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.