package bambunfc

import (
	"crypto/hkdf"
	"crypto/sha256"
	"fmt"
)

var kdfSalt = []byte{
	0x9a, 0x75, 0x9c, 0xf2, 0xc4, 0xf7, 0xca, 0xff,
	0x22, 0x2c, 0xb9, 0x76, 0x9b, 0x41, 0xbc, 0x96,
}

const kdfInfo = "RFID-A\x00"

// KeysA derives the 16 MIFARE Key-A values for a Bambu tag from its chip UID.
func KeysA(uid []byte) ([16][6]byte, error) {
	var keys [16][6]byte
	if len(uid) == 0 {
		return keys, fmt.Errorf("%w: empty", ErrUID)
	}
	okm, err := hkdf.Key(sha256.New, uid, kdfSalt, kdfInfo, 16*6)
	if err != nil {
		return keys, fmt.Errorf("%w: hkdf: %v", ErrUID, err)
	}
	for i := range 16 {
		copy(keys[i][:], okm[i*6:i*6+6])
	}
	return keys, nil
}