C Filter for Rc5

In this appendix we present an eecient implementation of the lter described in Section 3.1. One must place ciphertext diierences after (2r ?1)th round (one half-round before the last half-round) into DLevel + 1] and DLevel] before calling the lter. These diierences can be calculated (for RC5) from the known ciphertext diierences and the known rotation amount of the last round. A call to the lter function looks like: GoUP(Level). typedef unsigned long int WORD; /* 32-bit for RC5-XOR-32/12/16 */ #define Level 7 /* Depth of search */ int FibbLevel+1] = {1,2,3,5,8,13,13,13}; /* Fib. cuts for a search tree */ WORD DDLevel+1]; /* Array of differences */ int count = 0; /* Number of variants */ unsigned int HAM(); /* Returns Hamming weight */ WORD ROTR(); /* Rotation to the right */ /* This function carries out filtration of good pairs for RC5-XOR by checking*/ /* the structure of the Fibonacci weight sequence of the particular pair. */ int GoUP(depth) /* Returns 1 if the pair is good*/ int depth; /* Depth of recursion */ { int i,s, flag= 0; if (HAM(DDdepth]) > Fibbdepth]) return(0); /* Cut the search tree */ if(depth!=0) for(s=0; s<32; s++) /* Try all rotations 0..31 */ if(((ROTR(DDdepth+1],s)) &0x1F)==0){ DDdepth-1] = ROTR(DDdepth+1],s)^DDdepth]; if(GoUP(depth-1)) /* Recursive call */ flag=1; /* For correct count of variants*/ } else { count++; /* Accumulate num. of variants */ return(1); } return(flag); } This article was processed using the L a T E X macro package with LLNCS style /* Construct all Fibonacci sequences with up to mc corrections