Recently, Ma and Chen proposed a new authenticated encryption scheme with public verifiability. The signer can generate a signature with message recovery for a specified recipient. With a dispute, the recipient has ability to convert the signature into an ordinary one that can be verified by anyone without divulging her/his private key and the message. However, we point out that any adversary can forge a converted signature in this article.

In 2002, Hwang and Yeh proposed some improved schemes to mend several security flaws in the Peyravian-Zunic password transmission scheme and password change scheme. However, this article will point out that there still exist some security flaws in the Hwang-Yeh schemes; at the same time, we shall also propose some improved versions of their schemes.

Recently, the integration of wired and wireless networks has become an interesting issue. The introduction of extending Mobile IP to mobile ad hoc networks not only helps the mobile nodes connect to the Internet but also broadens the scope of the ad hoc networks and increases their application. However, these hybrid schemes faces several security problems from the inherent weakness of ad hoc routing. In this paper, we propose a hybrid authentication scheme of Mobile IP assistance for ad hoc routing security. The regular Mobile IP registration scheme has been refined to an ad hoc key-aided version and now incorporates a novel routing packet authentication mechanism in the ad hoc routing operation. A distinct character of this hybrid scheme is that a Mobile Agent can form a secure ad hoc network where the mobile hosts can be authorized and authenticated by the refined Mobile IP registration scheme. In these findings, we shall propose that the mobile hosts can follow a novel routing packet authentication mechanism to secure the routing packets by using the cryptography of the simple geometric properties of lines. Since the novel routing authentication mechanism does not need digital signatures for completing the routing packet integrity, in this hybrid authentication scheme, the cryptographic computation cost on the mobile hosts' side is relatively minimized.

In this article, we will present a modification attack and a dictionary attack to subvert the security of the Tseng scheme and the Ku-Wang scheme. As we know, no existing schemes of simple authenticated key agreement protocols can successfully withstand our modification attack.