decoder
Computing term for a device or program that converts encoded data back into its original form.
This decoder restores encrypted messages to their original form.
暗号化 is the process of encryption, whereas 復号器 is the tool or device that performs decryption.
Compound of 復号 (decoding) and 器 (device). The exact historical derivation is uncertain, but the spelling is conventionally associated with computing decoders.