code: func (mkc *memoryKeyCacher) Get(keyID string) (*jose.JSONWebKey, error) {
  	searchKey, ok := mkc.entries[keyID]
  	if ok {
  		if mkc.maxKeyAge == MaxKeyAgeNoCheck || !mkc.keyIsExpired(keyID) {
  			return &searchKey.JSONWebKey, nil
  		}
  		return nil, ErrKeyExpired
  	}
  	return nil, ErrNoKeyFound
  }
docs: // Get obtains a key from the cache, and checks if the key is expired