This book constitutes the thoroughly refereed post-workshop proceedings of the 26th International Workshop on Security Protocols, held in Cambridge, UK, in March 2018.

The volume consists of 17 thoroughly revised invited papers presented together with the respective transcripts of discussions. The theme of this year's workshop was fail-safe and fail-deadly concepts in protocol design. The topics covered included failures and attacks; novel protocols; threat models and incentives; cryptomoney; and the interplay of cryptography and dissent.