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/*
This file is part of GNUnet.
(C) 2014 Christian Grothoff (and other contributing authors)
GNUnet is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published
by the Free Software Foundation; either version 3, or (at your
option) any later version.
GNUnet is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with GNUnet; see the file COPYING. If not, write to the
Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.
*/
package org.gnunet.secretsharing;
import org.gnunet.construct.FixedSizeIntegerArray;
import org.gnunet.construct.Message;
import org.gnunet.util.BigIntegers;
import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.Arrays;
public class Plaintext implements Message {
@FixedSizeIntegerArray(signed = true, bitSize = 8, length = Parameters.elgamalBits / 8)
public byte[] bits;
public static Plaintext generate(BigInteger exp) {
BigInteger val;
val = Parameters.elgamalG.modPow(exp, Parameters.elgamalP);
Plaintext plaintext = new Plaintext();
plaintext.bits = BigIntegers.serializeUnsigned(val, Parameters.elgamalBits);
return plaintext;
}
/**
* Try all products of length l of the given generators.
*
* @param l number of factors
* @param generators different factors to chose from
* @return null if no solution was found, or an array of coefficients
*/
public long[] bruteForceDiscreteLog(final long l, BigInteger[] generators) {
BigInteger needle = new BigInteger(1, bits);
if (l == 0) {
if (needle.equals(BigInteger.ONE))
return new long[0];
return null;
}
long[] combo = new long[generators.length];
boolean success = bruteForceDiscreteLog(l, combo, needle, BigInteger.ONE, generators, 0);
if (success) {
return combo;
}
return null;
}
private static boolean bruteForceDiscreteLog(final long l, long[] combo,
final BigInteger needle, final BigInteger haystack,
BigInteger[] generators, final int genIdx) {
if (l == 0) {
combo[genIdx] = 0;
return haystack.equals(needle);
}
if (genIdx == generators.length - 1) {
combo[genIdx] = l;
BigInteger myHay = haystack.multiply(generators[genIdx].modPow(BigInteger.valueOf(l), Parameters.elgamalP)).mod(Parameters.elgamalP);
return myHay.equals(needle);
} else {
BigInteger myHay = haystack;
for (int i = 0; i <= l; i++) {
combo[genIdx] = i;
boolean success = bruteForceDiscreteLog(l - i, combo, needle, myHay, generators, genIdx + 1);
if (success) {
return true;
}
myHay = myHay.multiply(generators[genIdx]).mod(Parameters.elgamalP);
}
}
return false;
}
public Ciphertext encrypt(ThresholdPublicKey publicKey) {
SecureRandom secureRandom = new SecureRandom();
BigInteger c_1;
BigInteger c_2;
BigInteger y;
BigInteger m;
BigInteger h;
m = new BigInteger(1, this.bits);
h = new BigInteger(1, publicKey.bits);
do {
y = new BigInteger(Parameters.elgamalBits - 1, secureRandom);
} while (y.compareTo(BigInteger.ONE) == 0 || y.compareTo(Parameters.elgamalQ) >= 0);
c_1 = Parameters.elgamalG.modPow(y, Parameters.elgamalP);
c_2 = m.multiply(h.modPow(y, Parameters.elgamalP)).mod(Parameters.elgamalP);
Ciphertext ciphertext = new Ciphertext();
ciphertext.c_1 = BigIntegers.serializeUnsigned(c_1, Parameters.elgamalBits);
ciphertext.c_2 = BigIntegers.serializeUnsigned(c_2, Parameters.elgamalBits);
return ciphertext;
}
@Override
public String toString() {
return "Plaintext{" +
"bits=" + Arrays.toString(bits) +
'}';
}
}
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