path: root/src/lib/pubkey/ed25519/sc_reduce.cpp

/*
* Ed25519
* (C) 2017 Ribose Inc
*
* Based on the public domain code from SUPERCOP ref10 by
* Peter Schwabe, Daniel J. Bernstein, Niels Duif, Tanja Lange, Bo-Yin Yang
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/internal/ed25519_internal.h>

namespace Botan {

/*
Input:
  s[0]+256*s[1]+...+256^63*s[63] = s

Output:
  s[0]+256*s[1]+...+256^31*s[31] = s mod l
  where l = 2^252 + 27742317777372353535851937790883648493.
  Overwrites s in place.
*/

void sc_reduce(uint8_t* s)
   {
   const int64_t X20 = (1 << 20);
   const int64_t X21 = (1 << 21);


   int64_t s0 = 2097151 & load_3(s);
   int64_t s1 = 2097151 & (load_4(s + 2) >> 5);
   int64_t s2 = 2097151 & (load_3(s + 5) >> 2);
   int64_t s3 = 2097151 & (load_4(s + 7) >> 7);
   int64_t s4 = 2097151 & (load_4(s + 10) >> 4);
   int64_t s5 = 2097151 & (load_3(s + 13) >> 1);
   int64_t s6 = 2097151 & (load_4(s + 15) >> 6);
   int64_t s7 = 2097151 & (load_3(s + 18) >> 3);
   int64_t s8 = 2097151 & load_3(s + 21);
   int64_t s9 = 2097151 & (load_4(s + 23) >> 5);
   int64_t s10 = 2097151 & (load_3(s + 26) >> 2);
   int64_t s11 = 2097151 & (load_4(s + 28) >> 7);
   int64_t s12 = 2097151 & (load_4(s + 31) >> 4);
   int64_t s13 = 2097151 & (load_3(s + 34) >> 1);
   int64_t s14 = 2097151 & (load_4(s + 36) >> 6);
   int64_t s15 = 2097151 & (load_3(s + 39) >> 3);
   int64_t s16 = 2097151 & load_3(s + 42);
   int64_t s17 = 2097151 & (load_4(s + 44) >> 5);
   int64_t s18 = 2097151 & (load_3(s + 47) >> 2);
   int64_t s19 = 2097151 & (load_4(s + 49) >> 7);
   int64_t s20 = 2097151 & (load_4(s + 52) >> 4);
   int64_t s21 = 2097151 & (load_3(s + 55) >> 1);
   int64_t s22 = 2097151 & (load_4(s + 57) >> 6);
   int64_t s23 = (load_4(s + 60) >> 3);
   int64_t carry0;
   int64_t carry1;
   int64_t carry2;
   int64_t carry3;
   int64_t carry4;
   int64_t carry5;
   int64_t carry6;
   int64_t carry7;
   int64_t carry8;
   int64_t carry9;
   int64_t carry10;
   int64_t carry11;
   int64_t carry12;
   int64_t carry13;
   int64_t carry14;
   int64_t carry15;
   int64_t carry16;

   s11 += s23 * 666643;
   s12 += s23 * 470296;
   s13 += s23 * 654183;
   s14 -= s23 * 997805;
   s15 += s23 * 136657;
   s16 -= s23 * 683901;
   s23 = 0;

   s10 += s22 * 666643;
   s11 += s22 * 470296;
   s12 += s22 * 654183;
   s13 -= s22 * 997805;
   s14 += s22 * 136657;
   s15 -= s22 * 683901;
   s22 = 0;

   s9 += s21 * 666643;
   s10 += s21 * 470296;
   s11 += s21 * 654183;
   s12 -= s21 * 997805;
   s13 += s21 * 136657;
   s14 -= s21 * 683901;
   s21 = 0;

   s8 += s20 * 666643;
   s9 += s20 * 470296;
   s10 += s20 * 654183;
   s11 -= s20 * 997805;
   s12 += s20 * 136657;
   s13 -= s20 * 683901;
   s20 = 0;

   s7 += s19 * 666643;
   s8 += s19 * 470296;
   s9 += s19 * 654183;
   s10 -= s19 * 997805;
   s11 += s19 * 136657;
   s12 -= s19 * 683901;
   s19 = 0;

