The invention provides an encryption method and device, a decryption method and device and a terminal and relates to the field of network information security. The encryption method provided by the invention comprises the steps of operating key elements based on a key rule, thereby generating a key; carrying out encryption operation on a plaintext based on the key rule according to the key, thereby generating a ciphertext; carrying out encryption operation on the key rule based on a user password and/or a terminal identifier, thereby generating a key rule ciphertext; and uploading the key rule ciphertext to a cloud server for storage. Through application of the method, data is encrypted through adoption the key elements and the key rule, and the key rule and the key are separately stored. The encryption and decryption keys are generated by acting on the key elements through the key rule and the encryption and decryption keys are managed through the cloud server and a terminal respectively, the key is difficult to obtain when any part of information is stolen, the key steal risk is greatly reduced, and the security of the terminal data is improved.