The signer binary contains a ruleset engine, implemented with OttoVM

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:

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 ( == "0xae967917c465db8578ca9024c205720b1a3651a9") && {
		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 strings 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:

  • “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 library is 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

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 ?

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

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

function ApproveListing() {
	return "Approve"