  A proxy signature scheme allows a proxy signer to sign messages on behalf of
an original signer. Proxy signature schemes have found numerous practical
applications such as grid computing, mobile agent systems and cloud
applications. Recently, Jiang et al. proposed the first lattice-based proxy
signature scheme and claimed that their scheme provides all the security
properties of a secure proxy signature scheme. However, in this paper, we
disprove their claim and show that an original signer is able to forge a proxy
signature on any message.
