<pre>
  BIP: 157
  Layer: Peer Services
  Title: Client Side Block Filtering
  Author: Olaoluwa Osuntokun <laolu32@gmail.com>
          Alex Akselrod <alex@akselrod.org>
          Jim Posen <jimpo@coinbase.com>
  Comments-Summary: None yet
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0157
  Status: Draft
  Type: Standards Track
  Created: 2017-05-24
  License: CC0-1.0
</pre>


== Abstract ==

This BIP describes a new light client protocol in Bitcoin that improves upon
currently available options. The standard light client protocol in use today,
defined in BIP
37<ref>https://github.com/bitcoin/bips/blob/master/bip-0037.mediawiki</ref>, has
known flaws that weaken the security and privacy of clients and allow
denial-of-service attack vectors on full
nodes<ref>https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-May/012636.html</ref>.
The new protocol overcomes these issues by allowing light clients to obtain
compact probabilistic filters of block content from full nodes and download full
blocks if the filter matches relevant data.

New P2P messages empower light clients to securely sync the blockchain without
relying on a trusted source. This BIP also defines a filter header, which serves
as a commitment to all filters for previous blocks and provides the ability to
efficiently detect malicious or faulty peers serving invalid filters. The
resulting protocol guarantees that light clients with at least one honest peer
are able to identify the correct block filters.

== Motivation ==

Bitcoin light clients allow applications to read relevant transactions from the
blockchain without incurring the full cost of downloading and validating all
data. Such applications seek to simultaneously minimize the trust in peers and
the amount of bandwidth, storage space, and computation required. They achieve
this by downloading all block headers, verifying the proofs of work, and
following the longest proof-of-work chain. Since block headers are a fixed
80-bytes and are generated every 10 minutes on average, the bandwidth required
to sync the block headers is minimal. Light clients then download only the
blockchain data relevant to them directly from peers and validate inclusion in
the header chain. Though clients do not check the validity of all blocks in the
longest proof-of-work chain, they rely on miner incentives for security.

BIP 37 is currently the most widely used light client execution mode for
Bitcoin. With BIP 37, a client sends a Bloom filter it wants to watch to a full
node peer, then receives notifications for each new transaction or block that
matches the filter. The client then requests relevant transactions from the peer
along with Merkle proofs of inclusion in the blocks containing them, which are
verified against the block headers. The Bloom filters match data such as client
addresses and unspent outputs, and the filter size must be carefully tuned to
balance the false positive rate with the amount of information leaked to peer. It
has been shown, however, that most implementations available offer virtually
''zero privacy'' to wallets and other
applications<ref>https://eprint.iacr.org/2014/763.pdf</ref><ref>https://jonasnick.github.io/blog/2015/02/12/privacy-in-bitcoinj/</ref>.
Additionally, malicious full nodes serving light clients can omit critical data
with little risk of detection, which is unacceptable for some applications
(such as Lightning Network clients) that must respond to certain on-chain
events. Finally, honest nodes servicing BIP 37 light clients may incur
significant I/O and CPU resource usage due to maliciously crafted Bloom filters,
creating a denial-of-service (DoS) vector and disincentizing node operators from
supporting the
protocol<ref>https://github.com/bitcoin/bips/blob/master/bip-0111.mediawiki</ref>.

The alternative detailed in this document can be seen as the opposite of BIP 37:
instead of the client sending a filter to a full node peer, full nodes generate
deterministic filters on block data that are served to the client. A light
client can then download an entire block if the filter matches the data it is
watching for. Since filters are deterministic, they only need to be constructed
once and stored on disk, whenever a new block is connected to the chain. This
keeps the computation required to serve filters minimal, and eliminates the I/O
asymmetry that makes BIP 37 enabled nodes vulnerable. Clients also get better
assurance of seeing all relevant transactions because they can check the
validity of filters received from peers more easily than they can check
completeness of filtered blocks. Finally, client privacy is improved because
blocks can be downloaded from ''any source'', so that no one peer gets complete
information on the data required by a client. Extremely privacy conscious light
clients may opt to anonymously fetch blocks using advanced techniques such a
Private Information
Retrieval<ref>https://en.wikipedia.org/wiki/Private_information_retrieval</ref>.

== Definitions ==

<code>[]byte</code> represents a vector of bytes.

<code>[N]byte</code> represents a fixed-size byte array with length N.

