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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.digests;

import org.bouncycastle.util.Arrays;

public class XofUtils
{
    public static byte[] leftEncode(long strLen)
    {
        byte n = 1;

        long v = strLen;
        while ((v >>= 8) != 0)
        {
            n++;
        }

        byte[] b = new byte[n + 1];

        b[0] = n;

        for (int i = 1; i <= n; i++)
        {
            b[i] = (byte)(strLen >> (8 * (n - i)));
        }

        return b;
    }

    public static byte[] rightEncode(long strLen)
    {
        byte n = 1;

        long v = strLen;
        while ((v >>= 8) != 0)
        {
            n++;
        }

        byte[] b = new byte[n + 1];

        b[n] = n;

        for (int i = 0; i < n; i++)
        {
            b[i] = (byte)(strLen >> (8 * (n - i - 1)));
        }

        return b;
    }

    static byte[] encode(byte X)
    {
        return Arrays.concatenate(XofUtils.leftEncode(8), new byte[] { X });
    }

    static byte[] encode(byte[] in, int inOff, int len)
    {
        if (in.length == len)
        {
            return Arrays.concatenate(XofUtils.leftEncode(len * 8), in);
        }
        return Arrays.concatenate(XofUtils.leftEncode(len * 8), Arrays.copyOfRange(in, inOff, inOff + len));
    }
}