/*
 * RFC4226.java - et-otp: GPL Java TOTP soft token by Bernd Eckenfels.
 */
package org.signserver.validationservice.server;

import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;


/**
 * An implementation of the HOTP generator based on the reference implementation of RFC 4226.
 * <P>
 * Generates short passcodes that may be used in OTP and challenge-response protocols.
 * <P>
 * This code is based on the reference implementation contained in RFC 4226.
 */
public class RFC4226
{
    private static final int[] DIGITS_POWER
        // 0 1  2   3    4     5      6       7        8
        = {1,10,100,1000,10000,100000,1000000,10000000,100000000};

    // These are used to calculate the check-sum digits.
    private static final int[] doubleDigits =
                    { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

    public static byte[] hmac_sha1(byte[] keyBytes, byte[] text)
        throws NoSuchAlgorithmException, InvalidKeyException
    {
        Mac hmacSha1;
        try {
            hmacSha1 = Mac.getInstance("HmacSHA1");
        } catch (NoSuchAlgorithmException nsae) {
            hmacSha1 = Mac.getInstance("HMAC-SHA-1");
        }
        SecretKeySpec macKey =
            new SecretKeySpec(keyBytes, "RAW");
        hmacSha1.init(macKey);

        return hmacSha1.doFinal(text);
    }


    public static int calcChecksum(long numArg, int digitsArg)
    {
        boolean doubleDigit = true;
        int     total = 0;
        int digits = digitsArg;
        long num = numArg;
        while (digits-- > 0) {
            int digit = (int) (num % 10);
            num /= 10;
            if (doubleDigit) {
                digit = doubleDigits[digit];
            }
            total += digit;
            doubleDigit = !doubleDigit;
        }
        int result = total % 10;
        if (result > 0) {
            result = 10 - result;
        }
        return result;
    }

    public static String generateOTP(byte[] secret, long movingFactorArg, int codeDigits, boolean addChecksum, int truncationOffset)
        throws NoSuchAlgorithmException, InvalidKeyException
    {
        final int digits = addChecksum ? (codeDigits + 1) : codeDigits;

        // put movingFactor value into byte array (big endian)
        long movingFactor = movingFactorArg;
        byte[] text = new byte[8];
        for (int i = text.length - 1; i >= 0; i--) {
            text[i] = (byte) (movingFactor & 0xff);
            movingFactor >>= 8;
        }

        // compute hmac hash
        byte[] hash = hmac_sha1(secret, text);

        // put selected bytes into result int
        int offset = hash[hash.length - 1] & 0xf;
        if ( (0<=truncationOffset) &&
                (truncationOffset<(hash.length-4)) ) {
            offset = truncationOffset;
        }
        int binary =
            ((hash[offset] & 0x7f) << 24)
            | ((hash[offset + 1] & 0xff) << 16)
            | ((hash[offset + 2] & 0xff) << 8)
            | (hash[offset + 3] & 0xff);

        int otp = binary % DIGITS_POWER[codeDigits];
        if (addChecksum) {
            otp =  (otp * 10) + calcChecksum(otp, codeDigits);
        }

        String result = Integer.toString(otp);
        while (result.length() < digits) {
            result = "0" + result;
        }
        return result;
    }
}