''CompactSize'' is a compact encoding of unsigned integers used in the Bitcoin
P2P protocol.

''double-SHA256'' is a hash algorithm defined by two invocations of SHA-256:
<code>double-SHA256(x) = SHA256(SHA256(x))</code>.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.

== Specification ==

=== Filter Types ===

For the sake of future extensibility and reducing filter sizes, there are
multiple ''filter types'' that determine which data is included in a block
filter as well as the method of filter construction/querying. In this model,
full nodes generate one filter per block per filter type supported.

Each type is identified by a one byte code, and specifies the contents and
serialization format of the filter. A full node MAY signal support for
particular filter types using service bits. The initial filter types are defined
separately in [[bip-0158.mediawiki|BIP 158]], and one service bit is allocated
to signal support for them.

=== Filter Headers ===

This proposal draws inspiration from the headers-first mechanism that Bitcoin
nodes use to sync the block
chain<ref>https://bitcoin.org/en/developer-guide#headers-first</ref>. Similar to
how block headers have a Merkle commitment to all transaction data in the block,
we define filter headers that have commitments to the block filters. Also like
block headers, filter headers each have a commitment to the preceding one.
Before downloading the block filters themselves, a light client can download all
filter headers for the current block chain and use them to verify the
authenticity of the filters. If the filter header chains differ between multiple
peers, the client can identify the point where they diverge, then download the
full block and compute the correct filter, thus identifying which peer is
faulty.

The canonical hash of a block filter is the double-SHA256 of the serialized
filter. Filter headers are 32-byte hashes derived for each block filter. They
are computed as the double-SHA256 of the concatenation of the filter hash with
the previous filter header. The previous filter header used to calculate that of
the genesis block is defined to be the 32-byte array of 0's.

=== New Messages ===

==== getcfilters ====
<code>getcfilters</code> is used to request the compact filters of a particular
type for a particular range of blocks. The message contains the following
fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Filter type for which headers are requested
|-
| StartHeight
| uint32
| 4
| The height of the first block in the requested range
|-
| StopHash
| [32]byte
| 32
| The hash of the last block in the requested range
|}

# Nodes SHOULD NOT send <code>getcfilters</code> unless the peer has signaled support for this filter type. Nodes receiving <code>getcfilters</code> with an unsupported filter type SHOULD NOT respond.
# StopHash MUST be known to belong to a block accepted by the receiving peer. This is the case if the peer had previously sent a <code>headers</code> or <code>inv</code> message with that block or any descendents. A node that receives <code>getcfilters</code> with an unknown StopHash SHOULD NOT respond.
# The height of the block with hash StopHash MUST be greater than or equal to StartHeight, and the difference MUST be strictly less than 1000.
# The receiving node MUST respond to valid requests by sending one <code>cfilter</code> message for each block in the requested range, sequentially in order by block height.

==== cfilter ====
<code>cfilter</code> is sent in response to <code>getcfilters</code>, one for
each block in the requested range. The message contains the following fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Byte identifying the type of filter being returned
|-
| BlockHash
| [32]byte
| 32
| Block hash of the Bitcoin block for which the filter is being returned
|-
| NumFilterBytes
| CompactSize
| 1-5
| A variable length integer representing the size of the filter in the following field
|-
| FilterBytes
| []byte
| NumFilterBytes
| The serialized compact filter for this block
|}

# The FilterType SHOULD match the field in the <code>getcfilters</code> request, and BlockHash must correspond to a block that is an ancestor of StopHash with height greater than or equal to StartHeight.

==== getcfheaders ====
<code>getcfheaders</code> is used to request verifiable filter headers for a
range of blocks. The message contains the following fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Filter type for which headers are requested
|-
| StartHeight
| uint32
| 4
| The height of the first block in the requested range
|-
| StopHash
| [32]byte
| 32
| The hash of the last block in the requested range
|}

# Nodes SHOULD NOT send <code>getcfheaders</code> unless the peer has signaled support for this filter type. Nodes receiving <code>getcfheaders</code> with an unsupported filter type SHOULD NOT respond.
# StopHash MUST be known to belong to a block accepted by the receiving peer. This is the case if the peer had previously sent a <code>headers</code> or <code>inv</code> message with that block or any descendents. A node that receives <code>getcfheaders</code> with an unknown StopHash SHOULD NOT respond.
# The height of the block with hash StopHash MUST be greater than or equal to StartHeight, and the difference MUST be strictly less than 2,000.

