Currently, electronic documents are commonly exchanged between/among government offices in many countries. When a government office would like to transmit a high-security-level-electronic document to another office, the sending end officer needs to encrypt it so as to protect the document from being known to hackers. AES and DES have been commonly and widely invoked to protect documents in recent years. However, the two algorithms have so far faced the threats of Brute-Force cracks. To avoid the threats, in this study, we proposed a new data encryption approach, called the Secure Data Encryption Method (SeDEM for short), in which plaintext and system keys are encrypted by using a sequential-logic style encryption approach which further employs a three-dimensional operation and a feedback encryption mechanism to effectively protect encrypted data from brute-force and cryptanalysis attacks. The feedback encryption mechanism is a feedback process in which each of its calculation iteration generates three internally-used dynamic feedback keys for the next calculation iteration. The purpose is to effectively improve the security level and unpredictability of generated ciphertext. The three-dimensional operation is employed to further increase the computational complexity of the encryption technique so as to enhance the security level of the ciphertext, and difficulty of cracking the keys.