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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.

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package org.bouncycastle.crypto.generators;

import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.DerivationParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.DigestDerivationFunction;
import org.bouncycastle.crypto.OutputLengthException;
import org.bouncycastle.crypto.params.ISO18033KDFParameters;
import org.bouncycastle.crypto.params.KDFParameters;
import org.bouncycastle.util.Pack;

/**
 * Basic KDF generator for derived keys and ivs as defined by IEEE P1363a/ISO
 * 18033 

 * This implementation is based on ISO 18033/P1363a.
 */
public class BaseKDFBytesGenerator
    implements DigestDerivationFunction
{
    private int    counterStart;
    private Digest digest;
    private byte[] shared;
    private byte[] iv;

    /**
     * Construct a KDF Parameters generator.
     * 
     * 
     * @param counterStart
     *            value of counter.
     * @param digest
     *            the digest to be used as the source of derived keys.
     */
    protected BaseKDFBytesGenerator(int counterStart, Digest digest)
    {
        this.counterStart = counterStart;
        this.digest = digest;
    }

    public void init(DerivationParameters param)
    {
        if (param instanceof KDFParameters)
        {
            KDFParameters p = (KDFParameters)param;

            shared = p.getSharedSecret();
            iv = p.getIV();
        }
        else if (param instanceof ISO18033KDFParameters)
        {
            ISO18033KDFParameters p = (ISO18033KDFParameters)param;

            shared = p.getSeed();
            iv = null;
        }
        else
        {
            throw new IllegalArgumentException("KDF parameters required for generator");
        }
    }

    /**
     * return the underlying digest.
     */
    public Digest getDigest()
    {
        return digest;
    }

    /**
     * fill len bytes of the output buffer with bytes generated from the
     * derivation function.
     * 
     * @throws IllegalArgumentException
     *             if the size of the request will cause an overflow.
     * @throws DataLengthException
     *             if the out buffer is too small.
     */
    public int generateBytes(byte[] out, int outOff, int len) throws DataLengthException,
            IllegalArgumentException
    {
        if ((out.length - len) < outOff)
        {
            throw new OutputLengthException("output buffer too small");
        }

        long oBytes = len;
        int outLen = digest.getDigestSize();

        //
        // this is at odds with the standard implementation, the
        // maximum value should be hBits * (2^32 - 1) where hBits
        // is the digest output size in bits. We can't have an
        // array with a long index at the moment...
        //
        if (oBytes > ((2L << 32) - 1))
        {
            throw new IllegalArgumentException("Output length too large");
        }

        int cThreshold = (int)((oBytes + outLen - 1) / outLen);

        byte[] dig = new byte[digest.getDigestSize()];

        byte[] C = new byte[4];
        Pack.intToBigEndian(counterStart, C, 0);

        int counterBase = counterStart & ~0xFF;

        for (int i = 0; i < cThreshold; i++)
        {
            digest.update(shared, 0, shared.length);
            digest.update(C, 0, C.length);

            if (iv != null)
            {
                digest.update(iv, 0, iv.length);
            }

            digest.doFinal(dig, 0);

            if (len > outLen)
            {
                System.arraycopy(dig, 0, out, outOff, outLen);
                outOff += outLen;
                len -= outLen;
            }
            else
            {
                System.arraycopy(dig, 0, out, outOff, len);
            }

            if (++C[3] == 0)
            {
                counterBase += 0x100;
                Pack.intToBigEndian(counterBase, C, 0);
            }
        }

        digest.reset();

        return (int)oBytes;
    }
}