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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 and up.
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package org.bouncycastle.crypto.prng.drbg;

import java.util.Hashtable;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.prng.EntropySource;
import org.bouncycastle.util.Arrays;
import org.bouncycastle.util.Integers;

/**
 * A SP800-90A Hash DRBG.
 */
public class HashSP800DRBG
    implements SP80090DRBG
{
    private final static byte[]     ONE = { 0x01 };

    private final static long       RESEED_MAX = 1L << (48 - 1);
    private final static int        MAX_BITS_REQUEST = 1 << (19 - 1);

    private final static Hashtable  seedlens = new Hashtable();

    static
    {
        seedlens.put("SHA-1", Integers.valueOf(440));
        seedlens.put("SHA-224", Integers.valueOf(440));
        seedlens.put("SHA-256", Integers.valueOf(440));
        seedlens.put("SHA-512/256", Integers.valueOf(440));
        seedlens.put("SHA-512/224", Integers.valueOf(440));
        seedlens.put("SHA-384", Integers.valueOf(888));
        seedlens.put("SHA-512", Integers.valueOf(888));
    }

    private Digest        _digest;
    private byte[]        _V;
    private byte[]        _C;
    private long          _reseedCounter;
    private EntropySource _entropySource;
    private int           _securityStrength;
    private int           _seedLength;

    /**
     * Construct a SP800-90A Hash DRBG.
     * 
     * Minimum entropy requirement is the security strength requested.
     * 
     * @param digest  source digest to use for DRB stream.
     * @param securityStrength security strength required (in bits)
     * @param entropySource source of entropy to use for seeding/reseeding.
     * @param personalizationString personalization string to distinguish this DRBG (may be null).
     * @param nonce nonce to further distinguish this DRBG (may be null).
     */
    public HashSP800DRBG(Digest digest, int securityStrength, EntropySource entropySource, byte[] personalizationString, byte[] nonce)
    {
        if (securityStrength > Utils.getMaxSecurityStrength(digest))
        {
            throw new IllegalArgumentException("Requested security strength is not supported by the derivation function");
        }

        if (entropySource.entropySize() < securityStrength)
        {
            throw new IllegalArgumentException("Not enough entropy for security strength required");
        }

        _digest = digest;
        _entropySource = entropySource;
        _securityStrength = securityStrength;
        _seedLength = ((Integer)seedlens.get(digest.getAlgorithmName())).intValue();

        // 1. seed_material = entropy_input || nonce || personalization_string.
        // 2. seed = Hash_df (seed_material, seedlen).
        // 3. V = seed.
        // 4. C = Hash_df ((0x00 || V), seedlen). Comment: Preceed V with a byte
        // of zeros.
        // 5. reseed_counter = 1.
        // 6. Return V, C, and reseed_counter as the initial_working_state

        byte[] entropy = getEntropy();
        byte[] seedMaterial = Arrays.concatenate(entropy, nonce, personalizationString);
        byte[] seed = Utils.hash_df(_digest, seedMaterial, _seedLength);

        _V = seed;
        byte[] subV = new byte[_V.length + 1];
        System.arraycopy(_V, 0, subV, 1, _V.length);
        _C = Utils.hash_df(_digest, subV, _seedLength);

        _reseedCounter = 1;
    }

    /**
     * Return the block size (in bits) of the DRBG.
     *
     * @return the number of bits produced on each internal round of the DRBG.
     */
    public int getBlockSize()
    {
        return _digest.getDigestSize() * 8;
    }

    /**
     * Populate a passed in array with random data.
     *
     * @param output output array for generated bits.
     * @param additionalInput additional input to be added to the DRBG in this step.
     * @param predictionResistant true if a reseed should be forced, false otherwise.
     *
     * @return number of bits generated, -1 if a reseed required.
     */
    public int generate(byte[] output, byte[] additionalInput, boolean predictionResistant)
    {
        // 1. If reseed_counter > reseed_interval, then return an indication that a
        // reseed is required.
        // 2. If (additional_input != Null), then do
        // 2.1 w = Hash (0x02 || V || additional_input).
        // 2.2 V = (V + w) mod 2^seedlen
        // .
        // 3. (returned_bits) = Hashgen (requested_number_of_bits, V).
        // 4. H = Hash (0x03 || V).
        // 5. V = (V + H + C + reseed_counter) mod 2^seedlen
        // .
        // 6. reseed_counter = reseed_counter + 1.
        // 7. Return SUCCESS, returned_bits, and the new values of V, C, and
        // reseed_counter for the new_working_state.
        int numberOfBits = output.length*8;

        if (numberOfBits > MAX_BITS_REQUEST)
        {
            throw new IllegalArgumentException("Number of bits per request limited to " + MAX_BITS_REQUEST);
        }

        if (_reseedCounter > RESEED_MAX)
        {
            return -1;
        }

        if (predictionResistant)
        {   
            reseed(additionalInput);
            additionalInput = null;
        }