==== cfheaders ====
<code>cfheaders</code> is sent in response to <code>getcfheaders</code>. Instead
of including the filter headers themselves, the response includes one filter
header and a sequence of filter hashes, from which the headers can be derived.
This has the benefit that the client can verify the binding links between the
headers. The message contains the following fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Filter type for which hashes are requested
|-
| StopHash
| [32]byte
| 32
| The hash of the last block in the requested range
|-
| PreviousFilterHeader
| [32]byte
| 32
| The filter header preceding the first block in the requested range
|-
| FilterHashesLength
| CompactSize
| 1-3
| The length of the following vector of filter hashes
|-
| FilterHashes
| [][32]byte
| FilterHashesLength * 32
| The filter hashes for each block in the requested range
|}

# The FilterType and StopHash SHOULD match the fields in the <code>getcfheaders</code> request.
# FilterHashesLength MUST NOT be greater than 2,000.
# FilterHashes MUST have one entry for each block on the chain terminating with tip StopHash, starting with the block at height StartHeight. The entries MUST be the filter hashes of the given type for each block in that range, in ascending order by height.
# PreviousFilterHeader MUST be set to the previous filter header of first block in the requested range.

==== getcfcheckpt ====
<code>getcfcheckpt</code> is used to request filter headers at evenly spaced
intervals over a range of blocks. Clients may use filter hashes from
<code>getcfheaders</code> to connect these checkpoints, as is described in the
[[#client-operation|Client Operation]] section below. The
<code>getcfcheckpt</code> message contains the following fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Filter type for which headers are requested
|-
| StopHash
| [32]byte
| 32
| The hash of the last block in the chain that headers are requested for
|}

# Nodes SHOULD NOT send <code>getcfcheckpt</code> unless the peer has signaled support for this filter type. Nodes receiving <code>getcfcheckpt</code> with an unsupported filter type SHOULD NOT respond.
# StopHash MUST be known to belong to a block accepted by the receiving peer. This is the case if the peer had previously sent a <code>headers</code> or <code>inv</code> message with any descendent blocks. A node that receives <code>getcfcheckpt</code> with an unknown StopHash SHOULD NOT respond.

==== cfcheckpt ====
<code>cfcheckpt</code> is sent in response to <code>getcfcheckpt</code>. The
filter headers included are the set of all filter headers on the requested chain
where the height is a positive multiple of 1,000. The message contains the
following fields:

{| class="wikitable"
! Field Name
! Data Type
! Byte Size
! Description
|-
| FilterType
| byte
| 1
| Filter type for which headers are requested
|-
| StopHash
| [32]byte
| 32
| The hash of the last block in the chain that headers are requested for
|-
| FilterHeadersLength
| CompactSize
| 1-3
| The length of the following vector of filter headers
|-
| FilterHeaders
| [][32]byte
| FilterHeadersLength * 32
| The filter headers at intervals of 1,000
|}

# The FilterType and StopHash SHOULD match the fields in the <code>getcfcheckpt</code> request.
# FilterHeaders MUST have exactly one entry for each block on the chain terminating in StopHash, where the block height is a multiple of 1,000 greater than 0. The entries MUST be the filter headers of the given type for each such block, in ascending order by height.

=== Node Operation ===

Full nodes MAY opt to support this BIP and generate filters for any of the
specified filter types. Such nodes SHOULD treat the filters as an additional
index of the blockchain. For each new block that is connected to the main chain,
nodes SHOULD generate filters for all supported types and persist them. Nodes
that are missing filters and are already synced with the blockchain SHOULD
reindex the chain upon start-up, constructing filters for each block from
genesis to the current tip. They also SHOULD keep every checkpoint header in
memory, so that <code>getcfcheckpt</code> requests do not result in many
random-access disk reads.

Nodes SHOULD NOT generate filters dynamically on request, as malicious peers may
be able to perform DoS attacks by requesting small filters derived from large
blocks. This would require an asymmetical amount of I/O on the node to compute
and serve, similar to attacks against BIP 37 enabled nodes noted in BIP 111.

Nodes MAY prune block data after generating and storing all filters for a block.

=== Client Operation ===

This section provides recommendations for light clients to download filters with
maximal security.

