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package org.gnunet.voting.simulation;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Random;
/**
* Miscellaneous helper functions.
*
* @author Florian Dold
*/
public class CryptoUtil {
public static final Random random = new Random();
/**
* Return a random BigInteger not less than 'min' and not greater than 'max' with uniform distribution.
*
* @param min the least value that may be generated
* @param max the greatest value that may be generated
* @return a random BigInteger value in the range [min,max]
*/
public static BigInteger createRandomInRange(BigInteger min,
BigInteger max) {
int cmp = min.compareTo(max);
if (cmp >= 0) {
if (cmp > 0) {
throw new IllegalArgumentException("'min' may not be greater than 'max'");
}
return min;
}
if (min.bitLength() > max.bitLength() / 2) {
return createRandomInRange(BigInteger.ZERO, max.subtract(min)).add(min);
}
for (int i = 0; i < 1000; ++i) {
BigInteger x = new BigInteger(max.bitLength(), random);
if (x.compareTo(min) >= 0 && x.compareTo(max) <= 0) {
return x;
}
}
// fall back to a faster (restricted) method
// (using only this distribution would lead to a non-uniform distribution, see the BigInteger constructor)
return new BigInteger(max.subtract(min).bitLength() - 1, random).add(min);
}
/**
* Evaluate a polynomial over Zp*. Uses Horner's scheme.
*
* @param coeffs coefficients of the polynomial, where coeffs[i] is the coefficient of x^i
* @param x the polynomial is evaluated at this value
* @param p what group are we operating in?
* @return the result of evaluating the polynomial at x
*/
public static BigInteger evaluatePolynomial(BigInteger[] coeffs, BigInteger x, BigInteger p) {
BigInteger z = BigInteger.ZERO;
for (int i = 0; i < coeffs.length; ++i) {
// z <- zx + c
z = z.multiply(x).add(coeffs[coeffs.length - i - 1]);
}
return z;
}
public static BigInteger hash(BigInteger... x) {
MessageDigest md;
try {
md = MessageDigest.getInstance("SHA-512");
} catch (NoSuchAlgorithmException e) {
throw new RuntimeException("no SHA-512 available");
}
for (BigInteger v : x) {
md.update(v.toByteArray());
}
return new BigInteger(md.digest());
}
}
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