/**
 * (C) 2007-22 - ntop.org and contributors
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not see see <http://www.gnu.org/licenses/>
 *
 */


/**
 * version 20081011
 * Matthew Dempsky
 * Public domain.
 * Derived from public domain code by D. J. Bernstein.
 * 20140216 tweak: Mask top bit of point input.
 *
 */


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);
}


void 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];
    work[31] &= 127;

    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];
}