be used.
	* If a rule responds with neither 'Approve' nor 'Reject' - the tx goes to manual processing. If the user
	* then accepts the transaction, this method will be called.
	*
	* TLDR; Use this method to keep track of signed transactions, instead of using the data in ApproveTx.
	*/
 	function OnApprovedTx(resp){
		var value = big(resp.tx.value)
		var txs = []
		// Load stored transactions
		var stored = storage.Get('txs');
		if(stored != ""){
			txs = JSON.parse(stored)
		}
		// Add this to the storage
		txs.push({tstamp: new Date().getTime(), value: value});
		storage.Put("txs", JSON.stringify(txs));
	}

```

## Example 2: allow destination

```javascript

	function ApproveTx(r){
		if(r.transaction.from.toLowerCase()=="0x0000000000000000000000000000000000001337"){ return "Approve"}
		if(r.transaction.from.toLowerCase()=="0x000000000000000000000000000000000000dead"){ return "Reject"}
		// Otherwise goes to manual processing
	}

```

## Example 3: Allow listing

```javascript

    function ApproveListing(){
        return "Approve"
    }

```