/* $OpenBSD: smult_curve25519_ref.c,v 1.2 2013/11/02 22:02:14 markus Exp $ */

/*

version 20081011

Matthew Dempsky

Public domain.

Derived from public domain code by D. J. Bernstein.

*/

int crypto_scalarmult_curve25519(unsigned char *, const unsigned char *, const unsigned char *);

static void add(unsigned int out[32],const unsigned int a[32],const unsigned int b[32])

{

  unsigned int j;

  unsigned int u;

  u = 0;

  for (j = 0;j < 31;++j) { u += a[j] + b[j]; out[j] = u & 255; u >>= 8; }

  u += a[31] + b[31]; out[31] = u;

}

static void sub(unsigned int out[32],const unsigned int a[32],const unsigned int b[32])

{

  unsigned int j;

  unsigned int u;

  u = 218;

  for (j = 0;j < 31;++j) {

    u += a[j] + 65280 - b[j];

    out[j] = u & 255;

    u >>= 8;

  }

  u += a[31] - b[31];

  out[31] = u;

}

static void squeeze(unsigned int a[32])

{

  unsigned int j;

  unsigned int u;

  u = 0;

  for (j = 0;j < 31;++j) { u += a[j]; a[j] = u & 255; u >>= 8; }

  u += a[31]; a[31] = u & 127;

  u = 19 * (u >> 7);

  for (j = 0;j < 31;++j) { u += a[j]; a[j] = u & 255; u >>= 8; }

  u += a[31]; a[31] = u;

}

static const unsigned int minusp[32] = {

 19, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 128

} ;

static void freeze(unsigned int a[32])

{

  unsigned int aorig[32];

  unsigned int j;

  unsigned int negative;

  for (j = 0;j < 32;++j) aorig[j] = a[j];

  add(a,a,minusp);

  negative = -((a[31] >> 7) & 1);

  for (j = 0;j < 32;++j) a[j] ^= negative & (aorig[j] ^ a[j]);

}

static void mult(unsigned int out[32],const unsigned int a[32],const unsigned int b[32])

{

  unsigned int i;

  unsigned int j;

  unsigned int u;

  for (i = 0;i < 32;++i) {

    u = 0;

    for (j = 0;j <= i;++j) u += a[j] * b[i - j];

    for (j = i + 1;j < 32;++j) u += 38 * a[j] * b[i + 32 - j];

    out[i] = u;

  }

  squeeze(out);

}

static void mult121665(unsigned int out[32],const unsigned int a[32])

{

  unsigned int j;

  unsigned int u;

  u = 0;

  for (j = 0;j < 31;++j) { u += 121665 * a[j]; out[j] = u & 255; u >>= 8; }

  u += 121665 * a[31]; out[31] = u & 127;

  u = 19 * (u >> 7);

  for (j = 0;j < 31;++j) { u += out[j]; out[j] = u & 255; u >>= 8; }

  u += out[j]; out[j] = u;

}

static void square(unsigned int out[32],const unsigned int a[32])

{

  unsigned int i;

  unsigned int j;

  unsigned int u;

  for (i = 0;i < 32;++i) {

    u = 0;

    for (j = 0;j < i - j;++j) u += a[j] * a[i - j];

    for (j = i + 1;j < i + 32 - j;++j) u += 38 * a[j] * a[i + 32 - j];

    u *= 2;

    if ((i & 1) == 0) {

      u += a[i / 2] * a[i / 2];

      u += 38 * a[i / 2 + 16] * a[i / 2 + 16];

    }

    out[i] = u;

  }

  squeeze(out);

}

static void select(unsigned int p[64],unsigned int q[64],const unsigned int r[64],const unsigned int s[64],unsigned int b)

{

  unsigned int j;

  unsigned int t;

  unsigned int bminus1;

  bminus1 = b - 1;

  for (j = 0;j < 64;++j) {

    t = bminus1 & (r[j] ^ s[j]);

    p[j] = s[j] ^ t;

    q[j] = r[j] ^ t;

  }

}

static void mainloop(unsigned int work[64],const unsigned char e[32])

{

  unsigned int xzm1[64];

  unsigned int xzm[64];

  unsigned int xzmb[64];

  unsigned int xzm1b[64];

  unsigned int xznb[64];

  unsigned int xzn1b[64];

  unsigned int a0[64];

  unsigned int a1[64];

  unsigned int b0[64];

  unsigned int b1[64];

  unsigned int c1[64];

  unsigned int r[32];

  unsigned int s[32];

  unsigned int t[32];

  unsigned int u[32];

  unsigned int j;

  unsigned int b;

  int pos;

  for (j = 0;j < 32;++j) xzm1[j] = work[j];

  xzm1[32] = 1;

  for (j = 33;j < 64;++j) xzm1[j] = 0;

  xzm[0] = 1;

  for (j = 1;j < 64;++j) xzm[j] = 0;

  for (pos = 254;pos >= 0;--pos) {

    b = e[pos / 8] >> (pos & 7);

    b &= 1;

    select(xzmb,xzm1b,xzm,xzm1,b);

    add(a0,xzmb,xzmb + 32);

    sub(a0 + 32,xzmb,xzmb + 32);

    add(a1,xzm1b,xzm1b + 32);

    sub(a1 + 32,xzm1b,xzm1b + 32);

    square(b0,a0);

    square(b0 + 32,a0 + 32);

    mult(b1,a1,a0 + 32);

    mult(b1 + 32,a1 + 32,a0);

    add(c1,b1,b1 + 32);

    sub(c1 + 32,b1,b1 + 32);

    square(r,c1 + 32);

    sub(s,b0,b0 + 32);

    mult121665(t,s);

    add(u,t,b0);

    mult(xznb,b0,b0 + 32);

    mult(xznb + 32,s,u);

    square(xzn1b,c1);

    mult(xzn1b + 32,r,work);

    select(xzm,xzm1,xznb,xzn1b,b);

