DTRU, a new variant of NTRU cryptosystem, has some highlighted advantages in comparison with original NTRU at the same security levels. However, the message-expansion of DTRU is considered rather larger than that of other cryptosystems. In this paper, by exploiting the random polynomials in encrypting function for storing a half of plain-text message, we propose a modification of DTRU, called M-DTRU, that has not only smaller message-expansion factor but also larger message-security.
