Yao et al. [1,2] claimed a potential limitation on the class of protocols that
could be securely implemented in the generalized universal composability (GUC)
framework proposed by Canetti et al. [3]. Specifically, Yao et al. presented a
concrete attack on a GUC Zero Knowledge (GUCZK) protocol, a natural adaptation
Blum's ZK proof for Directed Hamiltonicity using the general GUC feasibility of
Canetti et al. [3]. Interestingly, the attack was not analysed in the GUC model
in Yao et al. [2] but in the FUC model, a new UC-like framework proposed in the
same work. Nonetheless, Yao et al. [2] argued that, in light of this attack,
GUC would lose its concurrent general composability and proof of knowledge
properties. Concretely, they argue that GUC composability would now be with
respect to some adversaries with limited access to external arbitrary
protocols.In this work, we show that the claimed attack from Yao et al. is
indeed harmless and does not contradict the security of the mentioned GUCZK
protocol, thus restoring the general feasibility for GUC.