HOTP Algorithm: Reference Implementation

OneTimePasswordAlgorithm.java

/*
 * OneTimePasswordAlgorithm.java
 * OATH Initiative,
 * HOTP one-time password algorithm
 *
 */

/* Copyright (C) 2004, OATH.  All rights reserved.
 *
 * License to copy and use this software is granted provided that it
 * is identified as the "OATH HOTP Algorithm" in all material
 * mentioning or referencing this software or this function.
 *
 * License is also granted to make and use derivative works provided
 * that such works are identified as
 *  "derived from OATH HOTP algorithm"
 * in all material mentioning or referencing the derived work.
 *
 * OATH (Open AuTHentication) and its members make no
 * representations concerning either the merchantability of this
 * software or the suitability of this software for any particular
 * purpose.
 *
 * It is provided "as is" without express or implied warranty
 * of any kind and OATH AND ITS MEMBERS EXPRESSaLY DISCLAIMS
 * ANY WARRANTY OR LIABILITY OF ANY KIND relating to this software.
 *
 * These notices must be retained in any copies of any part of this
 * documentation and/or software.
 */

package org.openauthentication.otp;

import java.io.IOException;
import java.io.File;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.lang.reflect.UndeclaredThrowableException;

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

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

/**
 * This class contains static methods that are used to calculate the
 * One-Time Password (OTP) using
 * JCE to provide the HMAC-SHA-1.
 *
 * @author Loren Hart
 * @version 1.0
 */
public class OneTimePasswordAlgorithm {
    private OneTimePasswordAlgorithm() {
    }

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

    /**
     * Calculates the checksum using the credit card algorithm.
     * This algorithm has the advantage that it detects any single
     * mistyped digit and any single transposition of
     * adjacent digits.
     *
     * @param num    the number to calculate the checksum for
     * @param digits number of significant places in the number
     * @return the checksum of num
     */
    public static int calcChecksum(long num, int digits) {
        boolean doubleDigit = true;
        int total = 0;
        while (0 < digits--) {
            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;
    }

    /**
     * This method uses the JCE to provide the HMAC-SHA-1
     * algorithm.
     * HMAC computes a Hashed Message Authentication Code and
     * in this case SHA1 is the hash algorithm used.
     *
     * @param keyBytes the bytes to use for the HMAC-SHA-1 key
     * @param text     the message or text to be authenticated.
     * @throws NoSuchAlgorithmException if no provider makes
     *                                  either HmacSHA1 or HMAC-SHA-1
     *                                  digest algorithms available.
     * @throws InvalidKeyException      The secret provided was not a valid HMAC-SHA-1 key.
     */
    public static byte[] hmac_sha1(byte[] keyBytes, byte[] text)
            throws NoSuchAlgorithmException, InvalidKeyException {
//        try {
        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);
//        } catch (GeneralSecurityException gse) {
//            throw new UndeclaredThrowableException(gse);
//        }
    }

    private static final int[] DIGITS_POWER
            // 0 1  2   3    4     5      6       7        8
            = {1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000};

    /**
     * This method generates an OTP value for the given
     * set of parameters.
     *
     * @param secret           the shared secret
     * @param movingFactor     the counter, time, or other value that
     *                         changes on a per use basis.
     * @param codeDigits       the number of digits in the OTP, not
     *                         including the checksum, if any.
     * @param addChecksum      a flag that indicates if a checksum digit
     *                         should be appended to the OTP.
     * @param truncationOffset the offset into the MAC result to
     *                         begin truncation.  If this value is out of
     *                         the range of 0 ... 15, then dynamic
     *                         truncation  will be used.
     *                         Dynamic truncation is when the last 4
     *                         bits of the last byte of the MAC are
     *                         used to determine the start offset.
     * @return A numeric String in base 10 that includes
     * {@link codeDigits} digits plus the optional checksum
     * digit if requested.
     * @throws NoSuchAlgorithmException if no provider makes
     *                                  either HmacSHA1 or HMAC-SHA-1
     *                                  digest algorithms available.
     * @throws InvalidKeyException      The secret provided was not
     *                                  a valid HMAC-SHA-1 key.
     */
    static public String generateOTP(byte[] secret,
                                     long movingFactor,
                                     int codeDigits,
                                     boolean addChecksum,
                                     int truncationOffset)
            throws NoSuchAlgorithmException, InvalidKeyException {
        // put movingFactor value into text byte array
        String result = null;
        int digits = addChecksum ? (codeDigits + 1) : codeDigits;
        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);
        }
        result = Integer.toString(otp);
        while (result.length() < digits) {
            result = "0" + result;
        }
        return result;
    }
}