The first line of a Pigeon message header is the `author` entry.
Every Pigeon database has an "identity". An identity is an ED25519 key pair that prevents tampering by parties other than the database owner. An identity is publicly referenced using a "multihash". In the example above, the identity multihash was `@MF312A76JV8S1XWCHV1XR6ANRDMPAT2G5K8PZTGKWV354PR82CD0.ed25519`.
The steps to generate a valid identity are:
1. Perform [Crockford Base32 encoding](https://www.crockford.com/base32.html) on an ED25519 public key.
2. Add an `@` symbol to the beginning of the string from step 1.
3. Add a `.ed25519` string to the end of the string from step 2.
The second line of the header is the `kind` entry. This entry is user definable. The `kind` entry is used as a means of signalling intent to applications that will consume the message.
It must meet the following criteria:
* Must be 1-90 characters in length
* Cannot contain whitespace or control characters
* May contain any of the following characters:
* alphanumeric characters
* dashes (`-`)
* underscores (`_`)
* Symbols used for multihashes, such as `@`, `&` and `%` (covered later).
A Pigeon message feed is a unidirectional chain of documents where the newest document points back to the document that came before it in the chain ([example diagram](diagram1.png)).
To create this chain, a Pigeon message uses the `prev` field. The `prev` field contains a message multihash. In this case, the multihash is `%ZV85NQS8B1BWQN7YAME1GB0G6XS2AVN610RQTME507DN5ASP2S6G.sha256`.
Messages are content addressed. This is in contrast to protocols such as HTTP which use names to identify resources. Because Pigeon messages are addressed by content rather than by name, changing a message's content, even by just one character, has the effect of completely changing the message's multihash.
**For the first message of a feed, this value is set to `NONE`.**
Message multihashes are calculated as follows:
1. The first character is a `%` symbol, indicating that it is a `message` rather than an `identity`, `blob` or `string`.
2. The next 52 characters are a [Crockford base 32](https://www.crockford.com/base32.html) SHA512 hash of the previous message's content.
Pigeon messages exist in a linear sequence which only moves forward and never "forks".
Every message has a `depth` field to indicate its "place in line".
Because every message has an ever-increasing integer that never duplicates, every message in a Pigeon feed will have a unique hash. This is true even if messages have identical body content.
**THIS FIELD WAS WRITTEN INCORRECTLY. THIS WILL CHANGE SOON. YOU CAN SAFELY MOVE TO THE NEXT SECTION OF THE DOCS**
This concept was borrowed from the [Bamboo protocol](https://github.com/AljoschaMeyer/bamboo#links-and-entry-verification) and [Helger Lipmaa's thesis](https://kodu.ut.ee/~lipmaa/papers/thesis/thesis.pdf).
The `lipmaa` field (often called a "Lipmaa Link") is a special kind of `prev` field that allows partial verification of feeds. This field makes it possible to verify a single message (or subset of messages) without downloading the entire chain of messages.
Applications may attach files to messages in the form of blobs. Blobs are referenced using a blob multihash.
* Starts with a `&` character.
* Ends with `.sha256`
* Contains exactly 52 characters between the `&` and `.sha256` parts. This is a SHA256 hash of the blob's content, represented in Crockford Base 32 encoding.
A blob is referenced in a message's key or value. A client will include a blob's content in a "bundle" (explained later).