Clients SHOULD first sync the entire block header chain from peers using the
standard headers-first syncing mechanism before downloading any block filters or
filter headers. Clients configured with trusted checkpoints MAY only sync
headers started from the last checkpoint. Clients SHOULD disconnect any outbound
peers whose best chain has significantly less work than the known longest
proof-of-work chain.

Once a client's block headers are in sync, it SHOULD download and verify filter
headers for all blocks and filter types that it might later download. The client
SHOULD send <code>getcfheaders</code> messages to peers and derive and store the
filter headers for each block. The client MAY first fetch headers at evenly
spaced intervals of 1,000 by sending <code>getcfcheckpt</code>. The header
checkpoints allow the client to download filter headers for different intervals
from multiple peers in parallel, verifying each range of 1,000 headers against
the checkpoints.

Unless securely connected to a trusted peer that is serving filter headers, the
client SHOULD connect to multiple outbound peers that support each filter type
to mitigate the risk of downloading incorrect headers. If the client receives
conflicting filter headers from different peers for any block and filter type,
it SHOULD interrogate them to determine which is faulty. The client SHOULD use
<code>getcfheaders</code> and/or <code>getcfcheckpt</code> to first identify
the first filter headers that the peers disagree on. The client then SHOULD
download the full block from any peer and derive the correct filter and filter
header. The client SHOULD ban any peers that sent a filter header that does not
match the computed one.

Once the client has downloaded and verified all filter headers needed, ''and''
no outbound peers have sent conflicting headers, the client can download the
actual block filters it needs. The client MAY backfill filter headers before the
first verified one at this point if it only downloaded them starting at a later
point. Clients SHOULD persist the verified filter headers for last 100 blocks in
the chain (or whatever finality depth is desired), to compare against headers
received from new peers after restart. They MAY store more filter headers to
avoid redownloading them if a rescan is later necessary.

Starting from the first block in the desired range, the client now MAY download
the filters. The client SHOULD test that each filter links to its corresponding
filter header and ban peers that send incorrect filters. The client MAY download
multiple filters at once to increase throughput, though it SHOULD test the
filters sequentially. The client MAY check if a filter is empty before
requesting it by checking if the filter header commits to the hash of the empty
filter, saving a round trip if that is the case.

Each time a new valid block header is received, the client SHOULD request the
corresponding filter headers from all eligible peers. If two peers send
conflicting filter headers, the client should interrogate them as described
above and ban any peers that send an invalid header.

If a client is fetching full blocks from the P2P network, they SHOULD be downloaded
from outbound peers at random to mitigate privacy loss due to transaction
intersection analysis. Note that blocks may be downloaded from peers that do not
support this BIP.

== Rationale ==

The filter headers and checkpoints messages are defined to help clients identify
the correct filter for a block when connected to peers sending conflicting
information. An alternative solution is to require Bitcoin blocks to include
commitments to derived block filters, so light clients can verify authenticity
given block headers and some additional witness data. This would require a
network-wide change to the Bitcoin consensus rules, however, whereas this
document proposes a solution purely at the P2P layer.

The constant interval of 1,000 blocks between checkpoints was chosen so that,
given the current chain height and rate of growth, the size of a
<code>cfcheckpt</code> message is not drastically from a
<code>cfheaders</code> between two checkpoints. Also, 1,000 is a nice round
number, at least to those of us who think in decimal.

== Compatibility ==

This light client mode is not compatible with current node deployments and
requires support for the new P2P messages. The node implementation of this
proposal is not incompatible with the current P2P network rules (ie. doesn't
affect network topology of full nodes). Light clients may adopt protocols based
on this as an alternative to the existing BIP 37. Adoption of this BIP may
result in reduced network support for BIP 37.

== Acknowledgments ==

We would like to thank bfd (from the bitcoin-dev mailing list) for bringing the
basis of this BIP to our attention, Joseph Poon for suggesting the filter header
chain scheme, and Pedro Martelletto for writing the initial indexing code for
<code>btcd</code>.

We would also like to thank Dave Collins, JJ Jeffrey, Eric Lombrozo, and Matt
Corallo for useful discussions.

== Reference Implementation ==

Light client: [https://github.com/lightninglabs/neutrino]

Full-node indexing: https://github.com/Roasbeef/btcd/tree/segwit-cbf

Golomb-Rice Coded sets: https://github.com/Roasbeef/btcutil/tree/gcs/gcs

== References ==

<references/>

== Copyright ==

This document is licensed under the  Creative Commons CC0 1.0 Universal license.
