HKDF Salt in Key Expansion

This weekend I made another addition to age-pkcs11, to follow best practices for HKDF key expansion from the shared secret at the core of the program. I’d been wanting to do this for a while, after reviewing some stuff I wrote about age and looking at the new V1 API there. If you recall back in June when I went into detail on the X25519 cryptography in Age, near the end Age builds up a salt which, when combined with a label and supplied to the HKDF function ties the derived key to a specific context.