The Decrypt service is meant to be used with the Encrypt service. It uses symmetric encryption which is designed to allow anyone with access to the encryption key to decrypt and obtain the original input value. It can decrypt encrypted strings provided the decryption key is the same used to encrypt the string. Often times passwords and query strings are encrypted so that even if they are visible to the user, without the encryption key used to encrypt the text, the original input value remains hidden. If the decrypt service is used to decrypt text, but is using the wrong encryption key, the service will throw an exception.