   s6 += s18 * 666643;
   s7 += s18 * 470296;
   s8 += s18 * 654183;
   s9 -= s18 * 997805;
   s10 += s18 * 136657;
   s11 -= s18 * 683901;
   s18 = 0;

   carry6 = (s6 + X20) >> 21;
   s7 += carry6;
   s6 -= carry6 * X21;
   carry8 = (s8 + X20) >> 21;
   s9 += carry8;
   s8 -= carry8 * X21;
   carry10 = (s10 + X20) >> 21;
   s11 += carry10;
   s10 -= carry10 * X21;
   carry12 = (s12 + X20) >> 21;
   s13 += carry12;
   s12 -= carry12 * X21;
   carry14 = (s14 + X20) >> 21;
   s15 += carry14;
   s14 -= carry14 * X21;
   carry16 = (s16 + X20) >> 21;
   s17 += carry16;
   s16 -= carry16 * X21;

   carry7 = (s7 + X20) >> 21;
   s8 += carry7;
   s7 -= carry7 * X21;
   carry9 = (s9 + X20) >> 21;
   s10 += carry9;
   s9 -= carry9 * X21;
   carry11 = (s11 + X20) >> 21;
   s12 += carry11;
   s11 -= carry11 * X21;
   carry13 = (s13 + X20) >> 21;
   s14 += carry13;
   s13 -= carry13 * X21;
   carry15 = (s15 + X20) >> 21;
   s16 += carry15;
   s15 -= carry15 * X21;

   s5 += s17 * 666643;
   s6 += s17 * 470296;
   s7 += s17 * 654183;
   s8 -= s17 * 997805;
   s9 += s17 * 136657;
   s10 -= s17 * 683901;
   s17 = 0;

   s4 += s16 * 666643;
   s5 += s16 * 470296;
   s6 += s16 * 654183;
   s7 -= s16 * 997805;
   s8 += s16 * 136657;
   s9 -= s16 * 683901;
   s16 = 0;

   s3 += s15 * 666643;
   s4 += s15 * 470296;
   s5 += s15 * 654183;
   s6 -= s15 * 997805;
   s7 += s15 * 136657;
   s8 -= s15 * 683901;
   s15 = 0;

   s2 += s14 * 666643;
   s3 += s14 * 470296;
   s4 += s14 * 654183;
   s5 -= s14 * 997805;
   s6 += s14 * 136657;
   s7 -= s14 * 683901;
   s14 = 0;

   s1 += s13 * 666643;
   s2 += s13 * 470296;
   s3 += s13 * 654183;
   s4 -= s13 * 997805;
   s5 += s13 * 136657;
   s6 -= s13 * 683901;
   s13 = 0;

   s0 += s12 * 666643;
   s1 += s12 * 470296;
   s2 += s12 * 654183;
   s3 -= s12 * 997805;
   s4 += s12 * 136657;
   s5 -= s12 * 683901;
   s12 = 0;

   carry0 = (s0 + X20) >> 21;
   s1 += carry0;
   s0 -= carry0 * X21;
   carry2 = (s2 + X20) >> 21;
   s3 += carry2;
   s2 -= carry2 * X21;
   carry4 = (s4 + X20) >> 21;
   s5 += carry4;
   s4 -= carry4 * X21;
   carry6 = (s6 + X20) >> 21;
   s7 += carry6;
   s6 -= carry6 * X21;
   carry8 = (s8 + X20) >> 21;
   s9 += carry8;
   s8 -= carry8 * X21;
   carry10 = (s10 + X20) >> 21;
   s11 += carry10;
   s10 -= carry10 * X21;

   carry1 = (s1 + X20) >> 21;
   s2 += carry1;
   s1 -= carry1 * X21;
   carry3 = (s3 + X20) >> 21;
   s4 += carry3;
   s3 -= carry3 * X21;
   carry5 = (s5 + X20) >> 21;
   s6 += carry5;
   s5 -= carry5 * X21;
   carry7 = (s7 + X20) >> 21;
   s8 += carry7;
   s7 -= carry7 * X21;
   carry9 = (s9 + X20) >> 21;
   s10 += carry9;
   s9 -= carry9 * X21;
   carry11 = (s11 + X20) >> 21;
   s12 += carry11;
   s11 -= carry11 * X21;

