This book constitutes the refereed post-conference proceedings of the 28th International Workshop on Security Protocols, held in Cambridge, UK, during March 27–28, 2023.Thirteen papers out of 23 submissions were selected for publication in this book, presented together with the respective transcripts of discussions. The theme of this year's workshop was “Humans in security protocols — are we learning from mistakes?”The topics covered are securing the human endpoint and proving humans correct.