qortal/src/org/whispersystems/curve25519/java/curve_sigs.java

package org.whispersystems.curve25519.java;

public class curve_sigs {

    public static void curve25519_keygen(byte[] curve25519_pubkey_out,
                           byte[] curve25519_privkey_in)
    {
      ge_p3 ed = new ge_p3(); /* Ed25519 pubkey point */
      //int[] ed_y = new int[10]; 
      int[] ed_y_plus_one = new int[10];
      int[] one_minus_ed_y = new int[10];
      int[] inv_one_minus_ed_y = new int[10];
      int[] mont_x = new int[10];

      /* Perform a fixed-base multiplication of the Edwards base point,
         (which is efficient due to precalculated tables), then convert
         to the Curve25519 montgomery-format public key.  In particular,
         convert Curve25519's "montgomery" x-coordinate into an Ed25519
         "edwards" y-coordinate:

         mont_x = (ed_y + 1) / (1 - ed_y)
         
         with projective coordinates:

         mont_x = (ed_y + ed_z) / (ed_z - ed_y)

         NOTE: ed_y=1 is converted to mont_x=0 since fe_invert is mod-exp
      */

      ge_scalarmult_base.ge_scalarmult_base(ed, curve25519_privkey_in);
      fe_add.fe_add(ed_y_plus_one, ed.Y, ed.Z);
      fe_sub.fe_sub(one_minus_ed_y, ed.Z, ed.Y);  
      fe_invert.fe_invert(inv_one_minus_ed_y, one_minus_ed_y);
      fe_mul.fe_mul(mont_x, ed_y_plus_one, inv_one_minus_ed_y);
      fe_tobytes.fe_tobytes(curve25519_pubkey_out, mont_x);
    }

    public static int curve25519_sign(Sha512 sha512provider, byte[] signature_out,
                        byte[] curve25519_privkey,
                        byte[] msg, int msg_len,
                        byte[] random)
    {
      ge_p3 ed_pubkey_point = new ge_p3(); /* Ed25519 pubkey point */
      byte[] ed_pubkey = new byte[32]; /* Ed25519 encoded pubkey */
      byte[] sigbuf = new byte[msg_len + 128]; /* working buffer */
      byte sign_bit = 0;

      /* Convert the Curve25519 privkey to an Ed25519 public key */
      ge_scalarmult_base.ge_scalarmult_base(ed_pubkey_point, curve25519_privkey);
      ge_p3_tobytes.ge_p3_tobytes(ed_pubkey, ed_pubkey_point);
      sign_bit = (byte)(ed_pubkey[31] & 0x80);

      /* Perform an Ed25519 signature with explicit private key */
      sign_modified.crypto_sign_modified(sha512provider, sigbuf, msg, msg_len, curve25519_privkey,
                                         ed_pubkey, random);
      System.arraycopy(sigbuf, 0, signature_out, 0, 64);

      /* Encode the sign bit into signature (in unused high bit of S) */
       signature_out[63] &= 0x7F; /* bit should be zero already, but just in case */
       signature_out[63] |= sign_bit;
       return 0;
    }

    public static int curve25519_verify(Sha512 sha512provider, byte[] signature,
                          byte[] curve25519_pubkey,
                          byte[] msg, int msg_len)
    {
      int[] mont_x = new int[10];
      int[] mont_x_minus_one = new int[10];
      int[] mont_x_plus_one = new int[10];
      int[] inv_mont_x_plus_one = new int[10];
      int[] one = new int[10];
      int[] ed_y = new int[10];
      byte[] ed_pubkey = new byte[32];
      long some_retval = 0;
      byte[] verifybuf = new byte[msg_len + 64]; /* working buffer */
      byte[] verifybuf2 = new byte[msg_len + 64]; /* working buffer #2 */

      /* Convert the Curve25519 public key into an Ed25519 public key.  In
         particular, convert Curve25519's "montgomery" x-coordinate into an
         Ed25519 "edwards" y-coordinate:

         ed_y = (mont_x - 1) / (mont_x + 1)

         NOTE: mont_x=-1 is converted to ed_y=0 since fe_invert is mod-exp

         Then move the sign bit into the pubkey from the signature.
      */
            
      fe_frombytes.fe_frombytes(mont_x, curve25519_pubkey);
      fe_1.fe_1(one);
      fe_sub.fe_sub(mont_x_minus_one, mont_x, one);
      fe_add.fe_add(mont_x_plus_one, mont_x, one);
      fe_invert.fe_invert(inv_mont_x_plus_one, mont_x_plus_one);
      fe_mul.fe_mul(ed_y, mont_x_minus_one, inv_mont_x_plus_one);
      fe_tobytes.fe_tobytes(ed_pubkey, ed_y);

      /* Copy the sign bit, and remove it from signature */
      ed_pubkey[31] &= 0x7F;  /* bit should be zero already, but just in case */
      ed_pubkey[31] |= (signature[63] & 0x80);
      System.arraycopy(signature, 0, verifybuf, 0, 64);
      verifybuf[63] &= 0x7F;

      System.arraycopy(msg, 0, verifybuf, 64, (int)msg_len);

      /* Then perform a normal Ed25519 verification, return 0 on success */
      /* The below call has a strange API: */
      /* verifybuf = R || S || message */
      /* verifybuf2 = java to next call gets a copy of verifybuf, S gets
         replaced with pubkey for hashing, then the whole thing gets zeroized
         (if bad sig), or contains a copy of msg (good sig) */
      return open.crypto_sign_open(sha512provider, verifybuf2, some_retval, verifybuf, 64 + msg_len, ed_pubkey);
    }
}