Subject: Re: [bitcoin-dev] BIP for Serverless Payjoin (AdamISZ)

From: Christopher Allen <[email-masked]>
Date: Fri, 11 Aug 2023 18:05:06 -0700

On Fri, Aug 11, 2023 at 3:29 PM symphonicbtc via bitcoin-dev <
[email-masked]> wrote:

> Quick little nit I noticed as well, are you sure base64 encoding is the
> best choice for the psk in the URI? You may find that having to urlencode
> the special characters in base64 it impacts readability and adds a layer of
> complexity if a human wanted to extract the psk from the URI for some
> reason. I suggest using something like [base64url](
> https://datatracker.ietf.org/doc/html/rfc4648#section-5) which modifies
> base64 slightly to be more suited to this purpose.

Yes, the URI version of base64 is better.

However, If you plan to display these via QRs, either will double the
density of the QR as QR libraries treat them as binary data (like hex of
hex data). Thus you may want to use UR encoding, which is what over a dozen
bitcoin wallets use to encode PSBTs. URs are very efficient with QRs, and
have the optional benefit that if the data carried becomes too large, they
can be animated. The have other advantages.

* A top level link about URs:
https://github.com/BlockchainCommons/crypto-commons/blob/master/Docs/README.md#uniform-resources-urs

* About the base64 encoding with QRs problem:
https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-003-uri-binary-compatibility.md

* The base UR tech spec:
https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-005-ur.md

* List of bitcoin wallets using UR for PSBTs:
https://github.com/blockchaincommons/gordian-developer-community#urs

* List of UR libraries:
https://github.com/BlockchainCommons/crypto-commons#bc-ur

Let me know if you’re interested.

— Christopher Allen