C code to implement RSA Algorithm(Encryption and Decryption)

C program to implement RSA algorithm. The given program will Encrypt and Decrypt a message using RSA Algorithm.

#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]); 
}

Output ff the above program:-

RSA Inputs

RSA Result

Related Programs:-

★ Encrypt and Decrypt a message using Substitution Cipher

★ Encrypt and Decrypt a message using Vernan Cipher

★ Encrypt and Decrypt a message using Transposition Cipher

★ Encrypt and Decrypt a message using PlayFair Cipher

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