lokinet/crypto/curve25519/ref10/fe_25_5/fe.h

/*
 Ignores top bit of h.
 */

void
fe25519_frombytes(fe25519 h, const unsigned char *s)
{
    int64_t h0 = load_4(s);
    int64_t h1 = load_3(s + 4) << 6;
    int64_t h2 = load_3(s + 7) << 5;
    int64_t h3 = load_3(s + 10) << 3;
    int64_t h4 = load_3(s + 13) << 2;
    int64_t h5 = load_4(s + 16);
    int64_t h6 = load_3(s + 20) << 7;
    int64_t h7 = load_3(s + 23) << 5;
    int64_t h8 = load_3(s + 26) << 4;
    int64_t h9 = (load_3(s + 29) & 8388607) << 2;

    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;

    carry9 = (h9 + (int64_t)(1L << 24)) >> 25;
    h0 += carry9 * 19;
    h9 -= carry9 * ((uint64_t) 1L << 25);
    carry1 = (h1 + (int64_t)(1L << 24)) >> 25;
    h2 += carry1;
    h1 -= carry1 * ((uint64_t) 1L << 25);
    carry3 = (h3 + (int64_t)(1L << 24)) >> 25;
    h4 += carry3;
    h3 -= carry3 * ((uint64_t) 1L << 25);
    carry5 = (h5 + (int64_t)(1L << 24)) >> 25;
    h6 += carry5;
    h5 -= carry5 * ((uint64_t) 1L << 25);
    carry7 = (h7 + (int64_t)(1L << 24)) >> 25;
    h8 += carry7;
    h7 -= carry7 * ((uint64_t) 1L << 25);

    carry0 = (h0 + (int64_t)(1L << 25)) >> 26;
    h1 += carry0;
    h0 -= carry0 * ((uint64_t) 1L << 26);
    carry2 = (h2 + (int64_t)(1L << 25)) >> 26;
    h3 += carry2;
    h2 -= carry2 * ((uint64_t) 1L << 26);
    carry4 = (h4 + (int64_t)(1L << 25)) >> 26;
    h5 += carry4;
    h4 -= carry4 * ((uint64_t) 1L << 26);
    carry6 = (h6 + (int64_t)(1L << 25)) >> 26;
    h7 += carry6;
    h6 -= carry6 * ((uint64_t) 1L << 26);
    carry8 = (h8 + (int64_t)(1L << 25)) >> 26;
    h9 += carry8;
    h8 -= carry8 * ((uint64_t) 1L << 26);

    h[0] = (int32_t) h0;
    h[1] = (int32_t) h1;
    h[2] = (int32_t) h2;
    h[3] = (int32_t) h3;
    h[4] = (int32_t) h4;
    h[5] = (int32_t) h5;
    h[6] = (int32_t) h6;
    h[7] = (int32_t) h7;
    h[8] = (int32_t) h8;
    h[9] = (int32_t) h9;
}

/*
 Preconditions:
 |h| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.

 Write p=2^255-19; q=floor(h/p).
 Basic claim: q = floor(2^(-255)(h + 19 2^(-25)h9 + 2^(-1))).

 Proof:
 Have |h|<=p so |q|<=1 so |19^2 2^(-255) q|<1/4.
 Also have |h-2^230 h9|<2^231 so |19 2^(-255)(h-2^230 h9)|<1/4.

 Write y=2^(-1)-19^2 2^(-255)q-19 2^(-255)(h-2^230 h9).
 Then 0<y<1.

 Write r=h-pq.
 Have 0<=r<=p-1=2^255-20.
 Thus 0<=r+19(2^-255)r<r+19(2^-255)2^255<=2^255-1.

 Write x=r+19(2^-255)r+y.
 Then 0<x<2^255 so floor(2^(-255)x) = 0 so floor(q+2^(-255)x) = q.

 Have q+2^(-255)x = 2^(-255)(h + 19 2^(-25) h9 + 2^(-1))
 so floor(2^(-255)(h + 19 2^(-25) h9 + 2^(-1))) = q.
*/

static void
fe25519_reduce(fe25519 h, const fe25519 f)
{
    int32_t h0 = f[0];
    int32_t h1 = f[1];
    int32_t h2 = f[2];
    int32_t h3 = f[3];
    int32_t h4 = f[4];
    int32_t h5 = f[5];
    int32_t h6 = f[6];
    int32_t h7 = f[7];
    int32_t h8 = f[8];
    int32_t h9 = f[9];

    int32_t q;
    int32_t carry0, carry1, carry2, carry3, carry4, carry5, carry6, carry7, carry8, carry9;

    q = (19 * h9 + ((uint32_t) 1L << 24)) >> 25;
    q = (h0 + q) >> 26;
    q = (h1 + q) >> 25;
    q = (h2 + q) >> 26;
    q = (h3 + q) >> 25;
    q = (h4 + q) >> 26;
    q = (h5 + q) >> 25;
    q = (h6 + q) >> 26;
    q = (h7 + q) >> 25;
    q = (h8 + q) >> 26;
    q = (h9 + q) >> 25;

