package seal

// The salt of each key that DeriveKeys derives. Each key has a salt of its
// own, so no two uses share a key.
const (
	sessionHashKeySalt  = "pixa-session-hash-v1"
	sessionBlockKeySalt = "pixa-session-block-v1"
	encryptedURLKeySalt = "pixa-urlenc-v1"
	csrfKeySalt         = "pixa-csrf-v1"
)

// Keys holds the keys derived from the signing key, one for each use. The
// signing key itself is used for two things only: it is the HMAC key of signed
// URLs and the password of the login page.
type Keys struct {
	// SessionHashKey signs the session cookie.
	SessionHashKey [KeySize]byte
	// SessionBlockKey encrypts the session cookie.
	SessionBlockKey [KeySize]byte
	// EncryptedURLKey encrypts the token of an encrypted URL.
	EncryptedURLKey [KeySize]byte
	// CSRFKey signs the CSRF token cookie of the login and generator pages.
	CSRFKey [KeySize]byte
}

// DeriveKeys derives each key of Keys from signingKey, with DeriveKey and the
// salt of that key.
func DeriveKeys(signingKey string) (Keys, error) {
	masterKey := []byte(signingKey)

	sessionHashKey, err := DeriveKey(masterKey, sessionHashKeySalt)
	if err != nil {
		return Keys{}, err
	}

	sessionBlockKey, err := DeriveKey(masterKey, sessionBlockKeySalt)
	if err != nil {
		return Keys{}, err
	}

	encryptedURLKey, err := DeriveKey(masterKey, encryptedURLKeySalt)
	if err != nil {
		return Keys{}, err
	}

	csrfKey, err := DeriveKey(masterKey, csrfKeySalt)
	if err != nil {
		return Keys{}, err
	}

	return Keys{
		SessionHashKey:  sessionHashKey,
		SessionBlockKey: sessionBlockKey,
		EncryptedURLKey: encryptedURLKey,
		CSRFKey:         csrfKey,
	}, nil
}