Inscriptions
============

Inscriptions inscribe sats with arbitrary content, creating bitcoin-native
digital artifacts, more commonly known as NFTs. Inscriptions do not require a
sidechain or separate token.

These inscribed sats can then be transferred using bitcoin transactions, sent
to bitcoin addresses, and held in bitcoin UTXOs. These transactions, addresses,
and UTXOs are normal bitcoin transactions, addresses, and UTXOS in all
respects, with the exception that in order to send individual sats,
transactions must control the order and value of inputs and outputs according
to ordinal theory.

The inscription content model is that of the web. An inscription consists of a
content type, also known as a MIME type, and the content itself, which is a
byte string. This allows inscription content to be returned from a web server,
and for creating HTML inscriptions that use and remix the content of other
inscriptions.

Inscription content is entirely on-chain, stored in taproot script-path spend
scripts. Taproot scripts have very few restrictions on their content, and
additionally receive the witness discount, making inscription content storage
relatively economical.

Since taproot script spends can only be made from existing taproot outputs,
inscriptions are made using a two-phase commit/reveal procedure. First, in the
commit transaction, a taproot output committing to a script containing the
inscription content is created. Second, in the reveal transaction, the output
created by the commit transaction is spent, revealing the inscription content
on-chain.

Inscription content is serialized using data pushes within unexecuted
conditionals, called "envelopes". Envelopes consist of an `OP_FALSE OP_IF …
OP_ENDIF` wrapping any number of data pushes. Because envelopes are effectively
no-ops, they do not change the semantics of the script in which they are
included, and can be combined with any other locking script.

A text inscription containing the string "Hello, world!" is serialized as
follows:

```
OP_FALSE
OP_IF
  OP_PUSH "ord"
  OP_PUSH 1
  OP_PUSH "text/plain;charset=utf-8"
  OP_PUSH 0
  OP_PUSH "Hello, world!"
OP_ENDIF
```

First the string `ord` is pushed, to disambiguate inscriptions from other uses
of envelopes.

`OP_PUSH 1` indicates that the next push contains the content type, and
`OP_PUSH 0`indicates that subsequent data pushes contain the content itself.
Multiple data pushes must be used for large inscriptions, as one of taproot's
few restrictions is that individual data pushes may not be larger than 520
bytes.

The inscription content is contained within the input of a reveal transaction,
and the inscription is made on the first sat of its input if it has no pointer
field. This sat can then be tracked using the familiar rules of ordinal
theory, allowing it to be transferred, bought, sold, lost to fees, and recovered.

Content
-------

The data model of inscriptions is that of a HTTP response, allowing inscription
content to be served by a web server and viewed in a web browser.

Fields
------

Inscriptions may include fields before an optional body. Each field consists of
two data pushes, a tag and a value.

Currently, there are six defined fields:

- `content_type`, with a tag of `1`, whose value is the MIME type of the body.
- `pointer`, with a tag of `2`, see [pointer docs](inscriptions/pointer.md).
- `parent`, with a tag of `3`, see [provenance](inscriptions/provenance.md).
- `metadata`, with a tag of `5`, see [metadata](inscriptions/metadata.md).
- `metaprotocol`, with a tag of `7`, whose value is the metaprotocol identifier.
- `content_encoding`, with a tag of `9`, whose value is the encoding of the body.
- `delegate`, with a tag of `11`, see [delegate](inscriptions/delegate.md).

The beginning of the body and end of fields is indicated with an empty data
push.

Unrecognized tags are interpreted differently depending on whether they are
even or odd, following the "it's okay to be odd" rule used by the Lightning
Network.

Even tags are used for fields which may affect creation, initial assignment, or
transfer of an inscription. Thus, inscriptions with unrecognized even fields
must be displayed as "unbound", that is, without a location.

Odd tags are used for fields which do not affect creation, initial assignment,
or transfer, such as additional metadata, and thus are safe to ignore.

Inscription IDs
---------------

The inscriptions are contained within the inputs of a reveal transaction. In
order to uniquely identify them they are assigned an ID of the form:

`521f8eccffa4c41a3a7728dd012ea5a4a02feed81f41159231251ecf1e5c79dai0`

The part in front of the `i` is the transaction ID (`txid`) of the reveal
transaction. The number after the `i` defines the index (starting at 0) of new inscriptions
being inscribed in the reveal transaction.

Inscriptions can either be located in different inputs, within the same input or
a combination of both. In any case the ordering is clear, since a parser would
go through the inputs consecutively and look for all inscription `envelopes`.

| Input | Inscription Count | Indices    |
|:-----:|:-----------------:|:----------:|
| 0     | 2                 | i0, i1     |
| 1     | 1                 | i2         |
| 2     | 3                 | i3, i4, i5 |
| 3     | 0                 |            |
| 4     | 1                 | i6         |

Inscription Numbers
-------------------

Inscriptions are assigned inscription numbers starting at zero, first by the
order reveal transactions appear in blocks, and the order that reveal envelopes
appear in those transactions.

Due to a historical bug in `ord` which cannot be fixed without changing a great
many inscription numbers, inscriptions which are revealed and then immediately
spent to fees are numbered as if they appear last in the block in which they
are revealed.

Inscriptions which are cursed are numbered starting at negative one, counting
down. Cursed inscriptions on and after the jubilee at block 824544 are
vindicated, and are assigned positive inscription numbers.

Sandboxing
----------

HTML and SVG inscriptions are sandboxed in order to prevent references to
off-chain content, thus keeping inscriptions immutable and self-contained.

This is accomplished by loading HTML and SVG inscriptions inside `iframes` with
the `sandbox` attribute, as well as serving inscription content with
`Content-Security-Policy` headers.

Self-Reference
--------------

The content of the inscription with ID `INSCRIPTION_ID` must served from the
URL path `/content/<INSCRIPTION_ID>`.

This allows inscriptions to retrieve their own inscription ID with:

```js
let inscription_id = window.location.pathname.split("/").pop();
```

If an inscription with ID X delegates to an inscription with ID Y, that is to
say, if inscription X contains a delegate field with value Y, the content of
inscription X must be served from the URL path `/content/X`, *not*
`/content/Y`.

This allows delegating inscriptions to use their own inscription ID as a seed
for generative delegate content.

Reinscriptions
--------------

Previously inscribed sats can be reinscribed with the `--reinscribe` command if
the inscription is present in the wallet. This will only append an inscription to
a sat, not change the initial inscription.

Reinscribe with satpoint:
`ord wallet inscribe --fee-rate <FEE_RATE> --reinscribe --file <FILE> --satpoint <SATPOINT>`

Reinscribe on a sat (requires sat index):
`ord --index-sats wallet inscribe --fee-rate <FEE_RATE> --reinscribe --file <FILE> --sat <SAT>`
