Rules

Rules in Clef are sets of conditions that determine whether a given action can be approved automatically without requiring manual intervention from the user. This can be useful for automatically approving transactions between a user's own accounts, or approving patterns that are commonly used by applications. Automatic signing also requires Clef to have access to account passwords which is configured independently of the ruleset.

Rules can define arbitrary conditions such as:

Because the rules are Javascript files they can be customized to implement any arbitrary logic on the available request data.

This page will explain how rules are implemented in Clef and how best to manage credentials when automatic rulesets are enabled.

Rule Implementation

The ruleset engine acts as a gatekeeper to the command line interface - it auto-approves any requests that meet the conditions defined in a set of authenticated rule files. This prevents the user from having to manually approve or reject every request - instead they can define common patterns in a rule file and abstract that task away to the ruleset engine. The general architecture is as follows:

When Clef receives a request, the ruleset engine evaluates a Javascript file for each method defined in the internal UI API docs. For example the code snippet below is an example ruleset that calls the function ApproveTx. The call to ApproveTx is invoking the ui_approveTx JSON_RPC API endpoint. Every time an RPC method is invoked the Javascript code is executed in a freshly instantiated virtual machine.

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, the decision to approve/reject
	// 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"}
}

When a request is made via the external API, the logic flow is as follows:

There are three possible outcomes from the ruleset engine that are handled in different ways:

There are some additional noteworthy implementation details that are important for defining rules correctly in ruleset.js:

Credential management

The ability to auto-approve transaction requires that the signer has the necessary credentials, i.e. account passwords, to decrypt keyfiles. These are stored encrypted as follows:

When the signer is started it generates a seed that is locked with a user specified password. The seed is saved to a location that defaults to $HOME/.clef/masterseed.json. The seed itself is a blob of bytes.

The signer uses the seed to:

config.json stores the hashes of any attested rulesets. credentials.json stores encrypted account passwords. The masterseed is required to decrypt these files. The decrypted account passwords can then be used to decrypt keyfiles.

Security

The Javascript VM

The downside of the very flexible rule implementation included in Clef is that the signer binary needs to contain a Javascript engine. This is an additional attack surface. The only viable attack is for an adversary to somehow extract cryptographic keys from memory during the Javascript VM execution. The hash-based rule attestation condition means the actual Javascript code executed by the Javascript engine is not a viable attack surface -- since if the attacker can control the ruleset, a much simpler attack would be to surreptitiously insert an attested "always-approve" rule instead of attempting to exploit the Javascript virtual machine. The Javascript engine is quite simple to implement and there are currently no known security vulnerabilities, not have there been any security problems identified for the similar Javascript VM implemented in Geth.

Writing rules

Since the user has complete freedom to write custom rules, it is plausible that those rules could create unintended security vulnerabilities. This can only really be protected by coding very carefully and trying to test rulesets (e.g. on a private testnet) before implementing them on a public network.

Javascript is very flexible but also easy to write incorrectly. For example, users might assume that javascript can handle large integers natively rather than explicitly using bigInt. This is an error commonly encountered in the Ethereum context when users attempt to multiply gas by gasCost.

It’s unclear whether any other language would be more secure - there is always the possibility of implementing an insecure rule.

Credential security

Clef implements a secure, encrypted vault for storing sensitive data. This vault is encrypted using a masterseed which the user is responsible for storing and backing up safely and securely. Since this masterseed is used to decrypt the secure vault, and its security is not handled by Clef, it could represent a security vulnerability if the user does not implement best practise in keeping it safe.

The same is also true for keys. Keys are not stored by Clef, they are only accessed using account passwords that Clef does store in its vault. The keys themselves are stored in an external keystore whose security is the responsibility of the user. If the keys are compromised, the account is not safe irrespective of the security benefits derived from Clef.

Ruleset examples

Below are some examples of ruleset.js files.

Example 1: Allow destination

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

Example 2: Allow listing

function ApproveListing() {
  return 'Approve';
}

Example 3: Approve signing data

function ApproveSignData(req) {
  if (req.address.toLowerCase() == '0xd9c9cd5f6779558b6e0ed4e6acf6b1947e7fa1f3') {
    if (req.messages[0].value.indexOf('bazonk') >= 0) {
      return 'Approve';
    }
    return 'Reject';
  }
  // Otherwise goes to manual processing
}

Example 4: 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 ?
  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));
}

Summary

Rules are sets of conditions encoded in Javascript files that enable certain actions to be auto-approved by Clef. This page outlined the implementation details and security considerations that will help to build suitable ruleset files. See the Clef GitHub for further reading.