# Rasterbar Software

## Software developement and consulting

# BitTorrent extension for arbitrary DHT store

Author: | Arvid Norberg, arvid@rasterbar.com
Version: | 1.0.0

Table of contents

- terminology
- messages
- immutable items
  - put message
  - get message
- mutable items
  - put message
  - get message
- signature verification
- expiration
- test vectors

This is a proposal for an extension to the BitTorrent DHT to allow storing and retrieving of arbitrary data.

It supports both storing immutable items, where the key is the SHA-1 hash of the data itself, and mutable items, where the key is the public key of the key pair used to sign the data.

There are two new proposed messages, `put` and `get`.

# terminology

In this document, a storage node refers to the node in the DHT to which an item is being announced and stored on. A subscribing node refers to a node which makes look-ups in the DHT to find the storage nodes, to request items from them, and possibly re-announce those items to keep them alive.

# messages

The proposed new messages `get` and `put` are similar to the existing `get_peers` and `announce_peer`.

Responses to `get` should always include `nodes` and `nodes6` has the same semantics as in its `get_peers` response. It should also include a write token, `token`, with the same semantics as `get_peers`.

The `id` field in these messages has the same semantics as the standard DHT messages, i.e. the node ID of the node sending the message, to maintain the structure of the DHT network.

The `token` field also has the same semantics as the standard DHT message `get_peers` and `announce_peer`, when requesting an item and to write an item respectively.

The `k` field is the PKCS#1 encoded 2048 bit RSA public key, which the signature can be authenticated with. When looking up a mutable item, the `target` field MUST be the SHA-1 hash of this key.

The distinction between storing mutable and immutable items is the inclusion of a public key, a sequence number and signature (`k`, `seq` and `sig`).

`get` requests for mutable items and immutable items cannot be distinguished from eachother. An implementation can either store mutable and immutable items in the same hash table internally, or in separate ones and potentially do two lookups for `get` requests.

The `v` field is the value to be stored. It is allowed to be any bencoded type (list, dict, string or integer). When it's being hashed (for verifying its signature or to calculate its key), its flattened, bencoded, form is used. It is important to use the exact bencoded representation as it appeared in the message. decoding and then re-encoding bencoded structures is not necessarily an identity operation.

Storing nodes SHOULD reject `put` requests where the bencoded form of `v` is longer than 767 bytes.

# immutable items

Immutable items are stored under their SHA-1 hash, and since they cannot be modified, there is no need to authenticate the origin of them. This makes immutable items simple.

A node making a lookup SHOULD verify the data it receives from the network, to verify that its hash matches the target that was looked up.

## put message

Request:

    {
            "a":
            {
                    "id": <20 byte id of sending node (string)>,
                    "v": <any bencoded type, whose encoded size < 768>
            },
            "t": <transaction-id (string)>,
            "y": "q",
            "q": "put"
    }

Response:

    {
            "r": { "id": <20 byte id of sending node (string)> },
            "t": <transaction-id (string)>,
            "y": "r",
    }

## get message

Request:

    {
            "a":
            {
                    "id": <20 byte id of sending node (string)>,
                    "target": <SHA-1 hash of item (string)>,
            },
            "t": <transaction-id (string)>,
            "y": "q",
            "q": "get"
    }

Response:

    {
       "r":
            {
                    "id": <20 byte id of sending node (string)>,
                    "token": <write token (string)>,
                    "v": <any bencoded type whose SHA-1 hash matches 'target'>,
                    "nodes": <IPv4 nodes close to 'target'>,
                    "nodes6": <IPv6 nodes close to 'target'>
            },
            "t": <transaction-id>,
            "y": "r",
    }

# mutable items

Mutable items can be updated, without changing their DHT keys. To authenticate that only the original publisher can update an item, it is signed by a private key generated by the original publisher. The target ID mutable items are stored under is the SHA-1 hash of the public key (as it appears in the `put` message).

In order to avoid a malicious node to overwrite the list head with an old version, the sequence number `seq` must be monotonically increasing for each update, and a node hosting the list node MUST not downgrade a list head from a higher sequence number to a lower one, only upgrade. The sequence number SHOULD not exceed `MAX_INT64`, (i.e. `0x7fffffffffffffff`. A client MAY reject any message with a sequence number exceeding this.

The signature is a 2048 bit RSA signature of the SHA-1 hash of the bencoded sequence number and `v` key. e.g. something like this:: `3:seqi4e1:v12:Hello world!`.

## put message

Request:

    {
            "a":
            {
                    "id": <20 byte id of sending node (string)>,
                    "k": <RSA-2048 public key (PKCS#1 encoded)>,
                    "seq": <monotonically increasing sequence number (integer)>,
                    "sig": <RSA-2048 signature (256 bytes string)>,
                    "token": <write-token (string)>,
                    "v": <any bencoded type, whose encoded size < 768>
            },
            "t": <transaction-id (string)>,
            "y": "q",
            "q": "put"
    }

Storing nodes receiving a `put` request where `seq` is lower than what's already stored on the node, MUST reject the request.

Response:

    {
            "r": { "id": <20 byte id of sending node (string)> },
            "t": <transaction-id (string)>,
            "y": "r",
    }

## get message

Request:

    {
            "a":
            {
                    "id": <20 byte id of sending node (string)>,
                    "target:" <20 byte SHA-1 hash of public key (string)>
            },
            "t": <transaction-id (string)>,
            "y": "q",
            "q": "get"
    }

Response:

    {
            "r":
            {
                    "id": <20 byte id of sending node (string)>,
                    "k": <RSA-2048 public key (268 bytes string)>,
                    "nodes": <IPv4 nodes close to 'target'>,
                    "nodes6": <IPv6 nodes close to 'target'>,
                    "seq": <monotonically increasing sequence number (integer)>,
                    "sig": <RSA-2048 signature (256 bytes string)>,
                    "token": <write-token (string)>,
                    "v": <any bencoded type, whose encoded size < 768>
            },
            "t": <transaction-id (string)>,
            "y": "r",
    }

# signature verification

In order to make it maximally difficult to attack the bencoding parser, signing and verification of the value and sequence number should be done as follows:

1. encode value and sequence number separately
2. concatenate "3:seqi" `seq` "e1:v" and the encoded value. sequence number 1 of value "Hello World!" would be converted to: 3:seqi1e1:v12:Hello World! In this way it is not possible to convince a node that part of the length is actually part of the sequence number even if the parser contains certain bugs. Furthermore it is not possible to have a verification failure if a bencoding serializer alters the order of entries in the dictionary.
3. hash the concatenated string with SHA-1
4. sign or verify the hash digest.

On the storage node, the signature MUST be verified before accepting the store command. The data MUST be stored under the SHA-1 hash of the public key (as it appears in the bencoded dict).

On the subscribing nodes, the key they get back from a `get` request MUST be verified to hash to the target ID the lookup was made for, as well as verifying the signature. If any of these fail, the response SHOULD be considered invalid.

# expiration

Without re-announcement, these items MAY expire in 2 hours. In order to keep items alive, they SHOULD be re-announced once an hour.

Subscriber nodes MAY help out in announcing items the are interested in to the DHT, to keep them alive.

# test vectors