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

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

public class fe_tobytes {

//CONVERT #include "fe.h"

/*
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> 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 << 26;
        carry1 = h1 >> 25;
        h2 += carry1;
        h1 -= carry1 << 25;
        carry2 = h2 >> 26;
        h3 += carry2;
        h2 -= carry2 << 26;
        carry3 = h3 >> 25;
        h4 += carry3;
        h3 -= carry3 << 25;
        carry4 = h4 >> 26;
        h5 += carry4;
        h4 -= carry4 << 26;
        carry5 = h5 >> 25;
        h6 += carry5;
        h5 -= carry5 << 25;
        carry6 = h6 >> 26;
        h7 += carry6;
        h6 -= carry6 << 26;
        carry7 = h7 >> 25;
        h8 += carry7;
        h7 -= carry7 << 25;
        carry8 = h8 >> 26;
        h9 += carry8;
        h8 -= carry8 << 26;
        carry9 = h9 >> 25;
        h9 -= carry9 << 25;
        /* h10 = carry9 */

  /*
  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.
  */

        s[0] = (byte) (h0);
        s[1] = (byte) (h0 >> 8);
        s[2] = (byte) (h0 >> 16);
        s[3] = (byte) ((h0 >> 24) | (h1 << 2));
        s[4] = (byte) (h1 >> 6);
        s[5] = (byte) (h1 >> 14);
        s[6] = (byte) ((h1 >> 22) | (h2 << 3));
        s[7] = (byte) (h2 >> 5);
        s[8] = (byte) (h2 >> 13);
        s[9] = (byte) ((h2 >> 21) | (h3 << 5));
        s[10] = (byte) (h3 >> 3);
        s[11] = (byte) (h3 >> 11);
        s[12] = (byte) ((h3 >> 19) | (h4 << 6));
        s[13] = (byte) (h4 >> 2);
        s[14] = (byte) (h4 >> 10);
        s[15] = (byte) (h4 >> 18);
        s[16] = (byte) (h5);
        s[17] = (byte) (h5 >> 8);
        s[18] = (byte) (h5 >> 16);
        s[19] = (byte) ((h5 >> 24) | (h6 << 1));
        s[20] = (byte) (h6 >> 7);
        s[21] = (byte) (h6 >> 15);
        s[22] = (byte) ((h6 >> 23) | (h7 << 3));
        s[23] = (byte) (h7 >> 5);
        s[24] = (byte) (h7 >> 13);
        s[25] = (byte) ((h7 >> 21) | (h8 << 4));
        s[26] = (byte) (h8 >> 4);
        s[27] = (byte) (h8 >> 12);
        s[28] = (byte) ((h8 >> 20) | (h9 << 6));
        s[29] = (byte) (h9 >> 2);
        s[30] = (byte) (h9 >> 10);
        s[31] = (byte) (h9 >> 18);
    }


}