/* RSA.java -- 
   Copyright (C) 2001, 2002, 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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your option) any later version.

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WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
USA

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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permission to link this library with independent modules to produce an
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package gnu.java.security.sig.rsa;

import gnu.java.security.Properties;
import gnu.java.security.util.PRNG;

import java.math.BigInteger;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.interfaces.RSAPrivateCrtKey;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;

/**
 * Utility methods related to the RSA algorithm.
 * <p>
 * References:
 * <ol>
 * <li><a
 * href="http://www.cosic.esat.kuleuven.ac.be/nessie/workshop/submissions/rsa-pss.zip">
 * RSA-PSS Signature Scheme with Appendix, part B.</a><br>
 * Primitive specification and supporting documentation.<br>
 * Jakob Jonsson and Burt Kaliski.</li>
 * <li><a href="http://www.ietf.org/rfc/rfc3447.txt">Public-Key Cryptography
 * Standards (PKCS) #1:</a><br>
 * RSA Cryptography Specifications Version 2.1.<br>
 * Jakob Jonsson and Burt Kaliski.</li>
 * <li><a href="http://crypto.stanford.edu/~dabo/abstracts/ssl-timing.html">
 * Remote timing attacks are practical</a><br>
 * D. Boneh and D. Brumley.</li>
 * </ol>
 */
public class RSA
{
  private static final BigInteger ZERO = BigInteger.ZERO;

  private static final BigInteger ONE = BigInteger.ONE;

  /** Our default source of randomness. */
  private static final PRNG prng = PRNG.getInstance();

  /** Trivial private constructor to enforce Singleton pattern. */
  private RSA()
  {
    super();
  }

  /**
   * An implementation of the <b>RSASP</b> method: Assuming that the designated
   * RSA private key is a valid one, this method computes a <i>signature
   * representative</i> for a designated <i>message representative</i> signed
   * by the holder of the designated RSA private key.
   * 
   * @param K the RSA private key.
   * @param m the <i>message representative</i>: an integer between
   *          <code>0</code> and <code>n - 1</code>, where <code>n</code>
   *          is the RSA <i>modulus</i>.
   * @return the <i>signature representative</i>, an integer between
   *         <code>0</code> and <code>n - 1</code>, where <code>n</code>
   *         is the RSA <i>modulus</i>.
   * @throws ClassCastException if <code>K</code> is not an RSA one.
   * @throws IllegalArgumentException if <code>m</code> (the <i>message
   *           representative</i>) is out of range.
   */
  public static final BigInteger sign(final PrivateKey K, final BigInteger m)
  {
    try
      {
        return RSADP((RSAPrivateKey) K, m);
      }
    catch (IllegalArgumentException x)
      {
        throw new IllegalArgumentException("message representative out of range");
      }
  }

  /**
   * An implementation of the <b>RSAVP</b> method: Assuming that the designated
   * RSA public key is a valid one, this method computes a <i>message
   * representative</i> for the designated <i>signature representative</i>
   * generated by an RSA private key, for a message intended for the holder of
   * the designated RSA public key.
   * 
   * @param K the RSA public key.
   * @param s the <i>signature representative</i>, an integer between
   *          <code>0</code> and <code>n - 1</code>, where <code>n</code>
   *          is the RSA <i>modulus</i>.
   * @return a <i>message representative</i>: an integer between <code>0</code>
   *         and <code>n - 1</code>, where <code>n</code> is the RSA
   *         <i>modulus</i>.
   * @throws ClassCastException if <code>K</code> is not an RSA one.
   * @throws IllegalArgumentException if <code>s</code> (the <i>signature
   *           representative</i>) is out of range.
   */
  public static final BigInteger verify(final PublicKey K, final BigInteger s)
  {
    try
      {
        return RSAEP((RSAPublicKey) K, s);
      }
    catch (IllegalArgumentException x)
      {
        throw new IllegalArgumentException("signature representative out of range");
      }
  }

  /**
   * An implementation of the <code>RSAEP</code> algorithm.
   * 
   * @param K the recipient's RSA public key.
   * @param m the message representative as an MPI.
   * @return the resulting MPI --an MPI between <code>0</code> and
   *         <code>n - 1</code> (<code>n</code> being the public shared
   *         modulus)-- that will eventually be padded with an appropriate
   *         framing/padding scheme.
   * @throws ClassCastException if <code>K</code> is not an RSA one.
   * @throws IllegalArgumentException if <code>m</code>, the message
   *           representative is not between <code>0</code> and
   *           <code>n - 1</code> (<code>n</code> being the public shared
   *           modulus).
   */
  public static final BigInteger encrypt(final PublicKey K, final BigInteger m)
  {
    try
      {
        return RSAEP((RSAPublicKey) K, m);
      }
    catch (IllegalArgumentException x)
      {
        throw new IllegalArgumentException("message representative out of range");
      }
  }

  /**
   * An implementation of the <code>RSADP</code> algorithm.
   * 
   * @param K the recipient's RSA private key.
   * @param c the ciphertext representative as an MPI.
   * @return the message representative, an MPI between <code>0</code> and
   *         <code>n - 1</code> (<code>n</code> being the shared public
   *         modulus).
   * @throws ClassCastException if <code>K</code> is not an RSA one.
   * @throws IllegalArgumentException if <code>c</code>, the ciphertext
   *           representative is not between <code>0</code> and
   *           <code>n - 1</code> (<code>n</code> being the shared public
   *           modulus).
   */
  public static final BigInteger decrypt(final PrivateKey K, final BigInteger c)
  {
    try
      {
        return RSADP((RSAPrivateKey) K, c);
      }
    catch (IllegalArgumentException x)
      {
        throw new IllegalArgumentException("ciphertext representative out of range");
      }
  }

