Learn in the textbookFundamentals of Encryption and Cipher Methods
DefinitionDecryption (復号) uses a key to turn ciphertext back into the original plaintext, the reverse of encryption.
ExplanationDecryption needs the correct key that matches the key used for encryption. The encryption procedure itself is usually public, so whether you can decrypt depends on whether you hold the key. If the key leaks, third parties can decrypt too, which is why key management decides how secure the system is.
AnalogyIt is like looking up the same substitution table in reverse to read the rewritten letter back into its original words.
Memory hookThe 復 in 復号 is the 復 of 復元 (restore) and 復旧 (recover). Think of it as returning ciphertext to its original form.