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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 Java 1.8 and later with debug enabled.
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package org.bouncycastle.crypto.generators;

import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.DerivationFunction;
import org.bouncycastle.crypto.DerivationParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.macs.HMac;
import org.bouncycastle.crypto.params.HKDFParameters;
import org.bouncycastle.crypto.params.KeyParameter;

/**
 * HMAC-based Extract-and-Expand Key Derivation Function (HKDF) implemented
 * according to IETF RFC 5869, May 2010 as specified by H. Krawczyk, IBM
 * Research & P. Eronen, Nokia. It uses a HMac internally to compute de OKM
 * (output keying material) and is likely to have better security properties
 * than KDF's based on just a hash function.
 */
public class HKDFBytesGenerator
    implements DerivationFunction
{

    private HMac hMacHash;
    private int hashLen;

    private byte[] info;
    private byte[] currentT;

    private int generatedBytes;

    /**
     * Creates a HKDFBytesGenerator based on the given hash function.
     *
     * @param hash the digest to be used as the source of generatedBytes bytes
     */
    public HKDFBytesGenerator(Digest hash)
    {
        this.hMacHash = new HMac(hash);
        this.hashLen = hash.getDigestSize();
    }

    public void init(DerivationParameters param)
    {
        if (!(param instanceof HKDFParameters))
        {
            throw new IllegalArgumentException(
                "HKDF parameters required for HKDFBytesGenerator");
        }

        HKDFParameters params = (HKDFParameters)param;
        if (params.skipExtract())
        {
            // use IKM directly as PRK
            hMacHash.init(new KeyParameter(params.getIKM()));
        }
        else
        {
            hMacHash.init(new KeyParameter(extractPRK(params.getSalt(), params.getIKM())));
        }

        info = params.getInfo();

        generatedBytes = 0;
        currentT = new byte[hashLen];
    }

    /**
     * Performs the extract part of the key derivation function.
     *
     * @param salt the salt to use
     * @param ikm  the input keying material
     * @return the PRK as KeyParameter
     */
    public byte[] extractPRK(byte[] salt, byte[] ikm)
    {
        if (salt == null)
        {
            // TODO check if hashLen is indeed same as HMAC size
            hMacHash.init(new KeyParameter(new byte[hashLen]));
        }
        else
        {
            hMacHash.init(new KeyParameter(salt));
        }

        hMacHash.update(ikm, 0, ikm.length);

        byte[] prk = new byte[hashLen];
        
        hMacHash.doFinal(prk, 0);

        return prk;
    }

    /**
     * Performs the expand part of the key derivation function, using currentT
     * as input and output buffer.
     *
     * @throws DataLengthException if the total number of bytes generated is larger than the one
     * specified by RFC 5869 (255 * HashLen)
     */
    private void expandNext()
        throws DataLengthException
    {
        int n = generatedBytes / hashLen + 1;
        if (n >= 256)
        {
            throw new DataLengthException(
                "HKDF cannot generate more than 255 blocks of HashLen size");
        }
        // special case for T(0): T(0) is empty, so no update
        if (generatedBytes != 0)
        {
            hMacHash.update(currentT, 0, hashLen);
        }
        hMacHash.update(info, 0, info.length);
        hMacHash.update((byte)n);
        hMacHash.doFinal(currentT, 0);
    }

    public Digest getDigest()
    {
        return hMacHash.getUnderlyingDigest();
    }

    public int generateBytes(byte[] out, int outOff, int len)
        throws DataLengthException, IllegalArgumentException
    {

        if (generatedBytes + len > 255 * hashLen)
        {
            throw new DataLengthException(
                "HKDF may only be used for 255 * HashLen bytes of output");
        }

        if (generatedBytes % hashLen == 0)
        {
            expandNext();
        }

        // copy what is left in the currentT (1..hash
        int toGenerate = len;
        int posInT = generatedBytes % hashLen;
        int leftInT = hashLen - generatedBytes % hashLen;
        int toCopy = Math.min(leftInT, toGenerate);
        System.arraycopy(currentT, posInT, out, outOff, toCopy);
        generatedBytes += toCopy;
        toGenerate -= toCopy;
        outOff += toCopy;

        while (toGenerate > 0)
        {
            expandNext();
            toCopy = Math.min(hashLen, toGenerate);
            System.arraycopy(currentT, 0, out, outOff, toCopy);
            generatedBytes += toCopy;
            toGenerate -= toCopy;
            outOff += toCopy;
        }

        return len;
    }
}