Introduction:
The Caesar Cipher technique is one of the earliest and simplest method of encryption technique. It’s simply a type of substitution cipher, i.e., each letter of a given text is replaced by a letter some fixed number of positions down the alphabet. The method is apparently named after Julius Caesar, who apparently used it to communicate with his officials.
Formula:
Ciphertext(C): E(k,p) = (p+k)mod26
Plaintext(p): D(k,C) = (C-k)mod26
Where k takes a value in the range 1 to 25.
Let us assign a numerical equivalent to each letter:
a
|
b
|
c
|
d
|
E
|
F
|
g
|
h
|
i
|
J
|
k
|
l
|
m
|
0
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
n
|
o
|
P
|
q
|
R
|
S
|
T
|
u
|
v
|
W
|
x
|
y
|
z
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
25
|
Example:
Key=3
Plain text: hello how are you
Cipher text: KHOOR KRZ DUH BRX
C code:
//CEASER-CIPHER
#include<stdio.h>
#include<conio.h>
void
main()
{
char
pt[20]={'\0'},ct[20]={'\0'},rt[20]={'\0'},st[20]={'\0'};
int key,i;
clrscr();
printf("\n enter the plain
text:");
gets(pt);
printf("\n enter teh key value
in numeric (0 to 25):");
scanf("\n %d",&key);
//converting plain text into cipher
text (encryption)
for(i=0;i<strlen(pt);i++)
{
ct[i]=((((pt[i]-'a')+key)%26)+'a');
}
printf("\n cipher text is:
%s",ct);
//converting cipher text into plain
text (decryption)
for(i=0;i<strlen(ct);i++)
{
if(ct[i]<pt[i])
{
rt[i]=(26+((ct[i]-97)-key)+97);
}
else
{
rt[i]=((((ct[i]-97)-key)%26)+97);
}
}
printf("\n \n plain text is:
%s",rt);
getch();
}
Output:
THANK YOU!!!!
