Title: FedProp: Cross-client Label Propagation for Federated Semi-supervised Learning. Abstract: Federated learning (FL) allows multiple clients to jointly train a machine learning model in such a way that no client has to share their data with any other participating party. In the supervised setting, where all client data is fully labeled, FL has been widely adopted for learning tasks that require data privacy. However, it is an ongoing research question how to best perform federated learning in a semi-supervised setting, where the clients possess data that is only partially labeled or even completely unlabeled. In this work, we propose a new method, FedProp, that follows a manifold-based approach to semi-supervised learning (SSL). It estimates the data manifold jointly from the data of multiple clients and computes pseudo-labels using cross-client label propagation. To avoid that clients have to share their data with anyone, FedProp employs two cryptographically secure yet highly efficient protocols: multi-party Hamming distance computation and secure aggregation. Experiments on three standard benchmarks show that FedProp achieves higher classification accuracy than previous federated SSL methods. Furthermore, as a pseudo-label-based technique, FedProp is complementary to other federated SSL approaches, in particular consistency-based ones. We demonstrate experimentally that further accuracy gains are possible by combining both.