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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.pqc.crypto.ntru;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.util.Arrays;

/**
 * An implementation of the Index Generation Function in IEEE P1363.1.
 */
public class IndexGenerator
{
    private byte[] seed;
    private int N;
    private int c;
    private int minCallsR;
    private int totLen;
    private int remLen;
    private BitString buf;
    private int counter;
    private boolean initialized;
    private Digest hashAlg;
    private int hLen;

    /**
     * Constructs a new index generator.
     *
     * @param seed   a seed of arbitrary length to initialize the index generator with
     * @param params NtruEncrypt parameters
     */
    IndexGenerator(byte[] seed, NTRUEncryptionParameters params)
    {
        this.seed = seed;
        N = params.N;
        c = params.c;
        minCallsR = params.minCallsR;

        totLen = 0;
        remLen = 0;
        counter = 0;
        hashAlg = params.hashAlg;

        hLen = hashAlg.getDigestSize();   // hash length
        initialized = false;
    }

    /*
     * Returns a number i such that 0 <= i < N.
     */
    int nextIndex()
    {
        if (!initialized)
        {
            buf = new BitString();
            byte[] hash = new byte[hashAlg.getDigestSize()];
            while (counter < minCallsR)
            {
                appendHash(buf, hash);
                counter++;
            }
            totLen = minCallsR * 8 * hLen;
            remLen = totLen;
            initialized = true;
        }

        while (true)
        {
            totLen += c;
            BitString M = buf.getTrailing(remLen);
            if (remLen < c)
            {
                int tmpLen = c - remLen;
                int cThreshold = counter + (tmpLen + hLen - 1) / hLen;
                byte[] hash = new byte[hashAlg.getDigestSize()];
                while (counter < cThreshold)
                {
                    appendHash(M, hash);
                    counter++;
                    if (tmpLen > 8 * hLen)
                    {
                        tmpLen -= 8 * hLen;
                    }
                }
                remLen = 8 * hLen - tmpLen;
                buf = new BitString();
                buf.appendBits(hash);
            }
            else
            {
                remLen -= c;
            }

            int i = M.getLeadingAsInt(c);   // assume c<32
            if (i < (1 << c) - ((1 << c) % N))
            {
                return i % N;
            }
        }
    }

    private void appendHash(BitString m, byte[] hash)
    {
        hashAlg.update(seed, 0, seed.length);

        putInt(hashAlg, counter);

        hashAlg.doFinal(hash, 0);

        m.appendBits(hash);
    }

    private void putInt(Digest hashAlg, int counter)
    {
        hashAlg.update((byte)(counter >> 24));
        hashAlg.update((byte)(counter >> 16));
        hashAlg.update((byte)(counter >> 8));
        hashAlg.update((byte)counter);
    }

    /**
     * Represents a string of bits and supports appending, reading the head, and reading the tail.
     */
    public static class BitString
    {
        byte[] bytes = new byte[4];
        int numBytes;   // includes the last byte even if only some of its bits are used
        int lastByteBits;   // lastByteBits <= 8

        /**
         * Appends all bits in a byte array to the end of the bit string.
         *
         * @param bytes a byte array
         */
        void appendBits(byte[] bytes)
        {
            for (int i = 0; i != bytes.length; i++)
            {
                appendBits(bytes[i]);
            }
        }

        /**
         * Appends all bits in a byte to the end of the bit string.
         *
         * @param b a byte
         */
        public void appendBits(byte b)
        {
            if (numBytes == bytes.length)
            {
                bytes = copyOf(bytes, 2 * bytes.length);
            }

            if (numBytes == 0)
            {
                numBytes = 1;
                bytes[0] = b;
                lastByteBits = 8;
            }
            else if (lastByteBits == 8)
            {
                bytes[numBytes++] = b;
            }
            else
            {
                int s = 8 - lastByteBits;
                bytes[numBytes - 1] |= (b & 0xFF) << lastByteBits;
                bytes[numBytes++] = (byte)((b & 0xFF) >> s);
            }
        }

        /**
         * Returns the last numBits bits from the end of the bit string.
         *
         * @param numBits number of bits
         * @return a new BitString of length numBits
         */
        public BitString getTrailing(int numBits)
        {
            BitString newStr = new BitString();
            newStr.numBytes = (numBits + 7) / 8;
            newStr.bytes = new byte[newStr.numBytes];
            for (int i = 0; i < newStr.numBytes; i++)
            {
                newStr.bytes[i] = bytes[i];
            }

            newStr.lastByteBits = numBits % 8;
            if (newStr.lastByteBits == 0)
            {
                newStr.lastByteBits = 8;
            }
            else
            {
                int s = 32 - newStr.lastByteBits;
                newStr.bytes[newStr.numBytes - 1] = (byte)(newStr.bytes[newStr.numBytes - 1] << s >>> s);
            }

            return newStr;
        }

        /**
         * Returns up to 32 bits from the beginning of the bit string.
         *
         * @param numBits number of bits
         * @return an int whose lower numBits bits are the beginning of the bit string
         */
        public int getLeadingAsInt(int numBits)
        {
            int startBit = (numBytes - 1) * 8 + lastByteBits - numBits;
            int startByte = startBit / 8;

            int startBitInStartByte = startBit % 8;
            int sum = (bytes[startByte] & 0xFF) >>> startBitInStartByte;
            int shift = 8 - startBitInStartByte;
            for (int i = startByte + 1; i < numBytes; i++)
            {
                sum |= (bytes[i] & 0xFF) << shift;
                shift += 8;
            }

            return sum;
        }

        public byte[] getBytes()
        {
            return Arrays.clone(bytes);
        }
    }

    private static byte[] copyOf(byte[] src, int len)
    {
        byte[] tmp = new byte[len];

        System.arraycopy(src, 0, tmp, 0, len < src.length ? len : src.length);

        return tmp;
    }
}