Improvement of Harn´s digital signature for Diffie-Hellman public keys

Recently, Harn proposed a digital signature for Diffie-Hellman public keys without using a one-way function (see Electron. Lett, 1997, 33, (2), pp. 125-126). In the Letter, the authors show that Harn´s digital signature is not secure enough to withstand the Nyberg-Rueppel attack. That is, an adversary who discloses one session key subsequently distributed by the same key exchange protocol. An improvement to Harn´s digital signature that can overcome the weakness stated above is also proposed