Subject: [bitcoindev] Revisiting secp256r1 signatures (i.e. P256, mobile HSM support)

From: Josh Doman <[email-masked]>
Date: Tue, 22 Jul 2025 14:44:02 -0700 (PDT)

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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