[Lightning-dev] [DRAFT] BOLT 13(?): WatchTower protocol

Sergi Delgado Segura sergi.delgado.s at gmail.com
Wed Nov 27 20:20:21 UTC 2019


Hi all,

Patrick McCorry and I have been working on putting together a BOLT draft
for WatchTowers. So far we've seen a couple of implementation of
WatchTowers in the wild (lnd and Electrum) based on Tadge's Monitor
approach and we are also working on our own one. While all are similar,
they differ in some points that may make then non interoperable, so it felt
right time to put some effort and try to standardise this. We've borrowed
ideas from all implementations as well as from the original approach and
added some additional bits to extended with different quality of service.
The draft is still rough on the edges and have several open discussion
topics at the very end (#FIXME section).

Best,

# WatchTower protocol specification (BOLT DRAFT)

## Overview

All off-chain protocols assume the user remains online and synchronised
with the network. To alleviate this assumption, customers can hire a third
party watching service (a.k.a WatchTower) to watch the blockchain and
respond to channel breaches on their behalf.

At a high level, the client sends an encrypted justice transaction
alongside a transaction locator to the WatchTower. Both the encryption key
and the transaction locator are derived from the breach transaction id,
meaning that the WatchTower will be able to decrypt the justice transaction
only after the corresponding breach is seen on the blockchain. Therefore,
the WatchTower does not learn any information about the client's channel
unless there is a channel breach (channel-privacy).

Due to replace-by-revocation Lightning channels, the client should send
data to the WatchTower for every new update in the channel, otherwise the
WatchTower may not be able to respond to specific breaches.

Finally, optional QoS can be offered by the WatchTower to provide stronger
guarantees to the client, such as a signed receipt for every new job. The
rationale for the receipt is to build an _accountable_ WatchTower as the
customer can later use it as publicly verifiable evidence if the WatchTower
fails to protect them.

The scope of this document includes:

- A protocol for client/server communication.
- How to build appointments for the WatchTower, including key/locator
derivation and data encryption.
- A format for the signed receipt.

The scope of this bolt does not include:

 - A payment protocol between the customer and WatchTower.
 - WatchTower server discovery.

For the rest of this document we will refer to the WatchTower as server,
and the user/Lightning node as client.

## Table of Contents
* [WatchTower discovery](#watchtower-discovery)
* [WatchTower services](#watchtower-discovery)
* [Basic Service](#basic-service)
* [Quality of Service](#quality-of-service)
* [Sending and receiving appointments](#sending-and-receiving-appointments)
  * [The `appointment` message](#the-appointment-message)
  * [The `appointment_accepted` message](#the-appointment_accepted-message)
  * [The `appointment_rejected` message](#the-appointment_rejected-message)
* [Quality of Service data](#quality-of-service-data)
* [`accountability`](#accountability)
* [Transaction Locator and Encryption
Key](#transaction-locator-and-encryption-key)
* [Encryption Algorithms and
Parameters](#encryption-algorithms-and-parameters)
* [Payment Modes](#payment-modes)
* [No compression of justice
transaction](#no-compression-of-justice-transaction)

## WatchTower discovery

We have not defined how a client can find a list of servers to hire yet. We
assume the client has found a server and the server is offering a watching
service. The service can either be the basic service or an accountable
quality of service. To deal with pre-payments (when necessary), the client
may have an authentication token that the server can verify when accepting
the job (e.g. a blinded token).

## WatchTower services

### Basic Service
The customer can hire the WatchTower to watch for breaches on the
blockchain and relay a justice transaction on their behalf. The customer
receives an acknowledgement when the WatchTower has accepted the job, but
the hiring protocol does not guarantee the transaction inclusion.

### Quality of Service
Quality of Service (`qos`) builds on top of the basic service provided by a
tower and it's optionally provided. Different kinds of QoS can be offered
by the tower.

For now we are defining a single type of `qos`: `accountability`.

#### `accountability`

A WatchTower provides a signed receipt to the customer. This is considered
reputational accountability as the customer has publicly verifiable
cryptographic evidence the WatchTower was hired. The receipt can be used to
prove the WatchTower did not relay the justice transaction on their behalf
and/or request a refund.

## Sending and receiving appointments

Once the client is aware of the services provided by the server, the former
can start sending appointments to the latter.

+-------+                                    +-------+
|   A   |--(1)---      appointment      ---->|   B   |
|       |<-(2)---   accepted/rejected   -----|       |
+-------+                                    +-------+

- where node A is 'client' and node B is 'server'

### The `appointment` message

This message contains all the information regarding the appointment that
the client wants to arrange with the server.

