  Verification plays an essential role in the formal analysis of
safety-critical systems. Most current verification methods have specific
requirements when working on Deep Neural Networks (DNNs). They either target
one particular network category, e.g., Feedforward Neural Networks (FNNs), or
networks with specific activation functions, e.g., RdLU. In this paper, we
develop a model-agnostic verification framework, called DeepAgn, and show that
it can be applied to FNNs, Recurrent Neural Networks (RNNs), or a mixture of
both. Under the assumption of Lipschitz continuity, DeepAgn analyses the
reachability of DNNs based on a novel optimisation scheme with a global
convergence guarantee. It does not require access to the network's internal
structures, such as layers and parameters. Through reachability analysis,
DeepAgn can tackle several well-known robustness problems, including computing
the maximum safe radius for a given input, and generating the ground-truth
adversarial examples. We also empirically demonstrate DeepAgn's superior
capability and efficiency in handling a broader class of deep neural networks,
including both FNNs, and RNNs with very deep layers and millions of neurons,
than other state-of-the-art verification approaches.
