Cryptanalytic model extraction aims to reconstruct a functionally equivalent model through black-box interactions with the victim model. Under the fundamental assumption that the network architecture is completely known, existing attacks achieve the goal by recovering the model parameters. In this paper, we explore whether this assumption can be removed practically. Focusing on ReLU fully connected networks, which are widely studied in this field, we propose a guess-and-determine framework that jointly recovers the network architecture (including network depth and hidden-layer dimensions) and the model parameters. This framework is based on a simple yet effective high-level idea: after designing a parameter recovery attack under the known-architecture assumption, we can analyze the architecture-sensitive traces observed during parameter recovery to recover the network architecture. We identify two such traces in differential extraction attacks: (i) a \emph{zero suffix} in the merged weight vectors produced by signature recovery, whose length reveals the hidden layer dimension; and (ii) an \emph{equality pattern} in the preimage-based sign recovery, which occurs only under the true hidden layer dimension. These two signals give rise to two routes for network architecture recovery. For the second-to-last layer, we further propose two methods, one for identifying it, and one for recovering its dimension. Practical end-to-end attacks are implemented on a wide range of ReLU neural networks, including both expansive and non-expansive networks. To the best of our knowledge, this is the first time the feasibility of achieving functionally equivalent extraction on deep neural networks, after removing the known-architecture assumption, has been demonstrated in practice.

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