An encryption method and decryption method are provided. The encryption method divides an electronic file into a plurality of message blocks, wherein the message blocks have a sequence. The encryption method sets a checking vector as the last message block. The encryption method performs the following steps on each message block according to the sequence: generating an input block, deriving an output block by encrypting the input block by an encryption key, and deriving an encrypted block by applying XOR operation to the output block and the previous message block, wherein the input block is equivalent to applying XOR operation to the message block, the output block corresponding to the previous message block, and the message block before the previous one. The encryption method generates an electronic encrypted file by concatenating the encrypted blocks. The decryption method performs a series of operations corresponding to the above operations.