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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8.

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package org.bouncycastle.crypto.prng;

/**
 * Utility methods for making use of EntropySources.
 */
public class EntropyUtil
{
    /**
     * Generate numBytes worth of entropy from the passed in entropy source.
     *
     * @param entropySource the entropy source to request the data from.
     * @param numBytes the number of bytes of entropy requested.
     * @return a byte array populated with the random data.
     */
    public static byte[] generateSeed(EntropySource entropySource, int numBytes)
    {
        byte[] bytes = new byte[numBytes];

        if (numBytes * 8 <= entropySource.entropySize())
        {
            byte[] ent = entropySource.getEntropy();

            System.arraycopy(ent, 0, bytes, 0, bytes.length);
        }
        else
        {
            int entSize = entropySource.entropySize() / 8;

            for (int i = 0; i < bytes.length; i += entSize)
            {
                byte[] ent = entropySource.getEntropy();

                if (ent.length <= bytes.length - i)
                {
                    System.arraycopy(ent, 0, bytes, i, ent.length);
                }
                else
                {
                    System.arraycopy(ent, 0, bytes, i, bytes.length - i);
                }
            }
        }

        return bytes;
    }
}