A cryptographic concept, blind decoding is discussed: a client has a message encrypted with a server's public key and the client asks the server to decode the message without revealing what is the decoded plaintext nor learning the server's secret key. Blind decoding is a useful tool for protecting user's privacy in on-line shopping over the Internet. The RSA-based blind decoding is easily converted from the similar protocol as the Chaum's blind signature scheme, and a blind decoding protocol for the E1Gama] encryption scheme is newly proposed. Moreover, the practical gap between the known RSA-based blind decoding and our E1Gamal-based scheme is discussed in the application to protecting copyright matter of electronic documents. In blind decoding scheme, undeteetability of the decrypted message has both negative and positive aspects: a negative aspect is considered as the problem of spotting the oracle and a positive aspect is applicable to making undeniable signatures blind against the signer.