Provided are a method of generating a Message Authentication Code (MAC) using a stream cipher, and authentication/encryption and authentication/decryption methods using a stream cipher.According to the methods, authentication/encryption is performed using a MAC generated using a stream cipher as an initialization vector of the stream cipher. Therefore, it is unnecessary to use a random number generation algorithm to generate the initialization vector, and thus implementation efficiency can be improved.In addition, upon generation of a MAC, a plurality of key stream generators perform computation for a plurality of message blocks, respectively. Therefore, the message blocks are computed in parallel at a time, and thus computation efficiency is excellent.