Constant P256_SPKI_PREFIX 

pub const P256_SPKI_PREFIX: [u8; 26];

Prefix that turns a raw P-256 point into SubjectPublicKeyInfo DER.

A WebAuthn authenticator hands over coordinates, not a key file. This is the constant SEQUENCE { AlgorithmIdentifier { id-ecPublicKey, prime256v1 }, BIT STRING } header that precedes the uncompressed 04 ‖ X(32) ‖ Y(32) point, so building a usable key is concatenation rather than a DER writer. Measured against openssl’s own output before it was relied on (D39).