  }

  for (j = 0;j < 64;++j) work[j] = xzm[j];

}

static void recip(unsigned int out[32],const unsigned int z[32])

{

  unsigned int z2[32];

  unsigned int z9[32];

  unsigned int z11[32];

  unsigned int z2_5_0[32];

  unsigned int z2_10_0[32];

  unsigned int z2_20_0[32];

  unsigned int z2_50_0[32];

  unsigned int z2_100_0[32];

  unsigned int t0[32];

  unsigned int t1[32];

  int i;

  /* 2 */ square(z2,z);

  /* 4 */ square(t1,z2);

  /* 8 */ square(t0,t1);

  /* 9 */ mult(z9,t0,z);

  /* 11 */ mult(z11,z9,z2);

  /* 22 */ square(t0,z11);

  /* 2^5 - 2^0 = 31 */ mult(z2_5_0,t0,z9);

  /* 2^6 - 2^1 */ square(t0,z2_5_0);

  /* 2^7 - 2^2 */ square(t1,t0);

  /* 2^8 - 2^3 */ square(t0,t1);

  /* 2^9 - 2^4 */ square(t1,t0);

  /* 2^10 - 2^5 */ square(t0,t1);

  /* 2^10 - 2^0 */ mult(z2_10_0,t0,z2_5_0);

  /* 2^11 - 2^1 */ square(t0,z2_10_0);

  /* 2^12 - 2^2 */ square(t1,t0);

  /* 2^20 - 2^10 */ for (i = 2;i < 10;i += 2) { square(t0,t1); square(t1,t0); }

  /* 2^20 - 2^0 */ mult(z2_20_0,t1,z2_10_0);

  /* 2^21 - 2^1 */ square(t0,z2_20_0);

  /* 2^22 - 2^2 */ square(t1,t0);

  /* 2^40 - 2^20 */ for (i = 2;i < 20;i += 2) { square(t0,t1); square(t1,t0); }

  /* 2^40 - 2^0 */ mult(t0,t1,z2_20_0);

  /* 2^41 - 2^1 */ square(t1,t0);

  /* 2^42 - 2^2 */ square(t0,t1);

  /* 2^50 - 2^10 */ for (i = 2;i < 10;i += 2) { square(t1,t0); square(t0,t1); }

  /* 2^50 - 2^0 */ mult(z2_50_0,t0,z2_10_0);

  /* 2^51 - 2^1 */ square(t0,z2_50_0);

  /* 2^52 - 2^2 */ square(t1,t0);

  /* 2^100 - 2^50 */ for (i = 2;i < 50;i += 2) { square(t0,t1); square(t1,t0); }

  /* 2^100 - 2^0 */ mult(z2_100_0,t1,z2_50_0);

  /* 2^101 - 2^1 */ square(t1,z2_100_0);

  /* 2^102 - 2^2 */ square(t0,t1);

  /* 2^200 - 2^100 */ for (i = 2;i < 100;i += 2) { square(t1,t0); square(t0,t1); }

  /* 2^200 - 2^0 */ mult(t1,t0,z2_100_0);

  /* 2^201 - 2^1 */ square(t0,t1);

  /* 2^202 - 2^2 */ square(t1,t0);

  /* 2^250 - 2^50 */ for (i = 2;i < 50;i += 2) { square(t0,t1); square(t1,t0); }

  /* 2^250 - 2^0 */ mult(t0,t1,z2_50_0);

  /* 2^251 - 2^1 */ square(t1,t0);

  /* 2^252 - 2^2 */ square(t0,t1);

  /* 2^253 - 2^3 */ square(t1,t0);

  /* 2^254 - 2^4 */ square(t0,t1);

  /* 2^255 - 2^5 */ square(t1,t0);

  /* 2^255 - 21 */ mult(out,t1,z11);

}

int crypto_scalarmult_curve25519(unsigned char *q,

  const unsigned char *n,

  const unsigned char *p)

{

  unsigned int work[96];

  unsigned char e[32];

  unsigned int i;

  for (i = 0;i < 32;++i) e[i] = n[i];

  e[0] &= 248;

  e[31] &= 127;

  e[31] |= 64;

  for (i = 0;i < 32;++i) work[i] = p[i];

  mainloop(work,e);

  recip(work + 32,work + 32);

  mult(work + 64,work,work + 32);

  freeze(work + 64);

  for (i = 0;i < 32;++i) q[i] = work[64 + i];

  return 0;

}