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DGK_scheme.java
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Tue, Jan 7, 02:28

DGK_scheme.java
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/*
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* and open the template in the editor.
*/
package crypto.dgk;
import java.math.BigInteger;
import java.util.Random;
import crypto.CryptoException;
/**
*
* @author raisaro
*/
public class DGK_scheme {
// scheme parameter
private int k;
private int t;
private int l;
// public key
private BigInteger n;
private BigInteger g;
private BigInteger h;
private BigInteger u;
//private key
private Boolean canDecrypt;
private BigInteger p;
private BigInteger q;
private BigInteger vpvq;
//CRT parameters
private Boolean secretKeyValid;
private BigInteger pp_inv;
private BigInteger qq_inv;
public DGK_scheme(BigInteger[] pk, BigInteger[] sk, int ka, int ti, int ell){
if (sk.length > 3){
secretKeyValid = true;
pp_inv = sk[3];
qq_inv = sk[4];
}
else
secretKeyValid = false;
n = pk[0];
g = pk[1];
h = pk[2];
u = pk[3];
p = sk[0];
q = sk[1];
vpvq = sk[2];
k = ka;
t = ti;
l = ell;
canDecrypt = true;
}
public DGK_scheme(BigInteger[] pk, int ti, int ell, int ka){
secretKeyValid = false;
n = pk[0];
g = pk[1];
h = pk[2];
u = pk[3];
t = ti;
l = ell;
k = ka;
canDecrypt = false;
}
/**
* Standard DGK encryption
* @param message
* @return E[message, r] = cipher
*/
public BigInteger encryption(BigInteger message){
BigInteger tmp1;
BigInteger tmp2;
BigInteger cipher;
tmp1 = new BigInteger(t*2, new Random(System.currentTimeMillis()));
tmp2 = h.modPow(tmp1, n);
tmp1 = g.modPow(message, n);
cipher = tmp1.multiply(tmp2);
cipher = cipher.mod(n);
return cipher;
}
/**
* DGK encryption using CRT for the owner of the private key. It is faster
* than the standard DGK encryption.
* @param message
* @return E[message, r] = cipher
*/
public BigInteger ecryptionCRT(BigInteger message) throws CryptoException{
if (!secretKeyValid){
throw new CryptoException("Cannot encrypt value" +
message + "because the secret key is not valid");
}
BigInteger cipher;
BigInteger tmp1, tmp2, tmp3, tmp_cp, tmp_cq;
// Use Zv instead of a 2t bit number:
Random rnd = new Random(System.currentTimeMillis());
long rand;
rand = Math.abs( rnd.nextLong() ) % vpvq.longValue();
tmp3 = BigInteger.valueOf( rand );
// Calculate in Zp and Zq instead of Zn:
tmp1 = g.modPow(message, p);
tmp2 = h.modPow(tmp3, p);
tmp_cp = tmp1.multiply(tmp2);
tmp1 = g.modPow(message, q);
tmp2 = h.modPow(tmp3, q);
tmp_cq = tmp1.multiply(tmp2);
tmp1 = tmp_cp.multiply(qq_inv);
tmp2 = tmp_cq.multiply(pp_inv);
tmp1 = tmp1.add(tmp2);
cipher = tmp1.mod(n);
return cipher;
}
/**
* DGK "zero decryption".
* This method only checks if or not the ciphertext is an encryption of zero, or not
* @param cipher
* @return true if the cipher is an encryption of zero, false otherwise
* @throws CryptoException
*/
public boolean decryptionZero(BigInteger cipher) throws CryptoException{
if (!canDecrypt){
throw new CryptoException("Cannot dencrypt value because the secret "
+ "key is not stored in the scheme");
}
BigInteger tmp1;
tmp1 = cipher.modPow(vpvq, n);
int res = tmp1.compareTo(BigInteger.ONE);
return (res == 0);
}
/**
* DGK "zero decryption". Using CRT.
* This method only checks if or not the ciphertext is an encryption of zero, or not
* @param cipher
* @return true if the cipher is an encryption of zero, false otherwise
* @throws CryptoException
*/
public boolean decryptionZeroCRT(BigInteger cipher) throws CryptoException{
if (!canDecrypt){
throw new CryptoException("Cannot dencrypt value because the secret "
+ "key is not stored in the scheme");
}
BigInteger tmp1, tmp2, tmp_cp, tmp_cq;
tmp_cp = cipher.modPow(vpvq, p);
tmp_cq = cipher.modPow(vpvq, q);
tmp1 = tmp_cp.multiply(qq_inv);
tmp2 = tmp_cq.multiply(pp_inv);
tmp1 = tmp1.add(tmp2);
tmp2 = tmp1.mod(n);
int res = tmp2.compareTo(BigInteger.ONE);
return (res == 0);
}
/**
* DGK Decryption.
* This method can be used for decryption
* @param cipher
* @return
* @throws CryptoException
*/
public BigInteger decryption(BigInteger cipher) throws CryptoException{
if (!canDecrypt){
throw new CryptoException("Cannot dencrypt value because the secret "
+ "key is not stored in the scheme");
}
BigInteger tmp1, tmp2;
BigInteger message;
message = cipher.modPow(vpvq, n);
int uff = u.intValue();
tmp1 = g.modPow(vpvq, n);
for(int i=0;i<uff;i++){
tmp2 = tmp1.modPow(BigInteger.valueOf(i), n);
int res = message.compareTo(tmp2);
if(res == 0)
message = BigInteger.valueOf(i);
}
return message;
}
}

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