SPW 2013 Twenty-first International Workshop on Security Protocols, Sidney Sussex College, Cambridge, England, March 18-20, 2013. (Submissions due 7th January, 2013) The theme of this year's workshop is "What's Happening on the Other Channel?" Many protocols use a secondary channel, either explicitly (as in multichannel protocols) but more usually implicitly, for example to exchange master keys, or their hashes. The role of the Other Channel is fundamental, and often problematic, and yet protocol composers typically take them as a given. Sometimes the Other Channel really is completely covert, but sometimes it just has properties that are different. And it's not only security properties that are relevant here: bandwidth, latency and error rate are often important considerations too. Even a line-of-sight channel usually doesn't quite have the properties that we unthinkingly attributed to it. Moriarty has been subscribing to the Other Channel for years: perhaps it's time for Alice and Bob to tune in too. This theme is not intended to restrict the topic of your paper, but to help provide a particular perspective and to focus the discussions. Our intention is to stimulate discussion likely to lead to conceptual advances, or to promising new lines of investigation, rather than merely to consider finished work. For more information, please see: http://spw.stca.herts.ac.uk/