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From: Greg Tonoski <gregtonoski@gmail.com>
Date: Wed, 23 Jul 2025 10:49:55 +0200
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Subject: Re: [bitcoindev] Revisiting secp256r1 signatures (i.e. P256, mobile
 HSM support)
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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I can see another attempt to re-open the vulnerability OP_CAT which had
been closed in Bitcoin long time ago. It's pushed under the guise of
secp256r1  which has nothing to do with the OP_CAT. Also, HSM and other
technobabble is irrelevant. For example, mobile users already have "mobile
HSM support" - see Samsung Blockchain Keystore.

-- 
Greg Tonoski

On Wed, 23 Jul 2025, 00:03 Josh Doman, <joshsdoman@gmail.com> wrote:

> A brief search on gnusha.org
> <https://gnusha.org/pi/bitcoindev/?q=secp256r1> suggests that it's been
> over 10 years since the Bitcoin community last discussed adding secp256r1
> support (also known as P256). The most in-depth discussions I found were on
> BitcoinTalk in 2011 <https://bitcointalk.org/?topic=2699.0> and 2013
> <https://bitcointalk.org/index.php?topic=151120.0>.
>
> Since then, P256 has gained widespread adoption across the modern internet
> and on mobile. Most notably, millions of users now possess mobile devices
> capable of generating and storing private keys in secure enclaves (see
> Apple iCloud Keychain and Android Keystore). Millions of users might be
> able to immediately use this to start self-custodying bitcoin, except this
> hardware only supports P256 signatures, which is incompatible with the
> secp256k1 curve that Bitcoin currently uses.
>
> Reading through old discussions, it appears that the primary concern the
> community had with P256 is the possibility of a NIST backdoor. Putting the
> likelihood of this aside, it seems reasonable to me that in 2025, users
> should at least have the option of using P256, if they wish. Native HSM
> support would significantly improve the onboarding experience for new
> users, increase the security and accessibility of hot wallets, and
> potentially reduce the cost of collaborative multisigs. Meanwhile, the
> community can continue to use secp256k1 as the ideal curve for private keys
> secured in cold storage.
>
> At a technical level, Tapscript makes P256 mechanically straightforward to
> adopt, because it has built-in support for new types of public keys. For
> example, we could define a 33-byte public key as one requiring a P256 ECDSA
> signature, while continuing to use 32-bytes for keys requiring Schnorr
> signatures over secp256k1.
>
> A secondary concern that I came across is that P256 can be 2-3x slower to
> validate than secp256k1. Assuming this cannot be improved, we can account
> for slower validation by doubling or tripling the validation weight cost
> for a P256 signature. Users can then pre-commit in their script to this
> additional weight or commit to it in the annex, as intended by BIP341
> <https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki>.
>
> P256 support would grant apps the ability to use platform APIs to access
> the secure HSM on user's mobile devices, but alone, P256 is insufficient
> for non-custodial WebAuthn / passkey-based wallets. To verify a WebAuthn
> signature, we'd additionally need CSFS and CAT, so we can compute a
> WebAuthn message from a sighash and the necessary WebAuthn data on the
> stack. Alternatively, we could create a dedicated WebAuthn opcode to verify
> a WebAuthn message without enabling recursive covenants. Regardless, the
> ability to verify a P256 signature would be an important primitive.
>
> In summary, *given the widespread hardware adoption and industry usage,
> is it worth revisiting adding P256 support to Bitcoin?*
>
> Josh Doman
>
> --
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> To view this discussion visit
> https://groups.google.com/d/msgid/bitcoindev/8fbe1fe3-425d-4056-8387-993f6ecc1been%40googlegroups.com
> <https://groups.google.com/d/msgid/bitcoindev/8fbe1fe3-425d-4056-8387-993f6ecc1been%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
>

