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

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Curve25519 library for Java
There is a newer version: 0.5.0
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 >> 0);
  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 >> 0);
  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);
}


}