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package org.spongycastle.pqc.crypto.ntru;
import org.spongycastle.crypto.Digest;
import org.spongycastle.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 <code>i</code> such that <code>0 <= i < N</code>.
*
* @return
*/
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 <code>numBits</code> bits from the end of the bit string.
*
* @param numBits number of bits
* @return a new <code>BitString</code> of length <code>numBits</code>
*/
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 <code>int</code> whose lower <code>numBits</code> 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;
}
}
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