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<div dir=3D"auto">I can see another attempt to re-open the vulnerability OP=
_CAT which had been closed in Bitcoin long time ago. It&#39;s pushed under =
the guise of secp256r1=C2=A0 which has nothing to do with the OP_CAT. Also,=
 HSM and other technobabble is irrelevant. For example, mobile users alread=
y have &quot;mobile HSM support&quot; - see Samsung Blockchain Keystore.<br=
><br>--=C2=A0<div dir=3D"auto">Greg Tonoski<br><br><div class=3D"gmail_quot=
e gmail_quote_container" dir=3D"auto"><div dir=3D"ltr" class=3D"gmail_attr"=
>On Wed, 23 Jul 2025, 00:03 Josh Doman, &lt;<a href=3D"mailto:joshsdoman@gm=
ail.com">joshsdoman@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"=
gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(20=
4,204,204);padding-left:1ex">A brief search on <a href=3D"https://gnusha.or=
g/pi/bitcoindev/?q=3Dsecp256r1" target=3D"_blank" rel=3D"noreferrer">gnusha=
.org</a> suggests that it&#39;s been over 10 years since the Bitcoin commun=
ity last discussed adding secp256r1 support (also known as P256). The most =
in-depth discussions I found were on BitcoinTalk in <a href=3D"https://bitc=
ointalk.org/?topic=3D2699.0" target=3D"_blank" rel=3D"noreferrer">2011</a> =
and <a href=3D"https://bitcointalk.org/index.php?topic=3D151120.0" target=
=3D"_blank" rel=3D"noreferrer">2013</a>.<br><br>Since then, P256 has gained=
 widespread adoption across the modern internet and on mobile. Most notably=
, millions of users now possess mobile devices capable of generating and st=
oring private keys in secure enclaves (see Apple iCloud Keychain and Androi=
d Keystore). Millions of users might be able to immediately use this to sta=
rt self-custodying bitcoin, except this hardware only supports P256 signatu=
res, which is incompatible with the secp256k1 curve that Bitcoin currently =
uses.<br><br>Reading through old discussions, it appears that the primary c=
oncern the community had with P256 is the possibility of a NIST backdoor. P=
utting the likelihood of this aside, it seems reasonable to me that in 2025=
, users should at least have the option of using P256, if they wish. Native=
 HSM support would significantly improve the onboarding experience for new =
users, increase the security and accessibility of hot wallets, and potentia=
lly reduce the cost of collaborative multisigs. Meanwhile, the community ca=
n continue to use secp256k1 as the ideal curve for private keys secured in =
cold storage.<br><br>At a technical level, Tapscript makes P256 mechanicall=
y straightforward to adopt, because it has built-in support for new types o=
f public keys. For example, we could define a 33-byte public key as one req=
uiring a P256 ECDSA signature, while continuing to use 32-bytes for keys re=
quiring Schnorr signatures over secp256k1.<br><br>A secondary concern that =
I came across is that P256 can be 2-3x slower to validate than secp256k1. A=
ssuming this cannot be improved, we can account for slower validation by do=
ubling or tripling the validation weight cost for a P256 signature. Users c=
an then pre-commit in their script to this additional weight or commit to i=
t in the annex, as intended by <a href=3D"https://github.com/bitcoin/bips/b=
lob/master/bip-0341.mediawiki" target=3D"_blank" rel=3D"noreferrer">BIP341<=
/a>.<br><br>P256 support would grant apps the ability to use platform APIs =
to access the secure HSM on user&#39;s mobile devices, but alone, P256 is i=
nsufficient for non-custodial WebAuthn / passkey-based wallets. To verify a=
 WebAuthn signature, we&#39;d additionally need CSFS and CAT, so we can com=
pute a WebAuthn message from a sighash and the necessary WebAuthn data on t=
he stack. Alternatively, we could create a dedicated WebAuthn opcode to ver=
ify a WebAuthn message without enabling recursive covenants. Regardless, th=
e ability to verify a P256 signature would be an important primitive.<br><b=
r>In summary, <b>given the widespread hardware adoption and industry usage,=
 is it worth revisiting adding P256 support to Bitcoin?</b><br><div><b><br>=
</b></div><div>Josh Doman</div>

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