Announcing Bitcoin BOSD: Standardized Withdrawal Output Specification for L2s

[Jose Storopoli]

Dear Bitcoin Dev Community,

I am excited to introduce Bitcoin BOSD (Bitcoin Output Script Descriptor), a new specification and open source Rust implementation to simplify relay-safe (standardness-guarantee) on-chain withdrawals for Bitcoin Layer 2 solutions.

Key Features:

- Standardness-by-construction for withdrawal outputs
- Eliminates ad-hoc standardness rule implementations in L2s
- Compact representation

Problem Solved:
L2 operators currently risk being unable to process withdrawals if users request non-standard transactions (e.g., oversized OP_RETURN). BOSD ensures outputs are correct-by-construction according to Bitcoin network's standardness rules, abstracting away complex validation logic.

Available on crates.io

Get Involved:

Detailed Specification and Motivation/Rationale: SPECIFICATION.md
Repository: github.com/alpenlabs/bitcoin-bosd
Feedback welcome via GitHub Issues and Discussion; or the mailing list. 

This work emerged from practical challenges faced by myself and Trey Del Bonis (GitHub @delbonis) at Alpen Labs, while building validation logic for processing L2 withdrawals on-chain for Strata: a Bitcoin-native BitVM-based bridge to a ZK-rollup. We believe standardization in this domain will benefit the entire ecosystem by reducing fragmentation and improving interoperability.

The specification is has a CC0 public domain license, and the Rust implementation is MIT/Apache-2.0 double licensed.

The structure of a BOSD consists of:

- 1-byte tag representing a type ID.
- Arbitrary-length cryptographic payload (although practically bounded to like
  80 bytes).

Here's a table for the supported outputs. Also note that we can trivially extend it by adding more prefixes using the type it byte.

```

| Type | Payload Length(s) | Payload Interpretation | Spend Type    | Mainnet Address Prefix |
| ---- | ----------------- | ---------------------- | ------------- | ---------------------- |
| 0    | ..=80             | `OP_RETURN` payload    | (N/A)         | (N/A)                  |
| 1    | 20                | pubkey hash            | P2PKH         | `1...`                 |
| 2    | 20                | script hash            | P2SH          | `3...`                 |
| 3    | 20, 32            | SegWit v0 hash         | P2WPKH, P2WSH | `bc1q...`              |
| 4    | 32                | SegWit v1 public key   | P2TR          | `bc1p...`              |

```

Since we strived for compact representation both in-memory and also over-the-wire, BOSD does not support checksums or any native error correction.

PS: Technically Strata is not a defect L2, since it is a 1-of-N trust-minimized bridge, and not a fully trustless bridge.

[David A. Harding]

On 2025-02-15 01:37, Jose Storopoli wrote:
> Dear Bitcoin Dev Community,
>
> I am excited to introduce Bitcoin BOSD (Bitcoin Output Script
> Descriptor), a new specification and open source Rust implementation
> to simplify relay-safe (standardness-guarantee) on-chain withdrawals
> for Bitcoin Layer 2 solutions.
>
> Key Features:
> - Standardness-by-construction for withdrawal outputs
> - Eliminates ad-hoc standardness rule implementations in L2s
> - Compact representation

It's unclear to me why you don't simply implement your selected
standardness rules on top of a BIP385 raw() output script descriptor
(with the normal BIP380 checksum)? That gives you an efficient
representation, compatibility with other software, a compact checksum,
and local policy.

For the later point, policy being local, there exists software[1] that I
believe has a moderate userbase (including possibly some miner support)
that implements less restrictive standard transaction rules than Bitcoin
Core, specifically allowing OP_RETURN outputs of more than 83 bytes
cumulative.[2] If a user of your software runs that node and believes
his transactions will relay to a compatible miner, what options does he
have of circumventing BOSD type 0's limit of 80 push bytes if one of his
users wants to send him such a data carrier output? He can propose a
type 5, but you'll probably reject that because >80 bytes will be
non-standard for other users. You can create an extension numeric
range, but then you'll have to centrally coordinate number assignment
for every random proposal. Instead, I think it makes more sense to not
use versioning at all and just perform local verification on raw()
descriptors; that way users of your software can modify one line of code
on their side to accept >80 bytes and everyone else can use the default
tests.

In addition to the above technical criticism, I find the BOSD name
extremely conflationary with the longstanding use of "output script
descriptors", or "descriptors" for short, to refer to the language that
originated in 2018 from Bitcoin Core developers for describing output
scripts.[3] I think it would be helpful to everyone for your proposal
to use a more distinct name.

Thanks,

-Dave

[1] https://github.com/petertodd/bitcoin/tree/libre-relay-v28.1
[2]
https://github.com/bitcoin/bitcoin/commit/d67f4634e5395fbdf4383d7adcdfc92c0cca7fc9
[3] https://bitcoinops.org/en/topics/output-script-descriptors/

[Martin Habovštiak]

Hi,

good idea and a nice crate! You'll hear from me soon regarding the Rust side on your issue tracker.

Maybe you should reconsider the name. It's too similar to BSOD and myabe you haerd taht if fisrt and lsat lteter in a wrod are the smae yuor brian can dcedoe tehm anwyay. :)

Good luck!

Martin

Dňa so 15. 2. 2025, 13:02 Jose Storopoli <jo...@storopoli.io> napísal(a):

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