hi there,

     This might help you to encrypt your message :)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LIM 1000

void hexToDec(int a[],char *key, int n)
{
    int i;
    int keyLen;

    keyLen = strlen(key);
    for ( i = 0; i < keyLen; i++) {
        if( key[i] > 64 && key[i] < 71) {
            a[i] = key[i] - 55;
        } else {
            a[i] = key[i]-48;
        }
    }

    if ( i < n) {
        int value = 0;
        while( i != n) {
            a[i] = value;
            value++;
            i++;
            if ( value == 16) {
                value = 0;
            }
        }
    }

    return;
}

void fun(char *encryptedMessage, char *message, char *encryptionKey)
{

    int msgLen = strlen(message);
    int arr[msgLen];
    int i;

    hexToDec(arr,encryptionKey,msgLen); // used to fill the array arr to repective decimal value

    for ( i = 0; i < msgLen; i++) {
        encryptedMessage[i] = message[i] + arr[i];
    }
    encryptedMessage[i] = '\n';

    return;
}

int main()
{
    char message[LIM];
    char encryptionKey[LIM];
    char encryptedMessage[] = "";
    printf("Enter the message : ");
    scanf("%[^\n]%*c",message);

    printf("Enter the encryption key : ");
    scanf("%[^\n]%*c",encryptionKey);

    printf("Message : %s\n",message);
    fun(encryptedMessage,message,encryptionKey);
    printf("Encrypted Message : %s\n",encryptedMessage);

    return 0;
}