/**
    * Portions of this software was developed by employees of the National Institute
    * of Standards and Technology (NIST), an agency of the Federal Government and is
    * being made available as a public service. Pursuant to title 17 United States
    * Code Section 105, works of NIST employees are not subject to copyright
    * protection in the United States. This software may be subject to foreign
    * copyright. Permission in the United States and in foreign countries, to the
    * extent that NIST may hold copyright, to use, copy, modify, create derivative
    * works, and distribute this software and its documentation without fee is hereby
   * granted on a non-exclusive basis, provided that this notice and disclaimer
   * of warranty appears in all copies.
   *
   * THE SOFTWARE IS PROVIDED 'AS IS' WITHOUT ANY WARRANTY OF ANY KIND, EITHER
   * EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY
   * THAT THE SOFTWARE WILL CONFORM TO SPECIFICATIONS, ANY IMPLIED WARRANTIES OF
   * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND FREEDOM FROM
   * INFRINGEMENT, AND ANY WARRANTY THAT THE DOCUMENTATION WILL CONFORM TO THE
   * SOFTWARE, OR ANY WARRANTY THAT THE SOFTWARE WILL BE ERROR FREE.  IN NO EVENT
   * SHALL NIST BE LIABLE FOR ANY DAMAGES, INCLUDING, BUT NOT LIMITED TO, DIRECT,
   * INDIRECT, SPECIAL OR CONSEQUENTIAL DAMAGES, ARISING OUT OF, RESULTING FROM,
   * OR IN ANY WAY CONNECTED WITH THIS SOFTWARE, WHETHER OR NOT BASED UPON WARRANTY,
   * CONTRACT, TORT, OR OTHERWISE, WHETHER OR NOT INJURY WAS SUSTAINED BY PERSONS OR
   * PROPERTY OR OTHERWISE, AND WHETHER OR NOT LOSS WAS SUSTAINED FROM, OR AROSE OUT
   * OF THE RESULTS OF, OR USE OF, THE SOFTWARE OR SERVICES PROVIDED HEREUNDER.
   */
  
  package gov.nist.secauto.swid.client.totp;
  
  import java.nio.ByteBuffer;
  import java.security.InvalidKeyException;
  import java.security.NoSuchAlgorithmException;
  
  import javax.crypto.Mac;
  import javax.crypto.spec.SecretKeySpec;
  
  /**
   * A direct implementation of https://tools.ietf.org/html/rfc4226 and
   * https://tools.ietf.org/html/rfc6238 C = counter K = key Digits = length of OTP Note that a strict
   * implementation of RFC 4226 will only support HMAC-SHA-1.
   */
  public class Hotp {
  
    public static enum HashAlgorithm {
      SHA1("HMACSHA1"),
      SHA256("HMACSHA256"),
      SHA512("HMACSHA512");
  
      private String name;
  
      private HashAlgorithm(String name) {
        this.name = name;
      }
  
      public String getAlgorithmName() {
        return name;
      }
    }
  
    private static class Digits {
      private long value;
      private int length;
  
      /**
       * @param length
       *          the number of digits in the OTP
       */
      private Digits(int length) {
        if (length < 6 && length > 9) {
          // 6 is the minimum in the RFC and 19 is the maximum that a long
          // can represent
          // 9 is the maximum number of digits supported in the RFC
          throw new IllegalArgumentException("the length must be between 6 and 9 digits");
        }
        this.length = length;
        StringBuilder builder = new StringBuilder("1");
        for (int i = 0; i < length; i++) {
          builder.append('0');
        }
        value = Long.valueOf(builder.toString());
      }
  
      public int length() {
        return length;
      }
  
      public long mod(long input) {
        return input % value;
      }
    }
  
    /** the hash algorithm */
    private HashAlgorithm algorithm;
    /** the number of digits */
    private Digits digits;
  
    /**
     * Creates a new instance
     * 
     * @param algorithm
    *          the algorithm to use
    * @param length
    *          the number of digits
    */
   public Hotp(HashAlgorithm algorithm, int length) {
     this.algorithm = algorithm;
     this.digits = new Digits(length);
   }
 
   /**
    * Generates an HOTP value according to RFC 4226
    * 
    * @param key
    *          the shared secret ('K')
    * @param counter
    *          the counter ('C') value
    * @return the HOTP value
    */
   public String generate(byte[] key, long counter) {
     try {
       byte[] hs = hashString(key, counter);
       long hotp = truncate(hs);
       return pad(Long.toString(hotp), this.digits.length());
     } catch (InvalidKeyException e) {
       throw new RuntimeException("configuration error, invalid key", e);
     } catch (NoSuchAlgorithmException e) {
       throw new RuntimeException("configuration error, missing algorithm support", e);
     }
   }
 
   /**
    * A helper function for padding a String up to a minimum number of characters
    * 
    * @param s
    *          the string to pad
    * @param minLength
    *          the minimum
    * @return the string or a string padded up to the minimum length
    */
   private String pad(String s, int minLength) {
     int padding = minLength - s.length();
     if (padding <= 0) {
       // nothing to do
       return s;
     }
     StringBuilder builder = new StringBuilder();
     for (int i = 0; i < padding; i++) {
       builder.append("0");
     }
     builder.append(s);
     return builder.toString();
   }
 
   /**
    * Performs dynamic truncation as described in section 5.3
    * 
    * @param hash
    *          the input hash
    * @return the truncated value
    */
   private long truncate(byte[] hash) {
     // put selected bytes into result int
     int offset = hash[hash.length - 1] & 0xf;
     int binary = ((hash[offset] & 0x7f) << 24) | ((hash[offset + 1] & 0xff) << 16) | ((hash[offset + 2] & 0xff) << 8)
         | (hash[offset + 3] & 0xff);
     return digits.mod(binary);
   }
 
   /**
    * Generates HS as described in step 1. of section 5.3
    * 
    * @param key
    *          the secret Key or 'K' value
    * @param counter
    *          the counter or 'C' value
    * @return hash string
    * @throws NoSuchAlgorithmException
    * @throws InvalidKeyException
    */
   private byte[] hashString(byte[] key, long counter) throws NoSuchAlgorithmException, InvalidKeyException {
     Mac mac = Mac.getInstance(algorithm.getAlgorithmName());
     SecretKeySpec macKey = new SecretKeySpec(key, algorithm.getAlgorithmName());
     ByteBuffer buffer = ByteBuffer.allocate(Long.BYTES);
     buffer.putLong(counter);
     mac.init(macKey);
     return mac.doFinal(buffer.array());
   }
 
 }