双随机相位编码光学加密系统的唯密文攻击

Ciphertext-only attack on double random phase encoding optical encryption system

针对双随机相位编码光学加密系统的安全性分析表明,该系统属于线性对称分组密码系统,其线性性质为安全性留下极大隐患.在唯密文攻击下,仅根据密文估计出物面波函数的“支撑”(support),然后利用迭代相位恢复算法获得物面波函数(其振幅是明文信息),再根据物面波函数与频域密文的关系可推导出频谱平面的解密密钥.由于估计出来的物面波函数的“支撑”相对于真实的物面波函数的“支撑”有一定的平移,使得恢复的物面波函数与真实的物面波函数之间无论在振幅上还是相位上都存在平移,导致用推导出来的解密密钥去解密其他密文时所获得的明文与原始明文之间存在明显平移.然而,可依照这一先验信息,将估计出来的物面波函数的“支撑”在物面内遍历,从而找到逼近真实解密密钥的解.利用此解密密钥去解密其他密文时获得更好的解密效果.

Security analysis of optical encryption system based on double random phase encoding indicates that the system can be classified as a linear symmetric block-cipher cryptosystem, which way lead to a great vulnerability. Under the ciphertext-only attack （COA）, an opponent can attack such a cryptosystem only on the basis of estimated support of wave function in the object plane with iterative phase retrieval methods, and subsequently deduce the phase keys in the Fourier plane easily. The ciphertext- only attack （COA） requires much less resources than other types of attacks. Estimated support of wave function in the object plane could have some translations relative to the true support, so retrieved wave function could also have translations in both the amplitude and the phase, leading to a translation of retrieved plaintext relative to original plaintext. However, attackers can take this translation as a priori knowledge to traverse estimated support in the object plane until finding the best estimated keys, which bring about the best decryption quality.