C code to implement RSA Algorithm(Encryption and Decryption)

RSA Program Input

ENTER FIRST PRIME NUMBER
7

ENTER ANOTHER PRIME NUMBER
17

ENTER MESSAGE
hello

C Program

#include<stdio.h>
 
#include<conio.h>
 
#include<stdlib.h>
 
#include<math.h>
 
#include<string.h>
 
long int p,q,n,t,flag,e[100],d[100],temp[100],j,m[100],en[100],i;
 
char msg[100];
 
int prime(long int);
 
void ce();
 
long int cd(long int);
 
void encrypt();
 
void decrypt();
 
void main() {
 
 clrscr();
 
 printf("\nENTER FIRST PRIME NUMBER\n");
 
 scanf("%d",&p);
 
 flag=prime(p);
 
 if(flag==0) {
 
  printf("\nWRONG INPUT\n");
 
  getch();
 
  exit(1);
 
 }
 
 printf("\nENTER ANOTHER PRIME NUMBER\n");
 
 scanf("%d",&q);
 
 flag=prime(q);
 
 if(flag==0||p==q) {
 
  printf("\nWRONG INPUT\n");
 
  getch();
 
  exit(1);
 
 }
 
 printf("\nENTER MESSAGE\n");
 
 fflush(stdin);
 
 scanf("%s",msg);
 
 for (i=0;msg[i]!=NULL;i++)
 
 m[i]=msg[i];
 
 n=p*q;
 
 t=(p-1)*(q-1);
 
 ce();
 
 printf("\nPOSSIBLE VALUES OF e AND d ARE\n");
 
 for (i=0;i<j-1;i++)
 
 printf("\n%ld\t%ld",e[i],d[i]);
 
 encrypt();
 
 decrypt();
 
 getch();
 
}
 
int prime(long int pr) {
 
 int i;
 
 j=sqrt(pr);
 
 for (i=2;i<=j;i++) {
 
  if(pr%i==0)
 
      return 0;
 
 }
 
 return 1;
 
}
 
void ce() {
 
 int k;
 
 k=0;
 
 for (i=2;i<t;i++) {
 
  if(t%i==0)
 
      continue;
 
  flag=prime(i);
 
  if(flag==1&&i!=p&&i!=q) {
 
   e[k]=i;
 
   flag=cd(e[k]);
 
   if(flag>0) {
 
    d[k]=flag;
 
    k++;
 
   }
 
   if(k==99)
 
           break;
 
  }
 
 }
 
}
 
long int cd(long int x) {
 
 long int k=1;
 
 while(1) {
 
  k=k+t;
 
  if(k%x==0)
 
      return(k/x);
 
 }
 
}
 
void encrypt() {
 
 long int pt,ct,key=e[0],k,len;
 
 i=0;
 
 len=strlen(msg);
 
 while(i!=len) {
 
  pt=m[i];
 
  pt=pt-96;
 
  k=1;
 
  for (j=0;j<key;j++) {
 
   k=k*pt;
 
   k=k%n;
 
  }
 
  temp[i]=k;
 
  ct=k+96;
 
  en[i]=ct;
 
  i++;
 
 }
 
 en[i]=-1;
 
 printf("\nTHE ENCRYPTED MESSAGE IS\n");
 
 for (i=0;en[i]!=-1;i++)
 
 printf("%c",en[i]);
 
}
 
void decrypt() {
 
 long int pt,ct,key=d[0],k;
 
 i=0;
 
 while(en[i]!=-1) {
 
  ct=temp[i];
 
  k=1;
 
  for (j=0;j<key;j++) {
 
   k=k*ct;
 
   k=k%n;
 
  }
 
  pt=k+96;
 
  m[i]=pt;
 
  i++;
 
 }
 
 m[i]=-1;
 
 printf("\nTHE DECRYPTED MESSAGE IS\n");
 
 for (i=0;m[i]!=-1;i++)
 
 printf("%c",m[i]);
 
}
 

C code to Encrypt & Decrypt Message using Transposition Cipher

C Program

#include<stdio.h> 
#include<string.h> 
void cipher(int i,int c);
int findMin();
void makeArray(int,int);
char arr[22][22],darr[22][22],emessage[111],retmessage[111],key[55];
char temp[55],temp2[55];
int k=0;
int main() {
 char *message,*dmessage;
 int i,j,klen,emlen,flag=0;
 int r,c,index,min,rows;
 clrscr();
 printf("Enetr the key\n");
 fflush(stdin);
 gets(key);
 printf("\nEnter message to be ciphered\n");
 fflush(stdin);
 gets(message);
 strcpy(temp,key);
 klen=strlen(key);
 k=0;
 for (i=0; ;i++) {
  if(flag==1) 
      break;
  for (j=0;key[j]!=NULL;j++) {
   if(message[k]==NULL) {
    flag=1;
    arr[i][j]='-';
   } else {
    arr[i][j]=message[k++];
   }
  }
 }
 r=i;
 c=j;
 for (i=0;i<r;i++) {
  for (j=0;j<c;j++) {
   printf("%c ",arr[i][j]);
  }
  printf("\n");
 }
 k=0;
 for (i=0;i<klen;i++) {
  index=findMin();
  cipher(index,r);
 }
 emessage[k]='\0';
 printf("\nEncrypted message is\n");
 for (i=0;emessage[i]!=NULL;i++) 
    printf("%c",emessage[i]);
 printf("\n\n");
 //deciphering 
 emlen=strlen(emessage);
 //emlen is length of encrypted message 
 strcpy(temp,key);
 rows=emlen/klen;
 //rows is no of row of the array to made from ciphered message 
 rows;
 j=0;
 for (i=0,k=1;emessage[i]!=NULL;i++,k++) {
  //printf("\nEmlen=%d",emlen); 
  temp2[j++]=emessage[i];
  if((k%rows)==0) {
   temp2[j]='\0';
   index=findMin();
   makeArray(index,rows);
   j=0;
  }
 }
 printf("\nArray Retrieved is\n");
 k=0;
 for (i=0;i<r;i++) {
  for (j=0;j<c;j++) {
   printf("%c ",darr[i][j]);
   //retrieving message 
   retmessage[k++]=darr[i][j];
  }
  printf("\n");
 }
 retmessage[k]='\0';
 printf("\nMessage retrieved is\n");
 for (i=0;retmessage[i]!=NULL;i++) 
      printf("%c",retmessage[i]);
 getch();
 return(0);
}
void cipher(int i,int r) {
 int j;
 for (j=0;j<r;j++) { {
   emessage[k++]=arr[j][i];
  }
 }
 // emessage[k]='\0';
}
void makeArray(int col,int row) {
 int i,j;
 for (i=0;i<row;i++) {
  darr[i][col]=temp2[i];
 }
}
int findMin() {
 int i,j,min,index;
 min=temp[0];
 index=0;
 for (j=0;temp[j]!=NULL;j++) {
  if(temp[j]<min) {
   min=temp[j];
   index=j;
  }
 }
 temp[index]=123;
 return(index);
}