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40
41 /*
42 //
43 // Purpose:
44 // Cryptography Primitive. Modular Arithmetic Engine. General Functionality
45 //
46 // Contents:
47 // gsMontFactor()
48 //
49 */
50
51 #include "owndefs.h"
52 #include "owncp.h"
53 #include "pcpbnumisc.h"
54 #include "pcpbnuarith.h"
55 #include "gsmodstuff.h"
56 #include "pcptool.h"
57
58 /*
59 // montfomery factor k0 = -((modulus^-1 mod B) %B)
60 */
gsMontFactor(BNU_CHUNK_T m0)61 BNU_CHUNK_T gsMontFactor(BNU_CHUNK_T m0)
62 {
63 BNU_CHUNK_T y = 1;
64 BNU_CHUNK_T x = 2;
65 BNU_CHUNK_T mask = 2*x-1;
66
67 int i;
68 for(i=2; i<=BNU_CHUNK_BITS; i++, x<<=1) {
69 BNU_CHUNK_T rH, rL;
70 MUL_AB(rH, rL, m0, y);
71 if( x < (rL & mask) ) /* x < ((m0*y) mod (2*x)) */
72 y+=x;
73 mask += mask + 1;
74 }
75 return 0-y;
76 }
77