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40
41 /*
42 //
43 // Purpose:
44 // Cryptography Primitive. Modular Arithmetic Engine. General Functionality
45 //
46 // Contents:
47 // gs_inv()
48 //
49 */
50
51 #include "owndefs.h"
52 #include "owncp.h"
53 #include "pcpbnumisc.h"
54 #include "pcpbnuarith.h"
55 #include "gsmodstuff.h"
56
57 //tbcd: temporary excluded: #include <assert.h>
58
59 /*
60 // returns r =1/a
61 // a in desidue domain
62 // r in desidue domain
63 */
gs_inv(BNU_CHUNK_T * pr,const BNU_CHUNK_T * pa,gsModEngine * pME,alm_inv alm_inversion)64 BNU_CHUNK_T* gs_inv(BNU_CHUNK_T* pr, const BNU_CHUNK_T* pa, gsModEngine* pME, alm_inv alm_inversion)
65 {
66 int k = alm_inversion(pr, pa, pME);
67
68 if(0==k)
69 return NULL;
70
71 {
72 int mLen = MOD_LEN(pME);
73 int m = mLen*BNU_CHUNK_BITS;
74 mod_mul mon_mul = MOD_METHOD(pME)->mul;
75
76 BNU_CHUNK_T* t = gsModPoolAlloc(pME, 1);
77 //tbcd: temporary excluded: assert(NULL!=t);
78
79 if(k>m) {
80 ZEXPAND_BNU(t, 0, mLen);
81 t[0] = 1;
82 mon_mul(pr, pr, t, pME);
83 k -= m;
84 }
85 ZEXPAND_BNU(t, 0, mLen);
86 SET_BIT(t, m-k); /* t = 2^(m-k) */
87 mon_mul(pr, pr, t, pME);
88
89 gsModPoolFree(pME, 1);
90
91 return pr;
92 }
93 }
94