Feedback-ID: i525146e8:Fastmail
Date: Fri, 4 Aug 2023 17:39:03 +0000
From: Peter Todd <pete@petertodd.org>
To: bitcoin-dev@lists.linuxfoundation.org
Message-ID: <ZM03twumu88V2NFH@petertodd.org>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
 protocol="application/pgp-signature"; boundary="SB8PAdHnrhigqCwh"
Content-Disposition: inline
Subject: [bitcoin-dev] BIP-352 Silent Payments addresses should have an
	expiration time
Precedence: list


--SB8PAdHnrhigqCwh
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

tl;dr: Wallets don't last forever. They are often compromised or lost. When
this happens, the addresses generated from those wallets become a form of t=
oxic
data: funds sent to those addresses can be easily lost forever.

All Bitcoin addresses have this problem. But at least existing Bitcoin
addresses aren't supposed to be reused. Silent Payments are: the whole poin=
t is
to have a single address that you can safely pay to multiple times, without
privacy concerns. Failing to make Silent Payment addresses eventually expir=
e in
a reasonable amount of time is thus a particularly harmful mistake.

Fixing this is easy: add a 3 byte field to silent payments addresses, encod=
ing
the expiration date in terms of days after some epoch. 2^24 days is 45,000
years, more than enough. Indeed, 2 bytes is probably fine too: 2^16 days is=
 180
years. We'll be lucky if Bitcoin still exists in 180 years.

Wallets should pick a reasonable default, eg 1 year, for newly created
addresses. Attempts to pay an expired address should just fail with a simple
"address expired". Lightning invoices are a good example here: while invoic=
es
does not require expiration from a technical point of view, they do expire =
for
similar UX reasons as applies to silent payments.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

--SB8PAdHnrhigqCwh
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iQIzBAEBCgAdFiEE0RcYcKRzsEwFZ3N5Lly11TVRLzcFAmTNN7QACgkQLly11TVR
Lzcqag//XGfqhEuXQMff0t8/BdgkNuOlW0JVbDcbQkoqtlvjtKEQNSvhSCPO6CVk
8GXc59qBLGe0oz+O5BdRrwnymEGj3bu9cSLm9l9n/t/LPZ4i6/3TIG1t4f5VVvuD
jmFA/1/tneA7Gv6ZGj1pxbmpZ1XluhrGLRrLi52BPjbXExIIfzoJWjnVvhUSjzvO
X966+Kj3Y1iIe2BOUfncT/Ig0tViU0JfbGXsLD4skTc7Jn8VrUWvpp55DMy8cRuK
XtIu660reaFLJVsC67IsvZJdTJR++Jsa4lhIuoCNnRLU50YyVckdUlsoOiZcfI0v
W5pC28fIcf3CAIjUDJMV3sp7DdEhNH8MTFGngZHuGqNLnj3KRbQ55bBkaUlliKNn
NRxoLebKLenbGweO7mxT4BMa0K4N3GA/7Pan4HYF4/p7soGjvTO/j8Lh/EqYvoF2
c4KF0LlHfnnCaHlAflIgar1RsnQ0dBZtA439sDQhKaGc2dkevsztU+gj9PDqNPRv
eaW6NLwhgbU8VV1iR6qeF11tPe8nfP+fpK0Abz3zYWnsnPttG1OvUVhAWJGxjNrp
SD+zTi4Q5WSJ9JnFP70Eep4CXtC2qwWSRUxMKCg5jsJtGrhzi3yMRXzYegRweO+A
mc5dv37uTGnc5Le981X+TbQI77AKxbajXd5jY/irCK59WnoVFOg=
=2pUB
-----END PGP SIGNATURE-----

--SB8PAdHnrhigqCwh--