1. type: ? (`appointment`)
2. data:
   * [`16*byte`:`locator`]
   * [`u64 `:`start_block`]
   * [`u64 `:`end_block`]
   * [`u16`: `encrypted_blob_len`
   * [`encrypted_blob_len*byte`:`encrypted_blob`]
   * [`u16`:`cipher`]
   * [`u16`: `auth_token_len`]
   * [`auth_token_len*byte`: `auth_token`]
   * [`u16`: `qos_len`]
   * [`qos_len*byte`: `qos_data`]

#### Requirements

The client:

* MUST set `locator` as specified in [Transaction Locator and Encryption
Key](#transaction-locator-and-encryption-key).
* MUST set `start_block` to the current chain tip height.
* MUST set `end_block` to the block height at which he requests the server
to stop watching for breaches.
* MUST set `encrypted_blob` to the encryption of the `justice_transaction`
as specified in [Transaction Locator and Encryption
Key](#transaction-locator-and-encryption-key).
* MUST set `cipher` to the cipher used to create the `encrypted_blob`.
* MAY send an empty `auth_token` field.
* MUST set `auth_token_len` to the length of `auth_token`.
* MAY send an empty `qos_data` field.
* if `qos_data` is not empty:
*  MUST set `qos_data` according to [Quality of Service
data](#quality-of-service-data).
* MUST set `qos_len` equal to the length of `qos_data`.

The server:

* MUST reject the appointment if:
* Authentication is required and `auth_token` is not provided.
* Authentication is required and `auth_token` is invalid.
* `locator` is not a `16-byte` value.
* `start_block` is further than one block behind the current chain tip.
* `start_block` is further than one block ahead the current chain tip.
* `encrypted_blob` has unreasonable size.
* `cipher` is not among the ones he implements.

* SHOULD reject the appointment if`end_block` is too far away in the future.

* MUST:
* truncate the remainder of the package to `qos_len`.
* process `qos_data` according to [Quality of Service
data](#quality-of-service-data) if `qos_len` is not 0.

* MAY accept the appointment otherwise.

#### Rationale

We define appointment as the way that the WatchTower is hired / requested
by a client to do it's watching services.

WatchTowers may offer their services for free (`altruistic`) or they may
require a payment when accepting the job (`non-altruistic`). We have
defined `auth_token` as an authentication mechanism between the client and
server, so the client can prove they are entitled to the service. The
tokens are not required to be linked to any kind of identity (e.g. blinded
tokens), but their sole purpose is to confirm the client has already paid
for the service.

The transaction `locator` can be deterministically computed by both the
client and the server. Locators of wrong size are therefore invalid.

`start_block` can be either one block ahead or behind the tower tip due to
network delays. A tower must not accept appointments arbitrarily ahead or
behind the current tip since it could increase DoS vectors. A `start_block`
long behind would force the tower to rescan block data for those
appointments instead of watching block by block. On the other hand, a
`start_time` long ahead would imply storing information way before it being
needed.

Regarding the `end_block`, too far away is a subjective concept. The
further away a tower accepts appointment ends, the higher the potential
storage requirements may be, and the easier, and cheaper, it my be to
perform DoS.

The `encrypted_blob` should have been encrypted using `cipher`. Block
ciphers have a size multiple of the block length, which depends on the key
size. Therefore the `encrypted_blob` have to be at least as big as:

`cipher_block_size * ceil(minimum_viable_transaction_size /
cipher_block_size)`

and at most as big as:

`cipher_block_size * ceil(maximum_viable_transaction_size /
cipher_block_size`)

`minimum_viable_transaction_size` and `maximum_viable_transaction_size`
refer to the minimum/maximum size required to create a valid transaction.
Accepting `encrypted_blob` outside those boundaries will increase DoS
attacks on the server.

The client should have learn about the `ciphers` implemented by the
WatchTower and the `qos` that the tower is offering during the peer
discovery.

A tower must not accept appointments using a cipher it does not implement,
otherwise the decryption of the `encrypted_blob` will not be possible.

`qos` is optional and can include multiple services.

### The `appointment_accepted` message

This message contains information about the acceptance of an appointment by
the WatchTower.

1. type: ? (`appointment_accepted `)
2. data:
   * [`16*byte `:`locator`]
   * [`u16`: `qos_len`]
* [`qos_len*byte`: `qos_data`]

The server:

* MUST receive `appointment` before sending an `appointment_accepted`
message.
* MUST set the `locator` to match the one received in `appointment`.
* if `qos_data` was requested in `appointment`:
*  MUST set `qos_data` according to [Quality of Service
data](#quality-of-service-data).
* MUST set `qos_len` equal to the length of `qos_data`.

The client:

* MUST fail the connection  if `locator` does not match any of locators the
previously sent to the server:

* if `qos` was requested in `appointment`:
* MUST fail the connection if `qos_len` is 0.
* MUST process `qos_data` according to [Quality of Service
data](#quality-of-service-data).

