Question

Cryptographic hash functions are very commonly used in password verification. For example, when you use any online website which requires a user login, you enter your E-mail and password to authenticate that the account you are trying to use belongs to you. When the password is entered, a hash of the password is computed which is then sent to the server for verification of the password. The passwords stored on the server are actually computed hash values of the original passwords. This is done to ensure that when the password is sent from client to server, no sniffing is there. Implement this approach.

Please also do the following:

1. Implement a digit-folding approach in the hash function.

2. Assume that, the password are stored in a hash table. Apply a “double hashing” as a collision resolving technique in the open addressing.

3. Draw a class diagram to present the relations between classes.

4. User should be able to interact with your program and provide the input.

*Please code in Java*

Answer 1

import java.io.*;

class FoldingHashTable

{

private HashDataItem[] hashArray;

private int arraySize;

private HashDataItem nonItem;

public FoldingHashTable(int size)

{

arraySize = size;

hashArray = new HashDataItem[arraySize];

nonItem = new HashDataItem(-1); //deleted item key is -1

}

public void displayTable()

{

System.out.print("Table: ");

for(int j = 0; j < arraySize; j++)

{

if(hashArray[j] != null)

System.out.print(hashArray[j].getKey() + " ");

else

System.out.print("** ");

}

System.out.println("");

}

//folds the key into groups of digits and adds the groups

public int hashFunc(int key)

{

int groupSize = 1;

int temp = arraySize;

int hashVal = 0;

while(temp > 0)

{

temp /= 10;

groupSize *= 10;

}

while(key > 0)

{

hashVal += key % groupSize;

key /= groupSize;

}

return hashVal;

}

public void insert(HashDataItem item)

{

//assumes table not full

int key = item.getKey();

int hashVal = hashFunc(key);

while(hashArray[hashVal] != null && hashArray[hashVal].getKey() != -1)

{

++hashVal;

hashVal %= arraySize;

}

hashArray[hashVal] = item;

}

public HashDataItem delete(int key)

{

int hashVal = hashFunc(key);

while(hashArray[hashVal] != null)

{

if(hashArray[hashVal].getKey() == key)

{

HashDataItem temp = hashArray[hashVal];

hashArray[hashVal] = nonItem;

return temp;

}

++hashVal;

hashVal %= arraySize;

}

return null;

}

public HashDataItem find(int key)

{

int hashVal = hashFunc(key);

while(hashArray[hashVal] != null)

{

if(hashArray[hashVal].getKey() == key)

return hashArray[hashVal];

++hashVal;

hashVal %= arraySize;

}

return null;

}

} //end class HashTable

class FoldingHashTableApp

{

public static void main(String[] args) throws IOException

{

HashDataItem aDataItem;

int aKey, size, n, keysPerCell;

System.out.print("Enter size of hash table: ");

size = getInt();

System.out.print("Enter initial number of items: ");

n = getInt();

keysPerCell = 10;

HashTable theHashTable = new HashTable(size);

for(int j=0; j<n; j++)

{

aKey = (int)(java.lang.Math.random() * keysPerCell * size);

aDataItem = new HashDataItem(aKey);

theHashTable.insert(aDataItem);

}

while(true)

{

System.out.print("Enter first letter of show, insert, delete, or find: ");

char choice = getChar();

switch(choice)

{

case 's':

theHashTable.displayTable();

break;

case 'i':

System.out.print("Enter key value to insert: ");

aKey = getInt();

aDataItem = new HashDataItem(aKey);

theHashTable.insert(aDataItem);

break;

case 'd':

System.out.print("Enter key value to delete: ");

aKey = getInt();

theHashTable.delete(aKey);

break;

case 'f':

System.out.print("Enter key value to find: ");

aKey = getInt();

aDataItem = theHashTable.find(aKey);

if(aDataItem != null)

System.out.println("Found " + aKey);

else

System.out.println("Could not find " + aKey);

break;

default:

System.out.println("Invalid entry!");

}

}

}//end main

public static String getString() throws IOException

{

InputStreamReader isr = new InputStreamReader(System.in);

BufferedReader br = new BufferedReader(isr);

String s = br.readLine();

return s;

}

public static char getChar() throws IOException

{

String s = getString();

return s.charAt(0);

}

public static int getInt() throws IOException

{

String s = getString();

return Integer.parseInt(s);

}

} //end HashTableApp