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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 LTS provider but may also be used with other providers providing cryptographic services.

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package org.bouncycastle.bcpg;

import java.io.IOException;
import java.math.BigInteger;

import org.bouncycastle.util.BigIntegers;

/**
 * base class for an RSA Secret (or Private) Key.
 */
public class RSASecretBCPGKey 
    extends BCPGObject implements BCPGKey 
{
    MPInteger    d;
    MPInteger    p;
    MPInteger    q;
    MPInteger    u;
    
    BigInteger    expP, expQ, crt;
    
    /**
     * 
     * @param in
     * @throws IOException
     */
    public RSASecretBCPGKey(
        BCPGInputStream    in)
        throws IOException
    {
        this.d = new MPInteger(in);
        this.p = new MPInteger(in);
        this.q = new MPInteger(in);
        this.u = new MPInteger(in);

        expP = d.getValue().remainder(p.getValue().subtract(BigInteger.valueOf(1)));
        expQ = d.getValue().remainder(q.getValue().subtract(BigInteger.valueOf(1)));
        crt = BigIntegers.modOddInverse(p.getValue(), q.getValue());
    }
    
    /**
     * 
     * @param d
     * @param p
     * @param q
     */
    public RSASecretBCPGKey(
        BigInteger    d,
        BigInteger    p,
        BigInteger    q)
    {
        //
        // pgp requires (p < q)
        //
        int cmp = p.compareTo(q);
        if (cmp >= 0)
        {
            if (cmp == 0)
            {
                throw new IllegalArgumentException("p and q cannot be equal");
            }

            BigInteger tmp = p;
            p = q;
            q = tmp;
        }

        this.d = new MPInteger(d);
        this.p = new MPInteger(p);
        this.q = new MPInteger(q);
        this.u = new MPInteger(BigIntegers.modOddInverse(q, p));

        expP = d.remainder(p.subtract(BigInteger.valueOf(1)));
        expQ = d.remainder(q.subtract(BigInteger.valueOf(1)));
        crt = BigIntegers.modOddInverse(p, q);
    }
    
    /**
     * return the modulus for this key.
     * 
     * @return BigInteger
     */
    public BigInteger getModulus()
    {
        return p.getValue().multiply(q.getValue());
    }
    
    /**
     * return the private exponent for this key.
     * 
     * @return BigInteger
     */
    public BigInteger getPrivateExponent()
    {
        return d.getValue();
    }
    
    /**
     * return the prime P
     */
    public BigInteger getPrimeP()
    {
        return p.getValue();
    }
    
    /**
     * return the prime Q
     */
    public BigInteger getPrimeQ()
    {
        return q.getValue();
    }
    
    /**
     * return the prime exponent of p
     */
    public BigInteger getPrimeExponentP()
    {
        return expP;
    }
    
    /**
     * return the prime exponent of q
     */
    public BigInteger getPrimeExponentQ()
    {
        return expQ;
    }
    
    /**
     * return the crt coefficient
     */
    public BigInteger getCrtCoefficient()
    {
        return crt;
    }
    
    /**
     *  return "PGP"
     * 
     * @see org.bouncycastle.bcpg.BCPGKey#getFormat()
     */
    public String getFormat() 
    {
        return "PGP";
    }

    /**
     * return the standard PGP encoding of the key.
     * 
     * @see org.bouncycastle.bcpg.BCPGKey#getEncoded()
     */
    public byte[] getEncoded() 
    {
        try
        {
            return super.getEncoded();
        }
        catch (IOException e)
        {
            return null;
        }
    }
    
    public void encode(
        BCPGOutputStream    out)
        throws IOException
    {
        out.writeObject(d);
        out.writeObject(p);
        out.writeObject(q);
        out.writeObject(u);
    }
}