### The `appointment_rejected` message

This message contains information about the rejection of an appointment by
the WatchTower.

1. type: ? (`appointment_rejected `)
2. data:
   * [`16*byte `:`locator`]
   * [`u16`: `rcode`]
   * [`u16`: `reason_len`
   * [`reason_len*byte`: `reason`]

The server:

* MUST receive `appointment` before sending an `appointment_rejected`
message.
* MUST set the `locator` to match the one received in `appointment`.
* MUST set `rcode` to the rejection code.
* MAY set and empty `reason` field.
* MUST set `reason_len` to length of `reason`.

#### Rationale

The `appointment_rejected` message follows the approach taken by the
`error` message defined in [BOLT#1](
https://github.com/lightningnetwork/lightning-rfc/blob/master/01-messaging.md#the-error-message):
error codes are mandatory, whereas reasons are optional and implementation
dependant.

## Quality of Service data

`qos_data` is a list where each field specifies they type and associated
data of the offered/requested `qos`. The format is defined as follows:

* [`u16`: `qos_type`]
* [`u16`: `data_len`]
* [`data_len*byte`: `data`]

So far, only `accountability` is defined.

### `accountability`

The accountability `qos` defines a pair `qos_data` blobs, associated to a
pair of messages: The first one is `customer_evidence` and it is provided
by the `client` in the `appointment` message. The second one is
`tower_evidence`, and is provided by the WatchTower in the
`appointment_accepted` message.

#### `customer_evidence`

The format for the `customer_evidence` is defined as follows:

1. type: ? (`customer_evidence`)
2. data:
* [`u64 `:`dispute_delta`]
* [`u64`: `transaction_size`]
* [`u64`: `transaction_fee`]

If `accountability` is being requested, the client:

* MUST set `dispute_delta` to the CLTV value specified in the
`commitment_transaction`.
* MUST set `transaction_size` to the size of the serialized
`justice_transaction`, in bytes.
* MUST set `transaction_fee` to the fee set in the `justice_transaction`,
in satoshis.
* MUST set the `customer_signature_algorithm` to one of the signature
algorithms supported by the tower.
* MUST set `customer_signature` to the signature of the appointment using
`op_customer_signature_algorithm`.
* MUST set `customer_public_key` to the public key that matches the private
key used to create `op_customer_signature`.

If `accountability` is being offered, the server:

* MUST compute the `customer_signature` verification using
`customer_public_key`.
* SHOULD compute the `fee_rate` set in the `justice_tx` using
`transaction_size` and `transaction_fee`.

* MUST reject the appointment if:
* Any of the fields is missing.
* `transaction_size` is unreasonable.
* `customer_signature_algorithm` does not match any of the supported
signing algorithms.
* `customer_signature` cannot be verified using `customer_public_key`.

* SHOULD reject the appointment if:
* `dispute_delta` is too small.
* `fee_rate` is too low.

If `accountability` is NOT being offered:

* The server MUST reject the appointment.

Otherwise:

* The server SHOULD accept the appointment.

#### Rationale

The concept of too small for `dispute_delta` is subjective. The
`dispute_delta` defines the time (in blocks) that the tower has in order to
respond after a breach is seen. The smaller the value, the more the server
risks to fail the appointment.

`transaction_size` and `transaction_fee` help the WatchTower to decide on
the likelihood of an appointment being fulfilled. Appointments with
`fee_rate` too low may be rejected by the WatchTower. While a customer can
always fake this values, it should break ToS between the client and the
server and, therefore, release the WatchTower of any liability.

By accepting the request, the tower is offering a reputationally
accountable watching service. If `accountability` is not offered, then the
tower will not accept appointments requesting for it.

As well, the WatchTower must check the transaction details before deciding
whether it will accept it. If the decrypted justice transaction does not
satisfy the job details (e.g. too low fee), then the tower is not obliged
to fulfil the appointment.

#### `tower_evidence`

The format for the `tower_evidence` is defined as follows:

1. type: ? (`tower_evidence`)
2. data:
* [`u16 `:`receipt_len`]
* [`receipt_len*byte `: `receipt`]
* [`u16`: `wt_signature_algorithm`]
* [`u16`: `wt_signature_len`
* [`wt_signature_len*byte`: `wt_signature`]
* [`u16`: `wt_public_key_len`]
* [`wt_public_key_len*byte`: `wt_public_key`]

The server:

* MUST set `receipt` to a receipt built according to
[Receipt-Format](#receipt-format).
* MUST set `wt_signature_algorithm` to one of the signature algorithms he
has announced.
* MUST set `wt_signature` to the signature of the appointment using
`wt_signature_algorithm`.
* MUST set `wt_public_key` to the public key that matches the private key
used to create `wt_signature`.

