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<B>Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth)</B>
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<I>(Publisher: John Wiley & Sons, Inc.)</I>
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Author(s): Bruce Schneier
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ISBN: 0471128457
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Publication Date: 01/01/96
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<P><FONT SIZE="+1"><B><I>RDES</I></B></FONT></P>
<P>RDES is a variant that replaces swapping the left and right halves at the end of each round with a key-dependent swap [893]. The swappings are fixed, depending solely on the key. This means that the 15 key-dependent swaps occur with 2<SUP>15</SUP> possible instances, and that the variant is not resistant to differential cryptanalysis [816,894,112]. RDES has a large number of weak keys. In fact, almost every key is weaker than a typical DES key. This variant should not be used.</P>
<P>A better idea is to swap only within the right half, at the beginning of each round. Another better idea is to make the swapping dependent on the input data and not a static function of the key. There are a number of possible variants [813,815]. In RDES-1, there is a data-dependent swap of the 16-bit words at the beginning of each round. In RDES-2, there is a data-dependent swap of the bytes at the beginning of each round after the 16-bit swappings as in RDES-1. And so on through RDES-4. RDES-1 is secure against both differential cryptanalysis [815] and linear cryptanalysis [1136]. Presumably RDES-2 and greater are as well.</P>
<TABLE WIDTH="70%"><TR>
<TH CAPTION COLSPAN="12" ALIGN="CENTER">Table 12.15<BR>Differential Cryptanalysis Attacks against DES Variants
<TR>
<TD COLSPAN="2"><HR>
<TR>
<TH WIDTH="45%" VALIGN="TOP" ALIGN="CENTER">Modified Operation
<TH WIDTH="25%" VALIGN="TOP" ALIGN="CENTER">Chosen Plaintexts
<TR>
<TD COLSPAN="2"><HR>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">Full DES (no modification)
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>47</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">P permutation
<TD VALIGN="TOP" ALIGN="CENTER">Cannot strengthen
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> Identity permutation
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>19</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">Order of S-boxes
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>38</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">Replace XORs by additions
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>39,</SUP> 2<SUP>31</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">S-boxes:
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> Random
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>18</SUP>–2<SUP>20</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> Random permutations
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>33</SUP>–2<SUP>41</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> One entry
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>33</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> Uniform tables
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>26</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">Elimination of the E Expansion
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>26</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">Order of E and subkey XOR
<TD VALIGN="TOP" ALIGN="CENTER">2<SUP>44</SUP>
<TR>
<TD VALIGN="TOP" ALIGN="LEFT">GDES (width q = 8):
<TR>
<TD VALIGN="TOP" ALIGN="LEFT"> 16 rounds
<TD VALIGN="TOP" ALIGN="CENTER">6, 16
<TR>
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