SSB Binary Field Encodings Specification

Shortname: ssb-bfe-spec

Definition of binary encodings for common types such as message IDs and feed IDs to be used in binary feed formats.

The binary encoding is defined as the concatenation of three parts, often known as T-F-D:

• The type of thing as a UInt8 byte
• The format of the type as a UInt8 byte
• The data as a sequence of UInt8 bytes

Types

0. Feed ID formats

A feed ID TFD represents the public portion of a cryptographic keypair used to identify a feed, and verify message signatures.

Example

Given a sigil-based string encoding of a classic SSB feed:

  @6CAxOI3f+LUOVrbAl0IemqiS7ATpQvr9Mdw9LC4+Uv0=.ed25519
  │└─────────────────────┬────────────────────┘└───┬──┘
 sigil           base64 encoded data            suffix

Its BFE encoding is the following bytes displayed in hexadecimal:

  00 00 e8 20 31 38 8d df f8 b5 0e 56 b6 c0 97 42 1e 9a a8 92 ec 04 e9 42 fa fd 31 dc 3d 2c 2e 3e 52 fd
   │  │ └────────────────────┬────────────────────────────────────────────────────────────────────────┘
type  │                    data
     format

1. Message ID formats

A message ID TFD represents the hash that uniquely identifies a message published on a feed. Some message ID formats directly reference the hash algorithm utilized, while others leave it implicit in the specification.

Example

Given a sigil-based string encoding of a classic SSB message ID:

  %R8heq/tQoxEIPkWf0Kxn1nCm/CsxG2CDpUYnAvdbXY8=.sha256
  │└─────────────────────┬────────────────────┘└──┬──┘
 sigil           base64 encoded data            suffix

Its BFE encoding is the following bytes displayed in hexadecimal:

  01 00 47 c8 5e ab fb 50 a3 11 08 3e 45 9f d0 ac 67 d6 70 a6 fc 2b 31 1b 60 83 a5 46 27 02 f7 5b 5d 8f
   │  │ └────────────────────┬────────────────────────────────────────────────────────────────────────┘
type  │                    data
     format

2. Blob ID formats

A blob ID TFD represents the hash that uniquely identifies the blob.

Example

Given a sigil-based string encoding of a classic SSB blob ID:

  &S7+CwHM6dZ9si5Vn4ftpk/l/ldbRMqzzJos+spZbWf4=.sha256
  │└─────────────────────┬────────────────────┘└───┬─┘
 sigil           base64 encoded data            suffix

Its BFE encoding is the following bytes displayed in hexadecimal:

  02 00 4b bf 82 c0 73 3a 75 9f 6c 8b 95 67 e1 fb 69 93 f9 7f 95 d6 d1 32 ac f3 26 8b 3e b2 96 5b 59 fe
   │  │ └────────────────────┬────────────────────────────────────────────────────────────────────────┘
type  │                    data
     format

3. Encryption Key formats

Keys used for encryption

4. Signature formats

Example

Given a base64 string encoding of a Classic SSB ed25519 signature:

  nkY4Wsn9feosxvX7bpLK7OxjdSrw6gSL8sun1n2TMLXKySYK9L5itVQnV2nQUctFsrUOa2istD2vDk1B0uAMBQ==.sig.ed25519
  └─────────────────────────────────────┬────────────────────────────────────────────────┘└────┬─────┘
                             base64 encoded signature                                        suffix

Its BFE encoding is the following bytes displayed in hexadecimal:

  04 00 9e 46 38 5a c9 fd 7d ea 2c c6 f5 fb 6e 92 ca ec ec 63 75 2a f0 ea 04 8b f2 cb a7 d6 7d 93 30 b5 ca c9 26 0a f4 be 62 b5 54 27 57 69 d0 51 cb 45 b2 b5 0e 6b 68 ac b4 3d af 0e 4d 41 d2 e0 0c 05
   │  │ └────────────────────┬────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
type  │                    data
     format

5. Encrypted data formats

When content is encrypted (in other words, “boxed”) in SSB, it is provided as uninterpretable bytes, plus a tag that identifies which algorithm was used for encrypting it, such as box or box2.

6. Generic data formats

BFE supports encoding data types with no semantics attached to them. They are merely categorized into formats that represent their data type.

7. Identity formats

Identities are distinct from feedIds in that they are not a key bound to a single feed/ device, and they are never used for signing of messages.