### Vigenere decipher process

In an earlier post we discussed a method to encrypt a string using Vigenere Cipher, in this post we see the method to decrypt the encrypted message.

The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers based on the letters of a keyword.
It is a form of polyalphabetic substitution.
Even though the vigenere cipher is easy to understand and implement, for three centuries it resisted all attempts to break it.

In order to encrypt a given message, a table of alphabets can be used, termed a tabula recta, The following is the image of the Table.

the above table consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword.
Now let us consider an example to understand the process of decryption,
For example, suppose that the plain text to be decrypted is:
SIXZBELUGVDQKPYZK
Now we need a key that was used while encryption of the original mesage, Suppose the key is "LEMON", we repeat this key to match the length of our message
LEMONLEMONLEMONL
In order to decrypt the message we take the first 2 letters, one from the message and one from the key i.e 'S' and 'L' and in our table we take the column that starts with 'S' and in that column we go to the row that has 'L' and we replace our 'S' with the Starting letter of the row.
Message  :  SIXZBELUGVDQKPYZK
Key          :  LEMONLEMONLEMONL
Cipher     :   HELLOTHISISMYBLOG

Let us now see how to calculate the index of the letter that need to replace the letter from the message, for our program we do not construct the table rather we use a simple formula that gives us index of cipher letter, the below formula is used in our code

position = (indexof(letter_in_message) - index(letter_in_key) + 26) % 26

so for our above example we get
indexof('S') = 20
indexof('L') = 12
position = (20 - 12 + 26) % 26 = 8
and 8th letter is 'H', so we replace 'S' by 'H'.

Still confused, watch the following video to understand the process.

Algorithm
1. Accept encrypted message and the key that was used for encryption.
2. Create a empty string that will hold the decrypted message
3. Make the length of key same as the message by appending it to itself as seen in above example
4. Iterate over the message and apply the above formula to find the deciphered letter and append it to decrypted message string.

### Sample input and output to check the program

You might also be interested in

Vigenere Cipher Algorithm
Counting Sort
Quick Sort using Recursion
Data Encryption using Caesar Cipher
LCM of 2 numbers
Anagram Strings

### Infix to Prefix conversion using Stack

This post is about conversion of Infix expression to Prefix conversion. For this conversion we take help of stack data structure, we need to push and pop the operators in and out of the stack.

Infix expressions are the expressions that we normally use,eg. 5+6-7; a+b*c etc. Prefix expressions are the expressions in which the 2 operands are preceded by the operator eg. -+567 , +a*bc etc.

This method is very similar to the method that we used to convert Infix to Postfix but the only difference is that here we need to reverse the input string before conversion and then reverse the final output string before displaying it.

NOTE: This changes one thing that is instead of encountering the opening bracket we now first encounter the closing bracket and we make changes accordingly in our code.

So, to convert an infix expression to a prefix expression we follow the below steps
(we have 2 string, 1st is the input infix expression string 2nd is the output string which is empty initially)

We first revers…

Hashing is a technique used for storing , searching and removing elements in almost constant time. Hashing is done with help of a hash function that generates index for a given input, then this index can be used to search the elements, store an element, or remove that element from that index.

A hash function is a function that is used to map the data elements to their position in the data structure used. For example if we use an array to store the integer elements then the hash function will generate position for each element so that searching, storing and removing operation on the array can be done in constant time that is independent of the number of elements in the array. For better look at the example below.

now we face a problem if for 2 numbers same position is generated example consider elements 1 and 14

1 % 13 = 1

14 % 13 = 1

so when we get 1 we store it at the first position, but when we get 14 we see that the position 1 is already taken, this is a case of collision.

Inorder…