Abstract
In 2006, Pomykala and Barabasz proposed an elliptic curve based threshold proxy signature scheme with known signers. They claimed that their scheme was relatively simple in construction, had the properties of unforgeability and non-repudiation, and admitted the proactive security. Later, Li and Hu pointed out that the scheme of Pomykala and Barabasz was insecure against the conspiracy attack. And then, they proposed an improved scheme. In this paper, we not only show that the threshold proxy signature scheme of Pomykala and Barabasz is universal forgeable, but also show that the improvement of Li and Hu suffers from the rogue key attack.