Bendy Butt

What is Bendy Butt?

A new feed format designed for meta feeds

Why a new feed format, when we already have gabby grove and bamboo?

Because we would want to offer a feed format that is simple and easy to implement in any programming language by reusing an established format (bencode) with existing encoder libraries. Furthermore, as meta feeds makes it easier to have multiple feed formats, we wanted to show that creating a new feed format does not have to be hard.

Can't you just use the classic format?

That would mean clients would have to support reading and writing the classic format forever, even for applications that only use newer feed formats.

What is wrong with the classic format?

There are multiple problems, most of them come from the fact that it is very tied to the internals of the v8 engine for Javascript. JSON is good as an exchange format for values of data but not well suited to cryptographically sign messages where a canonical representation of each value is paramount.

Can I use Bendy Butt for purposes other than meta feeds?

Yes, but Bendy Butt was not designed as a general purpose feed format. We do not intend to extend Bendy Butt to cover any other use cases besides meta feeds, but if the feed format constraints are sufficient for your use cases, nothing stops you from using it. To preserve the simplicity of this spec, we recommend not sending patches to the spec to generalize it.

Definitions

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.

Notations for bencode and BFE

To avoid confusion with JSON, we shall use the notation { "a" => 1, "b" => 2 } to denote a bencode dictionary with two entries: key "a" mapping to value 1 and key "b" mapping to value 2. We shall use the notation [a, b, c] to denote a bencode list of three values a, b, and c.

For sequences of bytes, we use the notation <03 02 01> to represent the sequence of bytes 03 followed by 02 followed by 01. We use the notation a + b + c to denote byte concatenation of the byte sequences a followed by b followed by c. In other words <05 04> + <03 02 01> equals <05 04 03 02 01>.

For abstract substitutions, we use (something) to denote whatever something describes. In other words, anything wrapped in parentheses is just a placeholder for something else.

Specification

A Bendy Butt message is encoded in bencode as a list of message payload and signature, which we shall denote [payload, signature]. SSB Binary Field Encodings (SSB-BFE) is used to encode all non-list non-dictionary non-integer values contained in the payload and signature. BFE also helps to disambiguate binary data like feed IDs and message IDs from generic data types such as strings.

The message payload is a bencode list with 5 elements in this specific order, in other words, [author, sequence, previous, timestamp, contentSection]:

1. author, a BFE-encoded "feed ID"
2. sequence, a bencode integer greater than or equal to 1
3. previous, a BFE-encoded "message ID" of the previous message on the feed. For the first message this must be the BFE "nil" generic data type.
4. timestamp, a bencode integer representing the UNIX epoch timestamp of message creation.
5. contentSection can be one of two possible shapes:
   1. [content, contentSignature] such that content is any bencode dictionary representing the message contents, all of which are internally encoded with BFE; and contentSignature is a BFE-encoded "signature". The signature is applied on prefix + content, where prefix is the string "bendybutt" encoded as UTF8 bytes. The signature uses some cryptographic keypair which MAY be different from the author keypair (for example, under the meta feeds spec, this will be the subfeed keypair).
   2. BFE-encoded "encrypted data". Upon decryption, it takes the shape [content, contentSignature] as described above.

The message signature is applied on payload using the author's cryptographic keypair.

Both the signature and the contentSignature use the same HMAC signing capability (sodium.crypto_auth) and sodium.crypto_sign_detached as in the classic SSB format (ed25519).

The key or ID of a Bendy Butt message is the SHA256 hash of the bencoded bytes for [payload, signature], i.e. key = SHA256([payload, signature]).

Example

Following our [a, b] and { "a" => 1, "b" => 2} notation, which may look similar to JSON, here is a Bendy Butt message with the content dictionary equal to { "type" => 'greet', "text" => 'Good morning!' }.

