From: Johnson Lau <jl2012@xbt.hk>
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Date: Fri, 1 Jun 2018 02:53:01 +0800
To: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: [bitcoin-dev] Disallow insecure use of SIGHASH_SINGLE
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I=E2=80=99ve made a PR to add a new policy to disallow using =
SIGHASH_SINGLE without matched output:

https://github.com/bitcoin/bitcoin/pull/13360

Signature of this form is insecure, as it commits to no output while =
users might think it commits to one. It is even worse in non-segwit =
scripts, which is effectively SIGHASH_NOINPUT|SIGHASH_NONE, so any UTXO =
of the same key could be stolen. (It=E2=80=99s restricted to only one =
UTXO in segwit, but it=E2=80=99s still like a SIGHASH_NONE.)

This is one of the earliest unintended consensus behavior. Since these =
signatures are inherently unsafe, I think it does no harm to disable =
this unintended =E2=80=9Cfeature=E2=80=9D with a softfork. But since =
these signatures are currently allowed, the first step is to make them =
non-standard.=