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RSA program

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Dear fellows,

I am new to C.My tutor ask us to write a C program with limitations,for
decoding and encoding similiar to RSA.

The question are as follows:
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To encode a message,one needs n and e.The value n is a product of any two
prime numbers p and q.The value e is any number less than n that cannot be
evenly divided into y(i.e. y/e would have a reminder),where
y=(p-1)*(q-1).The values n and e can be published in a newspaper or posted
on the internet,so anybody can encrypt messages.The original character is
than encoded to a numerical value c using the formula:

c=m^e mod n
where m is a numerical representation of the original
character(65=A,66=B..etc)
Now,to decode a messages,one needs d.The value d is a number that statisfies
the formula:

(e*d) mod y ==1
where e and y are the values defined in the encoding step.The original
character m can be derived from the encrypted character c by using the
formula:
m=c^d mod n
To write a program that encodes and decodes message using this system,you
will need to generate the oublic and secret key pairs:
Public key (KU)=(e,n) Secret key KR=(d,n)

Limitations:
1.Only functions is allowed
2.Simple Calculations(+-*/) is allowed.No power (x^y) is allowed
3.For the calculations,((a mod n)*(b mod n)) mod n == (a* mod n


fwongmc Posted: Nov 5 2004, 05:17 PM



Newbie

Group: Members
Posts: 9
Member No.: 683
Joined: 5-November 04



(cont'd)

3.Hints(a mod n)*(b mod n)) mod n == (a*b)mod n

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Sample run:
Do you want to start? (Y/N) y
Please enter the public key e:5
Please enter the common key n:119
Please enter the private key e:77
Encrypt or Decrypt? (E/D) e
Please enter 5 characters.
abcde
The encrypted message in number format is 20 98 29 52 35
Do you want to play again? (y/n)y
Encrypt or Decrypt? (E/D) d
Please enter 5 numbers.
20 98 29 53 33
The decrypted message char format is a b c d e

Do you want to play again? (y/n)n
Thank you for using this program

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My code:
#include <stdio.h>

int prime=1;
int i,number;
int num1,num2,num3,num4,num5;
int e,n,d,y,p,q;
int is_prime(int n){
 for (i=2;i<n;i++){
  if(n%i==0)
   return 0;
   }
   return 1;
   }

int decrypt(int num){
int a,f=1;
int b,c,i,j;
{
 b=check_div(e);
 c=e/b;
 for (i=1;i<=b;i++){
  a=(num%n)*a;
  }
 for (j=1;j<=c;j++){
  f=(num%n)*f;}

  number=((a*f)%n);
  return (num);
  }
  return 0;
  }


int encrypt(int num){
int a,f=1;
int b,c,i,j;
{
 b=check_div(d);
 c=d/b;

 for (i=1;i<=b;i++){
  a=(num%n)*a;
 }
 for (j=1;j<=c;j++){
  f=(num%n)*f;}

  number=((a*f)%n);
  return (num);
  }
  return 0;
  }


int check_div(int x){
int i=2;
int a=0;
{
do {
  a=(n%i);
  i++;  }while (n%i>0);
  return (i);
  }
  return 0;
   }



int main (void){
char choice,choice2;
char choice3='y';
printf ("Do you want to start?");
 scanf("%c",&choice);
if ((choice=='n')||(choice=='N')){
 printf ("Thank you for using this program.");}

if ((choice=='y')||(choice=='Y')){
 printf ("Please enter the Public Key e:");
  scanf ("%d",&e);
 printf ("Please enter the common key n:");
  scanf ("%d",&n);
 printf ("Please enter the private key:");
 scanf ("%d",&d);

/*To check if p and q are prime numbers*/
if ((is_prime(p)==1)&&(is_prime(q)==1)){
 y=(p-1)*(q-1);}
if ((is_prime(p)==0)||(is_prime(q)==0)){
 printf ("Check your common key.");}
/*Prompt user to select encrypt nor decrypt*/
do {
printf ("Encrypt or Decrypt? (E/D)");
fflush(stdin);
scanf ("%c",&choice2);}



/*To find the factors of n QUESTION 2*/
if (check_div(n)>0){
 p=check_div(n);
 q=n/p;}

if ((choice2=='e')||(choice2=='E')){
  printf ("Please enter 5 characters.");
  scanf ("%d %d %d %d %d",&num1,&num2,&num3,&num4,&num5);
  printf ("The encrypted message in number format is %d %d %d %d
%d",decrypt(num1),decrypt(num2),decrypt(num3),decrypt(num4),decrypt(num5));
  printf ("Do you want to play again ?(Y/N)");
  fflush(stdin);
  scanf ("%c",&choice3);}


 if ((choice2=='d')||(choice2=='D')){
  printf ("Please enter 5 numbers.");
  scanf ("%d %d %d %d %d",&num1,&num2,&num3,&num4,&num5);
   if ((e*d)%y==1){
    printf ("The decrypted message char format is %c %c %c %c
%c",encrypt(num1),encrypt(num2),encrypt(num3),encrypt(num4),encrypt(num5));}
    printf ("Do you want to play again ?(Y/N)");
    fflush(stdin);
    scanf ("%c",&choice3);}

 }  while ((choice3=='y')||(choice3=='Y'));
printf ("Thank you for using this program.");
}


The above is my new code.I get stuck with the calculations.Here is little
discreption of the three functions(excluding is_prime)
1.For check_div,is to check the two factors of the user input 'n'.I used a
for loop to count on n%i and stops if the result are 0.But I don't know if I
wrote it right.
2.For the function decryption,is counting on (m^e)%n,/For the function
encryptionmis counting on (c^d)%n,using of power function is not allowed in
this program by out tutor,thus I use a for loop again to accumlate e times
for (m mod n).But since the output may out of range which C can store,thus I
use the check_div function to find the two factors of e,and calculation it
seperatly.(i.e.: ((a mod n)*(b mod n))mod n==(a*b) mod n
3.Since i used the check_div function in the decrypt/encrypt,but the Vc++
told me that it is a warning,why this happen,and what should I do for it?
4.I get stuck with the do-while loop.The question want us to continue to ask
the user for encryption/decryption and do unless if the user enter 'n'.But
whether I enter,for example,if I enter e for encryption,therefore if I enter
yes,it will prompt to ask for thee same type only..


I did tried one night and I cannot figure out what's the problem,can you
help me to debug my program? I will be very pleased if you do so.
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