Security in the wireless application protocol (WAP) is based on using public-key cryptosystems such as RSA cryptosystem. This system has two problems, the decryption process is slow and it is insecure due to some known attacks such as common modulus attack and low exponent attack. Tsuyoshi Takagi presented a new public-key cryptosystem with fast decryption named PKQ cryptosystem that solve the slowness of decryption process, but it still suffers from the above attacks. In This work we present a modification of the encryption algorithm for the PKQ cryptosystem based on the PKQ problem to use it in the WAP architecture to optimize security for the WAP. We prove that standard attacks such as the common modulus attack and the low exponent attack are not applicable on the PKQ cryptosystem after our modification. We also show that the decryption process of the PKQ cryptosystem after this modification is faster than that before.