        // 2.
        if (additionalInput != null)
        {
            byte[] newInput = new byte[1 + _V.length + additionalInput.length];
            newInput[0] = 0x02;
            System.arraycopy(_V, 0, newInput, 1, _V.length);
            // TODO: inOff / inLength
            System.arraycopy(additionalInput, 0, newInput, 1 + _V.length, additionalInput.length);
            byte[] w = hash(newInput);

            addTo(_V, w);
        }
        
        // 3.
        byte[] rv = hashgen(_V, numberOfBits);
        
        // 4.
        byte[] subH = new byte[_V.length + 1];
        System.arraycopy(_V, 0, subH, 1, _V.length);
        subH[0] = 0x03;
        
        byte[] H = hash(subH);
        
        // 5.
        addTo(_V, H);
        addTo(_V, _C);
        byte[] c = new byte[4];
        c[0] = (byte)(_reseedCounter >> 24);
        c[1] = (byte)(_reseedCounter >> 16);
        c[2] = (byte)(_reseedCounter >> 8);
        c[3] = (byte)_reseedCounter;
        
        addTo(_V, c);

        _reseedCounter++;

        System.arraycopy(rv, 0, output, 0, output.length);

        return numberOfBits;
    }

    private byte[] getEntropy()
    {
        byte[] entropy = _entropySource.getEntropy();
        if (entropy.length < (_securityStrength + 7) / 8)
        {
            throw new IllegalStateException("Insufficient entropy provided by entropy source");
        }
        return entropy;
    }

    // this will always add the shorter length byte array mathematically to the
    // longer length byte array.
    // be careful....
    private void addTo(byte[] longer, byte[] shorter)
    {
        int carry = 0;
        for (int i=1;i <= shorter.length; i++) // warning
        {
            int res = (longer[longer.length-i] & 0xff) + (shorter[shorter.length-i] & 0xff) + carry;
            carry = (res > 0xff) ? 1 : 0;
            longer[longer.length-i] = (byte)res;
        }
        
        for (int i=shorter.length+1;i <= longer.length; i++) // warning
        {
            int res = (longer[longer.length-i] & 0xff) + carry;
            carry = (res > 0xff) ? 1 : 0;
            longer[longer.length-i] = (byte)res;
        }
    }

    /**
      * Reseed the DRBG.
      *
      * @param additionalInput additional input to be added to the DRBG in this step.
      */
    public void reseed(byte[] additionalInput)
    {
        // 1. seed_material = 0x01 || V || entropy_input || additional_input.
        //
        // 2. seed = Hash_df (seed_material, seedlen).
        //
        // 3. V = seed.
        //
        // 4. C = Hash_df ((0x00 || V), seedlen).
        //
        // 5. reseed_counter = 1.
        //
        // 6. Return V, C, and reseed_counter for the new_working_state.
        //
        // Comment: Precede with a byte of all zeros.
        byte[] entropy = getEntropy();
        byte[] seedMaterial = Arrays.concatenate(ONE, _V, entropy, additionalInput);
        byte[] seed = Utils.hash_df(_digest, seedMaterial, _seedLength);

        _V = seed;
        byte[] subV = new byte[_V.length + 1];
        subV[0] = 0x00;
        System.arraycopy(_V, 0, subV, 1, _V.length);
        _C = Utils.hash_df(_digest, subV, _seedLength);

        _reseedCounter = 1;
    }
    
    private byte[] hash(byte[] input)
    {
        byte[] hash = new byte[_digest.getDigestSize()];
        doHash(input, hash);
        return hash;
    }

    private void doHash(byte[] input, byte[] output)
    {
        _digest.update(input, 0, input.length);
        _digest.doFinal(output, 0);
    }

    // 1. m = [requested_number_of_bits / outlen]
    // 2. data = V.
    // 3. W = the Null string.
    // 4. For i = 1 to m
    // 4.1 wi = Hash (data).
    // 4.2 W = W || wi.
    // 4.3 data = (data + 1) mod 2^seedlen
    // .
    // 5. returned_bits = Leftmost (requested_no_of_bits) bits of W.
    private byte[] hashgen(byte[] input, int lengthInBits)
    {
        int digestSize = _digest.getDigestSize();
        int m = (lengthInBits / 8) / digestSize;

        byte[] data = new byte[input.length];
        System.arraycopy(input, 0, data, 0, input.length);

        byte[] W = new byte[lengthInBits / 8];

        byte[] dig = new byte[_digest.getDigestSize()];
        for (int i = 0; i <= m; i++)
        {
            doHash(data, dig);

            int bytesToCopy = ((W.length - i * dig.length) > dig.length)
                    ? dig.length
                    : (W.length - i * dig.length);
            System.arraycopy(dig, 0, W, i * dig.length, bytesToCopy);

            addTo(data, ONE);
        }

        return W;
    }    
}