In the field of optical information security, the most attractive work is the so-called double-random-phase encoding encryption scheme proposed by Javidi. However, the security of this cryptosystem has not been analyzed thoroughly from the point of view of cryptoanalysis. In this article, the weakness of Javidi's optical security system is carefully analyzed with a known-plain text attack. It is shown that the double-random-phase encoding encryption scheme is a linear symmetric block cipher cryptosystem and its linearity opens avenues of attacks. Under the known-plaintext attack, attacker can obtain the phase key(s) in the input plane using the typical phase retrieval algorithms and subsequently deduce the phase key(s) in the Fourier domain easily. In addition, an optical implementation of known-plain text attack is also proposed.