affinecypher

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int checkcoprime(int num)
{
	int a,b,temp;
	a=num;
	b=26;
	
	while(b!=0)
	{
		temp=b;
		b=a%b;
		a=temp;
	
	}
	
	return a;
	
}
main()
{
	int a,b,check,numstr[1000];
	char str[1000];
	printf("Enter a ( 1 to 25) both included which is co-prime with 26\n");
	scanf("%d",&a);
	if(a>=1 && a<=25)
	{
		check=checkcoprime(a);
		if(check!=1)
		{
			printf("Enter the one which is co-prime with 26\n");
			exit(0);
		}
		
	}
	else
	{
		printf("Enter between 1 and 25\n ");
		exit(1);
	}
	
	printf("Enter b ( 0 to 25) both included \n");
	scanf("%d",&b);	
	if(b<1 || a>25)
	{
		printf("Enter b between 0 to 25 \n");
		exit(3);
		
	}
	
	printf("Enter the string to be encypted: \n");
	fflush(stdin);
	gets(str);
	
	// converting string into capital letters
	
	int i,j;
	
	for(i=0;i<strlen(str);i++)
	{
		if(str[i]!=' ')
		{
			str[i]=toupper(str[i]);
			
		}
		
	}

	str[i]='\0';
	
	printf("Entered string is: %s \n",str);
	
	for(i=0;i<strlen(str);i++)
	{
		if(str[i]!=' ')
		{
			numstr[i]=str[i]-'A';
		}
		
		else
		{
			numstr[i]=-20;
		}
	}

	printf("The affine cipher is : ");
	for(i=0;i<strlen(str);i++)
	{
		
		if(numstr[i]!=-20)
		{
			
			numstr[i]= ((a*numstr[i])+b)%26;
			printf("%c",numstr[i]+'A');
		}
		
		else{
			printf(" ");
		}
		
		
		
	}
	
	

	
	
}