Add `from_pkcs8_pem` or `from_pkcs8_der` for X25519 static secret

[imlk0]

Hi there! 👋

I'm currently working on loading an X25519 private key from a PEM file in PKCS#8 format (e.g., generated by openssl genpkey -algorithm X25519), and I noticed that while ed25519-dalek provides convenient methods like:

SigningKey::from_pkcs8_pem(pem_str)?

the x25519-dalek crate does not offer similar functionality. This makes interoperability with standard key formats (like those used in TLS, OpenSSL, or configuration files) more cumbersome.

It would be incredibly helpful if x25519-dalek could provide built-in support for parsing PKCS#8 PEM/DER, just like ed25519-dalek does, e.g.:

let secret = x25519_dalek::StaticSecret::from_pkcs8_pem(pem_str)?;
// or
let secret = x25519_dalek::StaticSecret::from_pkcs8_der(der_bytes)?;

This would greatly improve usability and reduce boilerplate code for users integrating with standard tooling (OpenSSL, Let's Encrypt-style keys, etc.).

Would you be open to adding this feature? I’d be happy to help contribute if needed!

[tarcieri]

We can add support for this. It should follow RFC8410:

https://datatracker.ietf.org/doc/html/rfc8410#section-7

You can reuse the implementation from the ed22519 crate. It's the same encoding. I guess just copy and paste it for now:

https://github.com/RustCrypto/signatures/blob/master/ed25519/src/pkcs8.rs

Note the AlgorithmIdentifier OIDs will be different. See Section 9.