[
  [
    <00 03 5c 27 ac 6e f0 cd fb d0 f8 9a 89 a1 b6 5a 36 04 77 a3 3e c7 9c b7 ab 14 cd 90 76 25 59 be e2 ff>,
    1,
    <06 02>,
    12345,
    [
      {
        "type" => <06 00 67 72 65 65 74>,
        "text" => <06 00 47 6f 6f 64 20 6d 6f 72 6e 69 6e 67 21>
      }
      <04 00 51 a6 7a 43 6a 66 f6 6d e0 3d 77 73 c0 b7 ba 98 84 61 32 46 c6 ee 6c 74 1b 1d 9e 59 18 24 b3 c7 1d a3 ec 35 bf e0 32 cf 86 55 7c f8 72 30 e9 56 ... 16 more bytes>
    ]
  ],
  <04 00 6d 57 9f 55 14 d2 d8 69 09 ad 7b 31 f8 24 4f a7 fc 6a 0d c1 1e f4 1a 92 71 86 fb 8d 1b fc d5 17 b3 88 05 f0 a6 48 aa ba 24 f4 46 b0 9e 65 64 b6 ... 16 more bytes>
]

Notice that is has the shape:

[
  [
    author,
    sequence,
    previous,
    timestamp,
    [
      content,
      contentSignature
    ]
  ],
  signature
]

Here is the same data as above, but displayed in raw binary (hexadecimal):

    Offset   00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
    000000   6C 6C 33 34 3A 00 03 5C 27 AC 6E F0 CD FB D0 F8   ll34:..\'¬nðÍûÐø
    000010   9A 89 A1 B6 5A 36 04 77 A3 3E C7 9C B7 AB 14 CD   ..¡¶Z6.w£>Ç.·«.Í
    000020   90 76 25 59 BE E2 FF 69 31 65 32 3A 06 02 69 31   .v%Y¾âÿi1e2:..i1
    000030   32 33 34 35 65 6C 64 34 3A 74 65 78 74 31 35 3A   2345eld4:text15:
    000040   06 00 47 6F 6F 64 20 6D 6F 72 6E 69 6E 67 21 34   ..Good morning!4
    000050   3A 74 79 70 65 37 3A 06 00 67 72 65 65 74 65 36   :type7:..greete6
    000060   36 3A 04 00 51 A6 7A 43 6A 66 F6 6D E0 3D 77 73   6:..Q¦zCjfömà=ws
    000070   C0 B7 BA 98 84 61 32 46 C6 EE 6C 74 1B 1D 9E 59   À·º..a2FÆîlt...Y
    000080   18 24 B3 C7 1D A3 EC 35 BF E0 32 CF 86 55 7C F8   .$³Ç.£ì5¿à2Ï.U|ø
    000090   72 30 E9 56 8E D5 7B 25 F6 77 FE 58 3B 17 3D BD   r0éV.Õ{%öwþX;.=½
    0000A0   E7 08 82 0F 65 65 36 36 3A 04 00 6D 57 9F 55 14   ç...ee66:..mW.U.
    0000B0   D2 D8 69 09 AD 7B 31 F8 24 4F A7 FC 6A 0D C1 1E   ÒØi.­{1ø$O§üj.Á.
    0000C0   F4 1A 92 71 86 FB 8D 1B FC D5 17 B3 88 05 F0 A6   ô..q.û..üÕ.³..ð¦
    0000D0   48 AA BA 24 F4 46 B0 9E 65 64 B6 9A DE 97 F9 18   Hªº$ôF°.ed¶.Þ.ù.
    0000E0   04 AF 5F 7A F3 5E 5D 4B FD 85 0B 65               .¯_zó^]Ký..e

Validation

For validation we differentiate between message validity and content validity. Since the content can be encrypted, there is potentially no way to validate that. This means a message should only be rejected before insertion into the local database if it fails the message validation rules. A message should not be included in the bendy butt feed if its content is not valid.

The message MUST conform to the following rules:

• Has the shape of a bencode list [payload, signature]
• The payload has the shape of bencode list [author, sequence, previous, timestamp, contentSection]
• The previous field matches the previous message's ID
• The signature field must be correctly signing the payload
• The SSB-BFE type-format for author is 0x0003 (there is no upgradeability in the middle of a feed)
• The SSB-BFE type-format for previous is 0x0104
• The author is the same as in previous messages on the feed
• The maximum size of a [payload, signature] in bytes must not exceed 8192 bytes

Under the Bendy Butt spec, the content is free-form, but under the meta feeds spec there are strict constraints applied to this bencode dictionary.