/**
 * Copyright (c) 2013 LEAP Encryption Access Project and contributers
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but 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.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
 package se.leap.leapclient;

import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Arrays;

import org.jboss.security.srp.SRPParameters;

/**
 * Implements all SRP algorithm logic.
 * 
 * It's derived from JBoss implementation, with adjustments to make it work with LEAP platform.
 * 
 * @author parmegv
 *
 */
public class LeapSRPSession {

	private SRPParameters params;
	private String username;
	private String password;
	private BigInteger N;
	private byte[] N_bytes;
	private BigInteger g;
	private BigInteger x;
	private BigInteger v;
	private BigInteger a;
	private BigInteger A;
	private byte[] K;
	private SecureRandom pseudoRng;
	/** The M1 = H(H(N) xor H(g) | H(U) | s | A | B | K) hash */
	private MessageDigest clientHash;
	/** The M2 = H(A | M | K) hash */
	private MessageDigest serverHash;

	private static int A_LEN;

	/** Creates a new SRP server session object from the username, password
	    verifier,
	    @param username, the user ID
	    @param password, the user clear text password
	    @param params, the SRP parameters for the session
	 */
	public LeapSRPSession(String username, String password, SRPParameters params)
	{
		this(username, password, params, null);
	}

	/** Creates a new SRP server session object from the username, password
	    verifier,
	    @param username, the user ID
	    @param password, the user clear text password
	    @param params, the SRP parameters for the session
	    @param abytes, the random exponent used in the A public key
	 */
	public LeapSRPSession(String username, String password, SRPParameters params,
			byte[] abytes) {
		this.params = params;
		this.g = new BigInteger(1, params.g);
		N_bytes = ConfigHelper.trim(params.N);
		this.N = new BigInteger(1, N_bytes);
		this.username = username;
		this.password = password;
		
		try {
			pseudoRng = SecureRandom.getInstance("SHA1PRNG");
		} catch (NoSuchAlgorithmException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		if( abytes != null ) {
			A_LEN = 8*abytes.length;
			/* TODO Why did they put this condition?
	         if( 8*abytes.length != A_LEN )
	            throw new IllegalArgumentException("The abytes param must be "
	               +(A_LEN/8)+" in length, abytes.length="+abytes.length);
			 */
			this.a = new BigInteger(abytes);
		}
		else
			A_LEN = 64;

		serverHash = newDigest();
		clientHash = newDigest();
	}

	/**
	 * Calculates the parameter x of the SRP-6a algorithm.
	 * @param username
	 * @param password
	 * @param salt the salt of the user
	 * @return x
	 */
	public byte[] calculatePasswordHash(String username, String password, byte[] salt)
	{
		//password = password.replaceAll("\\\\", "\\\\\\\\");
		// Calculate x = H(s | H(U | ':' | password))
		MessageDigest x_digest = newDigest();
		// Try to convert the username to a byte[] using ISO-8859-1
		byte[] user = null;
		byte[] password_bytes = null;
		byte[] colon = {};
		String encoding = "ISO-8859-1";
		try {
			user = ConfigHelper.trim(username.getBytes(encoding));
			colon = ConfigHelper.trim(":".getBytes(encoding));
			password_bytes = ConfigHelper.trim(password.getBytes(encoding));
		}
		catch(UnsupportedEncodingException e) {
			// Use the default platform encoding
			user = ConfigHelper.trim(username.getBytes());
			colon = ConfigHelper.trim(":".getBytes());
			password_bytes = ConfigHelper.trim(password.getBytes());
		}
		
		// Build the hash
		x_digest.update(user);
		x_digest.update(colon);
		x_digest.update(password_bytes);
		byte[] h = x_digest.digest();
		
		x_digest.reset();
		x_digest.update(salt);
		x_digest.update(h);
		byte[] x_digest_bytes = x_digest.digest();

		return x_digest_bytes;
	}

	/**
	 * Calculates the parameter V of the SRP-6a algorithm.
	 * @param k_string constant k predefined by the SRP server implementation.
	 * @return the value of V
	 */
	private BigInteger calculateV(String k_string) {
		BigInteger k = new BigInteger(k_string, 16);
		BigInteger v = k.multiply(g.modPow(x, N));  // g^x % N
		return v;
	}

	/**
	 * Calculates the trimmed xor from two BigInteger numbers
	 * @param b1 the positive source to build first BigInteger
	 * @param b2 the positive source to build second BigInteger
	 * @param length 
	 * @return
	 */
	public byte[] xor(byte[] b1, byte[] b2)
	{
		//TODO Check if length matters in the order, when b2 is smaller than b1 or viceversa
		byte[] xor_digest = new BigInteger(1, b1).xor(new BigInteger(1, b2)).toByteArray();
		return ConfigHelper.trim(xor_digest);
	}

	/**
	 * @returns The exponential residue (parameter A) to be sent to the server.
	 */
	public byte[] exponential() {
		byte[] Abytes = null;
		if(A == null) {
			/* If the random component of A has not been specified use a random
	         number */
			if( a == null ) {
				BigInteger one = BigInteger.ONE;
				do {
					a = new BigInteger(A_LEN, pseudoRng);
				} while(a.compareTo(one) <= 0);
			}
			A = g.modPow(a, N);
			Abytes = ConfigHelper.trim(A.toByteArray());
		}
		return Abytes;
	}

