pub fn encode_p256_pkcs8_der(key: &SecretKey) -> Result<[u8; 138], Pkcs8Error>Expand description
Encodes a P-256 secret key as PKCS#8 DER (the fixed 138-byte
PrivateKeyInfo template used by OpenSSL for secp256r1).
The embedded public key is derived from key, so the output always
contains a consistent private/public key pair.