The12thintheseriesofIMAConferencesonCryptographyandCodingwasheld at the Royal Agricultural College, Cirencester, December 15–17, 2009. The p- gram comprised 3 invited talks and 26 contributed talks. The contributed talks werechosenbyathoroughreviewingprocessfrom53submissions.Oftheinvited and contributed talks,28 arerepresentedaspapersin this volume. These papers are grouped loosely under the headings: Coding Theory, Symmetric Crypt- raphy, Security Protocols, Asymmetric Cryptography, Boolean Functions, and Side Channels and Implementations. Numerous people helped to make this conference a success. To begin with I would like to thank all members of the Technical Program Committee who put a great deal of e?ort into the reviewing process so as to ensure a hi- quality program. Moreover, I wish to thank a number of people, external to the committee, who also contributed reviews on the submitted papers. Thanks, of course,mustalso goto allauthorswho submitted papers to the conference,both those rejected and accepted. The review process was also greatly facilitated by the use of the Web-submission-and-review software, written by Shai Halevi of IBM Research, and I would like to thank him for making this package available to the community. The invited talks were given by Frank Kschischang, Ronald Cramer, and Alexander Pott, and two of these invitedtalksappearaspapersinthisvolume. A particular thanks goes to these invited speakers, each of whom is well-known, notonlyforbeingaworld-leaderintheir?eld,butalsofortheirparticularability to communicate their expertise in an enjoyable and stimulating manner.