#include "keyword.h"

void encode_keyword(char* plaintext, char* keyword)
{
	int i ;
	int j ;
	int len = strlen(keyword) ;
	int plen = strlen(plaintext) ;

	// reduce the keyword to a number that can be easily
	// added to the characters.
	for(i = 0 ; i < len ; i++)
	{
		keyword[i] -= 'a' ;
	}

	for(i = 0 ; i < plen ; i+= len)
	{
		for(j = 0 ; j < len ; j++)
		{
			plaintext[i+j] = plaintext[i+j] + keyword[j] ;

			while(plaintext[i+j] > 'z')
			{
				plaintext[i+j] -= 26 ;
			}

			if(i+j >= plen)
			{
				break ;
			}
		}
	}

	for(i = 0 ; i < len ; i++)
	{
		keyword[i] += 'a' ;
	}
	
	return ;
}

void decode_keyword(char* ciphertext, char* keyword)
{
	int i ;
	int j ;
	int len = strlen(keyword) ;
	int plen = strlen(ciphertext) ;

	// reduce the keyword to a number that can be easily
	// added to the characters.
	for(i = 0 ; i < len ; i++)
	{
		keyword[i] -= 'a' ;
	}

	for(i = 0 ; i < plen ; i+= len)
	{
		for(j = 0 ; j < len ; j++)
		{
			ciphertext[i+j] = ciphertext[i+j] - keyword[j] ;

			while(ciphertext[i+j] < 'a')
			{
				ciphertext[i+j] += 26 ;
			}

			if(i+j >= plen)
			{
				break ;
			}
		}
	}

	for(i = 0 ; i < len ; i++)
	{
		keyword[i] += 'a' ;
	}
	
	return ;
}