Abstract: For meeting the high security and high efficiency of cryptography schemes in Cloud Computing, a security enhanced cryptographic mode of operation named Cipher FeedBack one Block one Key (CFB-BK) is proposed, and this mode is implemented based on the combination of mathematical cryptography and optical cryptography. Optical cryptography module encrypts (or decrypts) data blocks in a one block one key way. Moreover, it provides its ciphertext as a feedback to the mathematical cryptography module, which uses it to generate keys for the next block encryption (or decryption) of the optical cryptography module. Security analysis shows that the only possible attack in the area of cryptography for the proposed scheme is exhaustive attack, indicating that no adversary could get a significant advantage against the scheme without spending a huge amount of recourses and time. Efficiency analysis shows that the scheme implementing CFB-BK mode works much faster than the existing modes implemented based on mathematical cryptography.