BBQr

BBQr - Better Bitcoin QR

Encodes larger files into a series of QR codes so they can cross air gaps.

Project Status: New & proposed — looking for feedback

Project Home on Github

Specification

See full spec BBQr.md.

Summary

This protocol enables files larger than can fit into a single QR to be sent as a series of QR codes (sometimes called an “animated QR”). The target file types are PSBT (BIP-174) and signed Bitcoin transactions, but it also supports CBOR, JSON and Text options for general purpose use.

We carefully consider the data inside QR codes, and apply a deep knowledge of how QR codes work, so that no pixel nor byte is ever wasted! Internally it supports HEX and Base32 serializations and a constrained ZLIB option for data compression. This is all done with an eye to embedded implementations on very contrained devices (ie. hardware wallets), which may not have enough memory to keep more than a single QR code around.

Here are some compression numbers, using the ZLIB encoding option. Even though Bitcoin files have relatively high entropy (with hashes and UTXO’s being non-compressable) we still see 30% typical size reduction.

File (see testing/data) Before After Compression Ratio
1in1000out.psbt 35644 22095 38.0%
1in100out.psbt 4142 2654 35.9%
1in10out.psbt 992 670 32.5%
1in20out.psbt 1342 897 33.2%
1in2out.psbt 675 458 32.1%
devils-txn.txn 666 356 46.5%
finalized-by-ckcc.txn 1932 807 58.2%
signed.txn 100757 77090 23.5%

By using Base32 character encoding inside a QR code with the unique “alphanumeric” encoding (where each character takes 5.5 bits of space, and encodes 5 bits of binary), we can acheive the smallest possible QR codes.

The largest possible (uncompressed) data payload in 3,470,600 bytes spread across in 1,295 version 40 QR codes.

Example Image

Example of BBQr Image

The above BBQr encodes the entire specification itself (so meta).

% bbqr make BBQr.md -v 21 -o example.png --scale 3
Detected file type: U -> Unicode Text
Need 8 QR's each of version 20.
Building QR images... done!
Created 'example.png' with 8 frames.

Code Examples

License

Public Domain code by Coinkite

Useful Pipelines

These will load your clipboard with example data suitable for the COLDCARD Q Simulator.

See also psbt_faker

% psbt_faker - | bbqr make - -t P | pbcopy
A single QR version 18 will be needed.

% bbqr make - --fake-data 2048000 -t P | pbcopy
A single QR version 35 will be needed.

% psbt_faker -n 200 - | bbqr make - -t P -r | pbcopy
Need 5 QR's each of version 37.

These are round-trip examples, where encode and decode are performed.

% psbt_faker -n 10 - | bbqr make - -t P -r | bbqr decode
A single QR version 25 will be needed.
PSBT File:
cHNidP....

Signing Transaction with COLDCARD Q

1) Using COLDCARD Q choose Scan Any QR Code from main menu and scan this Seed QR to temporarily import a new seed:

simulator_seed_qr

- This seed is the COLDCARD simulator key and is well known!
- DO NOT send any funds to this seed.

2) Confirm import of temporary seed and if Seed Vault is enabled you may save it there (not required).

3) Navigate to Scan Any QR Code again, and scan either of these transactions. You’ll be shown the details, and if accepted, the signed result will be shown as BBQr.

PSBT: Basic 1 input 2 outputs

tx-1in2out

PSBT: 10 in, 2 out with Locktimes

tx-10in2out-locktimes