The client:

* MUST compute the `wt_signature` verification using `wt_public_key`.

* MUST fail the connection if:
* Any of the fields is missing.
* `receipt` does not matches the format specified at
[Receipt-Format](#receipt-format)
* `receipt` fields do not match the ones sent in the `appointment` message.
* `wt_signature_algorithm` does not match any of the ones offered by the
server.
* `wt_signature` cannot be verified using `wt_public_key`.

#### Receipt Format

The server MUST create the receipt containing the following information:

txlocator
start_block
end_block
dispute_delta
encrypted_blob
transaction_size
transaction_fee
cipher
customer_signature
wt_public_key


#### Rationale

We assume the client has a well-known public key for the WatchTower.

The receipt contains, mainly, the information provided by the user. The
WatchTower will need to sign the receipt to provide evidence of agreement.

The `customer_signature` is included in the receipt to link both the client
request and the server response. Otherwise, the tower could sign a receipt
with different data that the one sent by the user, and the user would have
no way to prove whether that's true or not. By signing the customer
signature there the tower creates evidence of what the user sent, since the
tower cannot forge the client's signature.

#### Receipt serialization and signature

[FIXME: TBD]

## Transaction Locator and Encryption Key

Implementations MUST compute the `locator`, `encryption_key` and
`encryption_iv` from the commitment transaction as defined below:

- `locator`: first half of the commitment transaction id
(`commitment_txid(0,16]`)
- `master_key`: Hash of the second half of the commitment transaction id
(`SHA256(commitment_txid(16,32])`)
- `encryption_key`: first half of the master key (`master_key(0,16]`)
- `encryption_iv`: second half of the master key (`master_key(16,32]`)


The server relies on both the encryption key and iv to decrypt the justice
transaction. Furthermore, the transaction locator helps the WatchTower
identify a breach transaction on the blockchain.

## Encryption Algorithms and Parameters

All clients and servers MUST use one of the following encryption
algorithms:

- ChaCha20 (https://tools.ietf.org/html/rfc7539)
- AES-GCM-256 (https://tools.ietf.org/html/rfc5288)

Sample code (python) for the client to prepare the `encrypted_blob`:

from hashlib import sha256
from binascii import hexlify

def encrypt(justice_tx, commitment_txid):
   # master_key = SHA256(commitment_txid(16, 32])
   master_key = sha256(commitment_txid[16:]).digest()

   # The 16 MSB of the master key will serve as the AES-GCM-256 secret key.
The 16 LSB will serve as the IV.
   sk = master_key[:16]
   nonce = master_key[16:]

   # Encrypt the data
   aesgcm = AESGCM(sk)
   encrypted_blob = aesgcm.encrypt(nonce=iv, data=tx, associated_data=None)
   encrypted_blob = hexlify(encrypted_blob).decode()

   return encrypted_blob

## Payment modes

Although this BOLT does not enforce any specific payment method to be
adopted, it is worth mentioning the three most common ones:

**On-chain bounty**. An additional output is created in the justice
transaction that will reward the WatchTower.

**Micropayments**. A small payment is sent to the WatchTower for every new
job (e.g. over the lightning network)

**Subscription**. WatchTower is periodically rewarded / paid for their
service to the customer. (e.g. over the lightning network or fiat
subscription).

Both micropayments and subscriptions are favourable for a WatchTower. The
on-chain bounty approach is not ideal for a watching network, it lets the
customer hire many WatchTowers (O(N) storage for each tower) and only one
WatchTower will be rewarded upon collecting the bounty. On top of that, the
onchain bounty allows a network-wise DoS attack for free.

## No compression of justice transaction

The storage requirements for a WatchTower can be reduced (linearly) by
implementing [shachain](
https://github.com/rustyrussell/ccan/blob/master/ccan/crypto/shachain/design.txt),
therefore storing the parts required to build the transaction and the
corresponding signing key instead of the full transaction. For now, we have
decided to keep the hiring protocol simple. Storage is relatively cheap and
we can revisit this standard if it becomes a problem.

## FIXMES

- Define a proper tower discovery.
- Define authentication mechanism (macaroons maybe?).
- None of the message types have been defined (they have been left with ?).
- Define receipt serialization format.
- `qos_type` can be defined by ranges, in the same way that error messages
are. In that way a range of values can belong to a specific `qos`.
- Define an optional way of doing batch appointments / appointments in
bulk? That would break appointment unlinkability but would ease the data
management for the tower.
- The `customer_signature` could be optional if the client does not care
that much about the worst case. Dicuss whether that makes sense.
- Discuss whether to extend it with shachain.


The document can also be found here:
https://github.com/PISAresearch/pisa/blob/master/13-watchtower-API.md

-- 
Sergi.
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