  /**
   * Converts a <i>multi-precision integer</i> (MPI) <code>s</code> into an
   * octet sequence of length <code>k</code>.
   * 
   * @param s the multi-precision integer to convert.
   * @param k the length of the output.
   * @return the result of the transform.
   * @exception IllegalArgumentException if the length in octets of meaningful
   *              bytes of <code>s</code> is greater than <code>k</code>.
   */
  public static final byte[] I2OSP(final BigInteger s, final int k)
  {
    byte[] result = s.toByteArray();
    if (result.length < k)
      {
        final byte[] newResult = new byte[k];
        System.arraycopy(result, 0, newResult, k - result.length, result.length);
        result = newResult;
      }
    else if (result.length > k)
      { // leftmost extra bytes should all be 0
        final int limit = result.length - k;
        for (int i = 0; i < limit; i++)
          {
            if (result[i] != 0x00)
              throw new IllegalArgumentException("integer too large");
          }
        final byte[] newResult = new byte[k];
        System.arraycopy(result, limit, newResult, 0, k);
        result = newResult;
      }
    return result;
  }

  private static final BigInteger RSAEP(final RSAPublicKey K, final BigInteger m)
  {
    // 1. If the representative m is not between 0 and n - 1, output
    // "representative out of range" and stop.
    final BigInteger n = K.getModulus();
    if (m.compareTo(ZERO) < 0 || m.compareTo(n.subtract(ONE)) > 0)
      throw new IllegalArgumentException();
    // 2. Let c = m^e mod n.
    final BigInteger e = K.getPublicExponent();
    final BigInteger result = m.modPow(e, n);
    // 3. Output c.
    return result;
  }

  private static final BigInteger RSADP(final RSAPrivateKey K, BigInteger c)
  {
    // 1. If the representative c is not between 0 and n - 1, output
    // "representative out of range" and stop.
    final BigInteger n = K.getModulus();
    if (c.compareTo(ZERO) < 0 || c.compareTo(n.subtract(ONE)) > 0)
      throw new IllegalArgumentException();
    // 2. The representative m is computed as follows.
    BigInteger result;
    if (! (K instanceof RSAPrivateCrtKey))
      {
        // a. If the first form (n, d) of K is used, let m = c^d mod n.
        final BigInteger d = K.getPrivateExponent();
        result = c.modPow(d, n);
      }
    else
      {
        // from [3] p.13 --see class docs:
        // The RSA blinding operation calculates x = (r^e) * g mod n before
        // decryption, where r is random, e is the RSA encryption exponent, and
        // g is the ciphertext to be decrypted. x is then decrypted as normal,
        // followed by division by r, i.e. (x^e) / r mod n. Since r is random,
        // x is random and timing the decryption should not reveal information
        // about the key. Note that r should be a new random number for every
        // decryption.
        final boolean rsaBlinding = Properties.doRSABlinding();
        BigInteger r = null;
        BigInteger e = null;
        if (rsaBlinding)
          { // pre-decryption
            r = newR(n);
            e = ((RSAPrivateCrtKey) K).getPublicExponent();
            final BigInteger x = r.modPow(e, n).multiply(c).mod(n);
            c = x;
          }
        // b. If the second form (p, q, dP, dQ, qInv) and (r_i, d_i, t_i)
        // of K is used, proceed as follows:
        final BigInteger p = ((RSAPrivateCrtKey) K).getPrimeP();
        final BigInteger q = ((RSAPrivateCrtKey) K).getPrimeQ();
        final BigInteger dP = ((RSAPrivateCrtKey) K).getPrimeExponentP();
        final BigInteger dQ = ((RSAPrivateCrtKey) K).getPrimeExponentQ();
        final BigInteger qInv = ((RSAPrivateCrtKey) K).getCrtCoefficient();
        // i. Let m_1 = c^dP mod p and m_2 = c^dQ mod q.
        final BigInteger m_1 = c.modPow(dP, p);
        final BigInteger m_2 = c.modPow(dQ, q);
        // ii. If u > 2, let m_i = c^(d_i) mod r_i, i = 3, ..., u.
        // iii. Let h = (m_1 - m_2) * qInv mod p.
        final BigInteger h = m_1.subtract(m_2).multiply(qInv).mod(p);
        // iv. Let m = m_2 + q * h.
        result = m_2.add(q.multiply(h));
        if (rsaBlinding) // post-decryption
          result = result.multiply(r.modInverse(n)).mod(n);
      }
    // 3. Output m
    return result;
  }

  /**
   * Returns a random MPI with a random bit-length of the form <code>8b</code>,
   * where <code>b</code> is in the range <code>[32..64]</code>.
   * 
   * @return a random MPI whose length in bytes is between 32 and 64 inclusive.
   */
  private static final BigInteger newR(final BigInteger N)
  {
    final int upper = (N.bitLength() + 7) / 8;
    final int lower = upper / 2;
    final byte[] bl = new byte[1];
    int b;
    do
      {
        prng.nextBytes(bl);
        b = bl[0] & 0xFF;
      }
    while (b < lower || b > upper);
    final byte[] buffer = new byte[b]; // 256-bit MPI
    prng.nextBytes(buffer);
    return new BigInteger(1, buffer);
  }
}