key_cacher.go
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
}
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
}
// Get obtains a key from the cache, and checks if the key is expired