	/**
	 * Calculates the parameter M1, to be sent to the SRP server.
	 * It also updates hashes of client and server for further calculations in other methods.
	 * It uses a predefined k.
	 * @param salt_bytes
	 * @param Bbytes the parameter received from the server, in bytes
	 * @return the parameter M1
	 * @throws NoSuchAlgorithmException
	 */
	public byte[] response(byte[] salt_bytes, byte[] Bbytes) throws NoSuchAlgorithmException {
		// Calculate x = H(s | H(U | ':' | password))
		byte[] xb = calculatePasswordHash(username, password, ConfigHelper.trim(salt_bytes));
		this.x = new BigInteger(1, xb);

		// Calculate v = kg^x mod N
		String k_string = "bf66c44a428916cad64aa7c679f3fd897ad4c375e9bbb4cbf2f5de241d618ef0";
		this.v = calculateV(k_string);

		// H(N)
		byte[] digest_of_n = newDigest().digest(N_bytes);
		
		// H(g)
		byte[] digest_of_g = newDigest().digest(params.g);
		
		// clientHash = H(N) xor H(g)
		byte[] xor_digest = xor(digest_of_n, digest_of_g);
		clientHash.update(xor_digest);
		
		// clientHash = H(N) xor H(g) | H(U)
		byte[] username_digest = newDigest().digest(ConfigHelper.trim(username.getBytes()));
		username_digest = ConfigHelper.trim(username_digest);
		clientHash.update(username_digest);
		
		// clientHash = H(N) xor H(g) | H(U) | s
		clientHash.update(ConfigHelper.trim(salt_bytes));
		
		K = null;

		// clientHash = H(N) xor H(g) | H(U) | A
		byte[] Abytes = ConfigHelper.trim(A.toByteArray());
		clientHash.update(Abytes);
		
		// clientHash = H(N) xor H(g) | H(U) | s | A | B
		Bbytes = ConfigHelper.trim(Bbytes);
		clientHash.update(Bbytes);
		
		// Calculate S = (B - kg^x) ^ (a + u * x) % N
		BigInteger S = calculateS(Bbytes);
		byte[] S_bytes = ConfigHelper.trim(S.toByteArray());

		// K = SessionHash(S)
		String hash_algorithm = params.hashAlgorithm;
		MessageDigest sessionDigest = MessageDigest.getInstance(hash_algorithm);
		K = ConfigHelper.trim(sessionDigest.digest(S_bytes));
		
		// clientHash = H(N) xor H(g) | H(U) | A | B | K
		clientHash.update(K);
		
		byte[] M1 = ConfigHelper.trim(clientHash.digest());
		
		// serverHash = Astr + M + K
		serverHash.update(Abytes);
		serverHash.update(M1);
		serverHash.update(K);
		
		return M1;
	}

	/**
	 * It calculates the parameter S used by response() to obtain session hash K.
	 * @param Bbytes the parameter received from the server, in bytes
	 * @return the parameter S
	 */
	private BigInteger calculateS(byte[] Bbytes) {
		byte[] Abytes = ConfigHelper.trim(A.toByteArray());
		Bbytes = ConfigHelper.trim(Bbytes);
		byte[] u_bytes = getU(Abytes, Bbytes);
		
		BigInteger B = new BigInteger(1, Bbytes);
		BigInteger u = new BigInteger(1, u_bytes);
		
		BigInteger B_minus_v = B.subtract(v);
		BigInteger a_ux = a.add(u.multiply(x));
		BigInteger S = B_minus_v.modPow(a_ux, N);
		return S;
	}

	/**
	 * It calculates the parameter u used by calculateS to obtain S.
	 * @param Abytes the exponential residue sent to the server
	 * @param Bbytes the parameter received from the server, in bytes
	 * @return
	 */
	public byte[] getU(byte[] Abytes, byte[] Bbytes) {
		MessageDigest u_digest = newDigest();
		u_digest.update(ConfigHelper.trim(Abytes));
		u_digest.update(ConfigHelper.trim(Bbytes));
		byte[] u_digest_bytes = u_digest.digest();
		return ConfigHelper.trim(new BigInteger(1, u_digest_bytes).toByteArray());
	}

	/**
	 * @param M2 The server's response to the client's challenge
	 * @returns True if and only if the server's response was correct.
	 */
	public boolean verify(byte[] M2)
	{
		// M2 = H(A | M1 | K)
		M2 = ConfigHelper.trim(M2);
		byte[] myM2 = ConfigHelper.trim(serverHash.digest());
		boolean valid = Arrays.equals(M2, myM2);
		return valid;
	}

	/**
	 * @return a new SHA-256 digest.
	 */
	public MessageDigest newDigest()
	{
		MessageDigest md = null;
		try {
			md = MessageDigest.getInstance("SHA-256");
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
		}
		return md;
	}
}