The Fast Software Encryption 2005 Workshop was the twelfth in a series of annual workshops on symmetric cryptography, sponsored for the fourth year by the International Association for Cryptologic Research (IACR). The workshop concentratedonallaspectsoffastprimitivesforsymmetriccryptology,including thedesign,cryptanalysisandimplementationofblockandstreamciphersaswell as hash functions and message authentication codes. The ?rst FSE workshop was held in Cambridge in 1993, followed by Leuven in 1994, Cambridge in 1996, Haifain1997,Parisin1998,Romein1999,NewYorkin2000,Yokohamain2001, Leuven in 2002, Lund in 2003, and New Delhi in 2004. This year, a total of 96 submissions were received. After an extensive review by the Program Committee, 30 submissions were accepted. Two of these s- missions were merged into a single paper, yielding a total of 29 papers accepted for presentation at the workshop. Also, we were very fortunate to have in the programan invited talk byXuejia Laion Attacks andProtection ofHash Fu- tions and a very entertaining rump session that Bart Preneel kindly accepted to chair. These proceedings contain the revised versions of the accepted papers; the revised versions were not subsequently checked for correctness.