package seal

import (
	"crypto/hmac"
	"encoding/hex"
)

// Fingerprint is a keyed, stable digest: equal values give equal
// fingerprints and nothing else can be learned from one. An address is
// enumerable, so a plain hash of it is the address in another alphabet;
// HMAC under a purpose key is not.
func (s *Sealer) Fingerprint(purpose, value string) (string, error) {
	key, err := s.purposeKey(purpose)
	if err != nil {
		return "", err
	}
	mac := hmac.New(sha256New, key)
	mac.Write([]byte(value))
	return hex.EncodeToString(mac.Sum(nil)), nil
}