Path: news.io.com!news.fc.net!news.net99.net!sun.cais.com!news.charm.net!news. + sprintlink.net!hookup!news.mathworks.com!uunet!in1.uu.net!sparky! + paganini.sydney.sterling.com!paganini.sydney.sterling.com!not-for-mail From: Ralf Brown
Newsgroups: sci.crypt.research Subject: Sliding Feistel Networks Followup-To: sci.crypt.research Date: 1 May 1995 10:10:18 +1000 Organization: Sterling Software (Aust). Pty. Ltd. Lines: 69 Sender: firstname.lastname@example.org Approved: PGP Moose V0.9 19950501 sci.crypt.research + iQBVAgUBL6Qm6WzWcw3p062JAQFBQwIAsQ7d9r0TqrNWDuTYrLwlyff+dV3Ap18s + 0wqul0vhHihh1h4cG2RyVB7Sl0qgAfSaJhCVg8xrJch9NM27geyMWA== + =1Lkr Distribution: world Message-ID: <email@example.com> NNTP-Posting-Host: paganini.sydney.sterling.com In my previous article, I described a generalization of the standard Feistel network to use N subblocks instead of two. This article describes a further modification which allows the network to be "slid" along the input, effectively producing an unlimited block size. Consider the following BUAG of a generalized Feistel network with N=3 and one additional minor change: A B C D E \ |\ /| | | \ | \ | | | / \+ \+ | | C | | | | | | # | | \ |\ | | | \ | \ | / | \+ \+ | D | | | \ |\ | | \ | \ | | \+ \+ / | / : # / : / E \ |\ | \ | \ | \+ \+ At the points marked by #, instead of feeding a copy of the mixing function output back to the left edge of the network, we output a copy of that value and instead feed the next subblock of the plaintext into the left edge of the network. Thus, every K rounds (K >= N, as explained below), we effectively slide the network one position further down the plaintext, outputting a chunk of ciphertext. Decryption naturally runs this process in reverse. K, the number of rounds between shifts, needs to be at least as great as the number of subblocks processed at once, to ensure that every subblock of input is processed uniformly throughout the network. Advantages: 1. effectively unlimited block size; chaining is automatic 2. message need only be a multiple of the subblock size greater than N; no additional padding to a multiple of N*subblock is required 3. except for the very beginning and end of the messsage, all subblocks are processed through K*N rounds 4. except for the subblocks at the very end of the message, all subblocks of the input are diffused over K*N subblocks of the ciphertext Disadvantages: 1. the text at the very beginning and very end is processed through fewer rounds, which may provide a point of attack 2. the message must be decrypted from end to beginning, which limits usefulness for very large messages (the ideal application is probably smallish [up to 8K or so] fixed-size 'messages' such as disk sectors). 3. slightly greater computational overhead than the simple generalized Feistel network of my previous article using K*N rounds -- Internet: firstname.lastname@example.org -=- FIDO: Ralf Brown 1:129/26.1 Disclaimer? | Gilb's First Law of Computer Unreliability: Computers What's that? | are unreliable, but humans are even more unreliable.