Learn in the textbookFundamentals of Encryption and Cipher Methods
DefinitionSymmetric-key cryptography (共通鍵暗号方式) uses the same key for encryption and decryption, and the sender and receiver share that key while keeping it secret.
ExplanationUsing one key in both directions keeps the mechanism simple, but the key must first reach the other party safely, and this key distribution (鍵配送) is the hard part. Each relationship you want to keep private needs its own key, so every pair of people needs one key. AES is a typical example.
AnalogyThink of a lockbox with two identical keys, one for you and one for your partner. If the key is copied while it is being delivered, anyone can open the box.
Memory hookThe English name, symmetric, gives it away: the key on the sending side mirrors the one on the receiving side, so they are the same key.