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56 lines
2.1 KiB
C
56 lines
2.1 KiB
C
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#include "sc25519.h"
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/*Arithmetic modulo the group order n = 2^252 + 27742317777372353535851937790883648493
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* = 7237005577332262213973186563042994240857116359379907606001950938285454250989
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*/
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/* Contains order, 2*order, 4*order, 8*order, each represented in 4 consecutive unsigned long long */
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static const unsigned long long order[16] = {0x5812631A5CF5D3EDULL, 0x14DEF9DEA2F79CD6ULL,
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0x0000000000000000ULL, 0x1000000000000000ULL,
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0xB024C634B9EBA7DAULL, 0x29BDF3BD45EF39ACULL,
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0x0000000000000000ULL, 0x2000000000000000ULL,
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0x60498C6973D74FB4ULL, 0x537BE77A8BDE7359ULL,
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0x0000000000000000ULL, 0x4000000000000000ULL,
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0xC09318D2E7AE9F68ULL, 0xA6F7CEF517BCE6B2ULL,
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0x0000000000000000ULL, 0x8000000000000000ULL};
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static unsigned long long smaller(unsigned long long a,unsigned long long b)
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{
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unsigned long long atop = a >> 32;
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unsigned long long abot = a & 4294967295;
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unsigned long long btop = b >> 32;
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unsigned long long bbot = b & 4294967295;
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unsigned long long atopbelowbtop = (atop - btop) >> 63;
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unsigned long long atopeqbtop = ((atop ^ btop) - 1) >> 63;
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unsigned long long abotbelowbbot = (abot - bbot) >> 63;
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return atopbelowbtop | (atopeqbtop & abotbelowbbot);
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}
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void sc25519_from32bytes(sc25519 *r, const unsigned char x[32])
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{
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unsigned long long t[4];
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unsigned long long b;
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unsigned long long mask;
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int i, j;
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/* assuming little-endian */
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r->v[0] = *(unsigned long long *)x;
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r->v[1] = *(((unsigned long long *)x)+1);
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r->v[2] = *(((unsigned long long *)x)+2);
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r->v[3] = *(((unsigned long long *)x)+3);
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for(j=3;j>=0;j--)
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{
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b=0;
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for(i=0;i<4;i++)
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{
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b += order[4*j+i]; /* no overflow for this particular order */
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t[i] = r->v[i] - b;
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b = smaller(r->v[i],b);
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}
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mask = b - 1;
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for(i=0;i<4;i++)
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r->v[i] ^= mask & (r->v[i] ^ t[i]);
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}
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}
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