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Polyalphabetic cipher

Polyalphabetic cipher encryption- decryption.

PSEDOCODE:
Encryption
                  Ci=(Ti +Ki)modn
Ci - i-th character of the ciphertext
Ti - i-th character of the open text
Ki - i-th character of the key phrase (if the key phrase is shorter than the open text, which is usual, than the keyphrase is reapeated to math the length of the open text)
m - length of the alphabet


1.      To encrypt the open text, we have to sum together the first letters of the open text and key phrase, the second letters, third and so on.
2.      To encrypt the n-th letter of the open text (assume “L“) using the Vigenère square
3.      we find the letter on the horizontal axis of the table and we find n-th letter of the key phase on the vertical axis (assume “T“)
4.       At the intersection of the given row and column the resulting n-th letter of the ciphertext is located (“E“).
Decryption

                  Ti=(Ci-Ki)modn

Ci - i-th character of the ciphertext
Ti - i-th character of the open text
Ki - i-th character of the key phrase (if the key phrase is shorter than the open text, which is usual, than the keyphrase is reapeated to math the length of the open text)
m - length of the alphabet

1.      The decryption works the other way around.
2.      We find in the row corresponding to the n-th letter of the key phrase (“T”) a cell in which the n-th letter of the ciphertext (“E”) resides.

3.       Its column is denoted by the n-th letter of the open text (“L“).

//Program:
#include<stdio.h>

void main()
{
int i, kl,pl;
char p[50],k[50],cipher[50];
printf("Enter the length of the key stream. ");
scanf("%d",&kl);
printf("Enter the length of the plain text stream.(Without spaces) ");
scanf("%d",&pl);



printf("\nEnter the Key. ");
for(i=-1;i<kl;++i)
scanf("%c",&k[i]);

printf("\nEnter the Plain text. ");
for(i=-1;i<pl;++i)
scanf("%c",&p[i]);

int s[3][pl];

for(i=0;i<pl;++i)
{
if(65<=p[i] && p[i]<=91)
s[0][i]=p[i]%65;
else
s[0][i]=p[i]%97;
}

for(i=0;i<pl;++i)
printf("%d ",s[0][i]);
int count=0;
while(count<pl)
{
for(i=0;i<kl;++i)
{
if(65<=k[i] && k[i]<=91)
s[1][count+i]=k[i]%65;
else
s[1][count+i]=k[i]%97;
}
count=count+kl;
}
printf("\n");
for(i=0;i<pl;++i)
printf("%d ",s[1][i]);

//printf("\n");
//for(i=0;i<pl;++i)
//printf("%d ",s[2][i]);

printf("\n\nThe cipher text is: ");

for(i=0;i<pl;++i)
{
s[2][i]=(s[0][i]+s[1][i])%26;

cipher[i]=(char)(s[2][i]+65);
printf("%c ",cipher[i]);
}

}VIVA Questions:

1.      Explain monoalphabetic cipher and polyalphabetic cipher by giving an example
2.      What is Vigenere
3.      How to attack Polyaphabetic Cipher.
4.      What is vernam Cipher.
5.      Explain One Time Pad in detail and write an algorithm for it.
6.      What are the problems with one-time pad?
7.      What is the objective of attacking an encryption system? Write the two approaches to attack a conventional encryption scheme.
8.      What is the difference between an unconditionally secure cipher and a computationally secure cipher?
9.      Define the terms threat and attack. List and briefly define categories of security attacks.
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