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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.bcpg;

import java.io.*;
import java.math.BigInteger;

/**
 * a multiple precision integer
 */
public class MPInteger 
    extends BCPGObject
{
    BigInteger    value = null;
    
    public MPInteger(
        BCPGInputStream    in)
        throws IOException
    {
        int       length = (in.read() << 8) | in.read();
        byte[]    bytes = new byte[(length + 7) / 8];
        
        in.readFully(bytes);
        
        value = new BigInteger(1, bytes);
    }
    
    public MPInteger(
        BigInteger    value)
    {
        if (value == null || value.signum() < 0)
        {
            throw new IllegalArgumentException("value must not be null, or negative");
        }

        this.value = value;
    }
    
    public BigInteger getValue()
    {
        return value;
    }
    
    public void encode(
        BCPGOutputStream    out)
        throws IOException
    {
        int length = value.bitLength();
        
        out.write(length >> 8);
        out.write(length);
        
        byte[]    bytes = value.toByteArray();
        
        if (bytes[0] == 0)
        {
            out.write(bytes, 1, bytes.length - 1);
        }
        else
        {
            out.write(bytes, 0, bytes.length);
        }
    }
}