method of encrypting a file and decrypting a file to be encrypted

PURPOSE: A file encryption method and a file automatic decoding method are provided to block direct outflow of cryptographic keys through reverse engineering, by preventing direct insertion of the cryptographic key into a file. CONSTITUTION: An installation file extracts encrypted file stream, cryptographic key index and encryption algorithm information from a security file(S41). A decoding key is generated by computing the extracted cryptographic key index and the encrypted bit stream of a code book(S42). A decryption algorithm corresponding to an encryption algorithm is extracted(S43). An original file stream is restored by applying the encrypted file stream and the decryption key to the extracted decryption algorithm(S44). A program is installed in a client system by using the restored original file stream(S45).