    /* Goal: Output h-(2^255-19)q, which is between 0 and 2^255-20. */
    h0 += 19 * q;
    /* Goal: Output h-2^255 q, which is between 0 and 2^255-20. */

    carry0 = h0 >> 26;
    h1 += carry0;
    h0 -= carry0 * ((uint32_t) 1L << 26);
    carry1 = h1 >> 25;
    h2 += carry1;
    h1 -= carry1 * ((uint32_t) 1L << 25);
    carry2 = h2 >> 26;
    h3 += carry2;
    h2 -= carry2 * ((uint32_t) 1L << 26);
    carry3 = h3 >> 25;
    h4 += carry3;
    h3 -= carry3 * ((uint32_t) 1L << 25);
    carry4 = h4 >> 26;
    h5 += carry4;
    h4 -= carry4 * ((uint32_t) 1L << 26);
    carry5 = h5 >> 25;
    h6 += carry5;
    h5 -= carry5 * ((uint32_t) 1L << 25);
    carry6 = h6 >> 26;
    h7 += carry6;
    h6 -= carry6 * ((uint32_t) 1L << 26);
    carry7 = h7 >> 25;
    h8 += carry7;
    h7 -= carry7 * ((uint32_t) 1L << 25);
    carry8 = h8 >> 26;
    h9 += carry8;
    h8 -= carry8 * ((uint32_t) 1L << 26);
    carry9 = h9 >> 25;
    h9 -= carry9 * ((uint32_t) 1L << 25);

    h[0] = h0;
    h[1] = h1;
    h[2] = h2;
    h[3] = h3;
    h[4] = h4;
    h[5] = h5;
    h[6] = h6;
    h[7] = h7;
    h[8] = h8;
    h[9] = h9;
}

/*
 Goal: Output h0+...+2^255 h10-2^255 q, which is between 0 and 2^255-20.
 Have h0+...+2^230 h9 between 0 and 2^255-1;
 evidently 2^255 h10-2^255 q = 0.

 Goal: Output h0+...+2^230 h9.
 */

void
fe25519_tobytes(unsigned char *s, const fe25519 h)
{
    fe25519 t;

    fe25519_reduce(t, h);
    s[0]  = t[0] >> 0;
    s[1]  = t[0] >> 8;
    s[2]  = t[0] >> 16;
    s[3]  = (t[0] >> 24) | (t[1] * ((uint32_t) 1 << 2));
    s[4]  = t[1] >> 6;
    s[5]  = t[1] >> 14;
    s[6]  = (t[1] >> 22) | (t[2] * ((uint32_t) 1 << 3));
    s[7]  = t[2] >> 5;
    s[8]  = t[2] >> 13;
    s[9]  = (t[2] >> 21) | (t[3] * ((uint32_t) 1 << 5));
    s[10] = t[3] >> 3;
    s[11] = t[3] >> 11;
    s[12] = (t[3] >> 19) | (t[4] * ((uint32_t) 1 << 6));
    s[13] = t[4] >> 2;
    s[14] = t[4] >> 10;
    s[15] = t[4] >> 18;
    s[16] = t[5] >> 0;
    s[17] = t[5] >> 8;
    s[18] = t[5] >> 16;
    s[19] = (t[5] >> 24) | (t[6] * ((uint32_t) 1 << 1));
    s[20] = t[6] >> 7;
    s[21] = t[6] >> 15;
    s[22] = (t[6] >> 23) | (t[7] * ((uint32_t) 1 << 3));
    s[23] = t[7] >> 5;
    s[24] = t[7] >> 13;
    s[25] = (t[7] >> 21) | (t[8] * ((uint32_t) 1 << 4));
    s[26] = t[8] >> 4;
    s[27] = t[8] >> 12;
    s[28] = (t[8] >> 20) | (t[9] * ((uint32_t) 1 << 6));
    s[29] = t[9] >> 2;
    s[30] = t[9] >> 10;
    s[31] = t[9] >> 18;
}