   s0 += s12 * 666643;
   s1 += s12 * 470296;
   s2 += s12 * 654183;
   s3 -= s12 * 997805;
   s4 += s12 * 136657;
   s5 -= s12 * 683901;
   s12 = 0;

   carry0 = s0 >> 21;
   s1 += carry0;
   s0 -= carry0 * X21;
   carry1 = s1 >> 21;
   s2 += carry1;
   s1 -= carry1 * X21;
   carry2 = s2 >> 21;
   s3 += carry2;
   s2 -= carry2 * X21;
   carry3 = s3 >> 21;
   s4 += carry3;
   s3 -= carry3 * X21;
   carry4 = s4 >> 21;
   s5 += carry4;
   s4 -= carry4 * X21;
   carry5 = s5 >> 21;
   s6 += carry5;
   s5 -= carry5 * X21;
   carry6 = s6 >> 21;
   s7 += carry6;
   s6 -= carry6 * X21;
   carry7 = s7 >> 21;
   s8 += carry7;
   s7 -= carry7 * X21;
   carry8 = s8 >> 21;
   s9 += carry8;
   s8 -= carry8 * X21;
   carry9 = s9 >> 21;
   s10 += carry9;
   s9 -= carry9 * X21;
   carry10 = s10 >> 21;
   s11 += carry10;
   s10 -= carry10 * X21;
   carry11 = s11 >> 21;
   s12 += carry11;
   s11 -= carry11 * X21;

   s0 += s12 * 666643;
   s1 += s12 * 470296;
   s2 += s12 * 654183;
   s3 -= s12 * 997805;
   s4 += s12 * 136657;
   s5 -= s12 * 683901;
   s12 = 0;

   carry0 = s0 >> 21;
   s1 += carry0;
   s0 -= carry0 * X21;
   carry1 = s1 >> 21;
   s2 += carry1;
   s1 -= carry1 * X21;
   carry2 = s2 >> 21;
   s3 += carry2;
   s2 -= carry2 * X21;
   carry3 = s3 >> 21;
   s4 += carry3;
   s3 -= carry3 * X21;
   carry4 = s4 >> 21;
   s5 += carry4;
   s4 -= carry4 * X21;
   carry5 = s5 >> 21;
   s6 += carry5;
   s5 -= carry5 * X21;
   carry6 = s6 >> 21;
   s7 += carry6;
   s6 -= carry6 * X21;
   carry7 = s7 >> 21;
   s8 += carry7;
   s7 -= carry7 * X21;
   carry8 = s8 >> 21;
   s9 += carry8;
   s8 -= carry8 * X21;
   carry9 = s9 >> 21;
   s10 += carry9;
   s9 -= carry9 * X21;
   carry10 = s10 >> 21;
   s11 += carry10;
   s10 -= carry10 * X21;

   s[0] = s0 >> 0;
   s[1] = s0 >> 8;
   s[2] = (s0 >> 16) | (s1 << 5);
   s[3] = s1 >> 3;
   s[4] = s1 >> 11;
   s[5] = (s1 >> 19) | (s2 << 2);
   s[6] = s2 >> 6;
   s[7] = (s2 >> 14) | (s3 << 7);
   s[8] = s3 >> 1;
   s[9] = s3 >> 9;
   s[10] = (s3 >> 17) | (s4 << 4);
   s[11] = s4 >> 4;
   s[12] = s4 >> 12;
   s[13] = (s4 >> 20) | (s5 << 1);
   s[14] = s5 >> 7;
   s[15] = (s5 >> 15) | (s6 << 6);
   s[16] = s6 >> 2;
   s[17] = s6 >> 10;
   s[18] = (s6 >> 18) | (s7 << 3);
   s[19] = s7 >> 5;
   s[20] = s7 >> 13;
   s[21] = s8 >> 0;
   s[22] = s8 >> 8;
   s[23] = (s8 >> 16) | (s9 << 5);
   s[24] = s9 >> 3;
   s[25] = s9 >> 11;
   s[26] = (s9 >> 19) | (s10 << 2);
   s[27] = s10 >> 6;
   s[28] = (s10 >> 14) | (s11 << 7);
   s[29] = s11 >> 1;
   s[30] = s11 >> 9;
   s[31] = s11 >> 17;
   }

}