code: func (t *SecureTrie) GetKey(shaKey []byte) []byte {
  	if key, ok := t.getSecKeyCache()[string(shaKey)]; ok {
  		return key
  	}
  	key, _ := t.trie.db.preimage(common.BytesToHash(shaKey))
  	return key
  }
docs: // GetKey returns the sha3 preimage of a hashed key that was
  // previously used to store a value.