org.whispersystems.curve25519.java.ge_scalarmult Maven / Gradle / Ivy

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Curve25519 library for Java
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package org.whispersystems.curve25519.java;

import static org.whispersystems.curve25519.java.ge_scalarmult_base.equal;
import static org.whispersystems.curve25519.java.ge_scalarmult_base.negative;

public class ge_scalarmult {

    static void cmov(ge_cached t, ge_cached u, int b) {
        fe_cmov.fe_cmov(t.YplusX, u.YplusX, b);
        fe_cmov.fe_cmov(t.YminusX, u.YminusX, b);
        fe_cmov.fe_cmov(t.Z, u.Z, b);
        fe_cmov.fe_cmov(t.T2d, u.T2d, b);
    }

    static void select(ge_cached t, ge_cached[] pre, byte b) {
        ge_cached minust = new ge_cached();
        byte bnegative = negative(b);
        byte babs = (byte) (b - (((-bnegative) & b) << 1));

        fe_1.fe_1(t.YplusX);
        fe_1.fe_1(t.YminusX);
        fe_1.fe_1(t.Z);
        fe_0.fe_0(t.T2d);

        cmov(t, pre[0], equal(babs, (byte) 1));
        cmov(t, pre[1], equal(babs, (byte) 2));
        cmov(t, pre[2], equal(babs, (byte) 3));
        cmov(t, pre[3], equal(babs, (byte) 4));
        cmov(t, pre[4], equal(babs, (byte) 5));
        cmov(t, pre[5], equal(babs, (byte) 6));
        cmov(t, pre[6], equal(babs, (byte) 7));
        cmov(t, pre[7], equal(babs, (byte) 8));
        fe_copy.fe_copy(minust.YplusX, t.YminusX);
        fe_copy.fe_copy(minust.YminusX, t.YplusX);
        fe_copy.fe_copy(minust.Z, t.Z);
        fe_neg.fe_neg(minust.T2d, t.T2d);
        cmov(t, minust, bnegative);
    }

    /*
    h = a*A, where A is the input
    where a = a[0]+256*a[1]+...+256^31 a[31]

    Preconditions:
      a[31] <= 127
    */
    public static void ge_scalarmult(ge_p3 h, byte[] a, ge_p3 A) {
        byte[] e = new byte[64];
        byte carry;
        ge_p1p1 r = new ge_p1p1();
        ge_p2 s = new ge_p2();
        ge_p3 t0 = new ge_p3(), t1 = new ge_p3(), t2 = new ge_p3();
        ge_cached t = new ge_cached();
        ge_cached[] pre = new ge_cached[8];
        for (int i = 0; i < 8; i++) {
            pre[i] = new ge_cached();
        }
        int i;

        for (i = 0; i < 32; ++i) {
            e[2 * i] = (byte) ((a[i]) & 15);
            e[2 * i + 1] = (byte) ((a[i] >>> 4) & 15);
        }
        /* each e[i] is between 0 and 15 */
        /* e[63] is between 0 and 7 */

        carry = 0;
        for (i = 0; i < 63; ++i) {
            e[i] += carry;
            carry = (byte) (e[i] + 8);
            carry >>= 4;
            e[i] -= carry << 4;
        }
        e[63] += carry;
        /* each e[i] is between -8 and 8 */

        // Precomputation:
        ge_p3_to_cached.ge_p3_to_cached(pre[0], A); // A

        ge_p3_dbl.ge_p3_dbl(r, A);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t0, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[1], t0); // 2A

        ge_add.ge_add(r, A, pre[1]);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t1, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[2], t1); // 3A

        ge_p3_dbl.ge_p3_dbl(r, t0);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t0, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[3], t0); // 4A

        ge_add.ge_add(r, A, pre[3]);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t2, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[4], t2); // 5A

        ge_p3_dbl.ge_p3_dbl(r, t1);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t1, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[5], t1); // 6A

        ge_add.ge_add(r, A, pre[5]);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t1, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[6], t1); // 7A

        ge_p3_dbl.ge_p3_dbl(r, t0);
        ge_p1p1_to_p3.ge_p1p1_to_p3(t0, r);
        ge_p3_to_cached.ge_p3_to_cached(pre[7], t0); // 8A

        ge_p3_0.ge_p3_0(h);

        for (i = 63; i > 0; i--) {
            select(t, pre, e[i]);
            ge_add.ge_add(r, h, t);
            ge_p1p1_to_p2.ge_p1p1_to_p2(s, r);

            ge_p2_dbl.ge_p2_dbl(r, s);
            ge_p1p1_to_p2.ge_p1p1_to_p2(s, r);
            ge_p2_dbl.ge_p2_dbl(r, s);
            ge_p1p1_to_p2.ge_p1p1_to_p2(s, r);
            ge_p2_dbl.ge_p2_dbl(r, s);
            ge_p1p1_to_p2.ge_p1p1_to_p2(s, r);
            ge_p2_dbl.ge_p2_dbl(r, s);
            ge_p1p1_to_p3.ge_p1p1_to_p3(h, r);
        }
        select(t, pre, e[0]);
        ge_add.ge_add(r, h, t);
        ge_p1p1_to_p3.ge_p1p1_to_p3(h, r);
    }

}