package cryptocore

import (
	"crypto/sha256"
	"log"

	"golang.org/x/crypto/hkdf"
)

const (
	// "info" data that HKDF mixes into the generated key to make it unique.
	// For convenience, we use a readable string.
	HkdfInfoEMENames   = "EME filename encryption"
	hkdfInfoGCMContent = "AES-GCM file content encryption"
	hkdfInfoSIVContent = "AES-SIV file content encryption"
)

// hkdfDerive derives "outLen" bytes from "masterkey" and "info" using
// HKDF-SHA256 (RFC 5869).
// It returns the derived bytes or panics.
func HkdfDerive(masterkey []byte, info string, outLen int) (out []byte) {
	h := hkdf.New(sha256.New, masterkey, nil, []byte(info))
	out = make([]byte, outLen)
	n, err := h.Read(out)
	if n != outLen || err != nil {
		log.Panicf("hkdfDerive: hkdf read failed, got %d bytes, error: %v", n, err)
	}
	return out
}