This paper addresses the issue of collaborative deep learning with privacy constraints. Building upon differentially private decentralized semi-supervised learning, we introduce homomorphically encrypted operations to extend the set of threats considered so far. While previous methods relied on the existence of an hypothetical ‘trusted’ third party, we designed specific aggregation operations in the encrypted domain that allow us to circumvent this assumption. This makes our method practical to real-life scenario where data holders do not trust any third party to process their datasets. Crucially the computational burden of the approach is maintained reasonable, making it suitable to deep learning applications. In order to illustrate the performances of our method, we carried out numerical experiments using image datasets in a classification context.