Implementation of autokey cipher in c

autokey_cypher.c

/*
The MIT License (MIT)

Copyright (c) 2014 Tanay PrabhuDesai

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

#define VAL(PTR) (*PTR)

typedef char* STRING;
typedef char** STRING_REF;

void autokey_encrypt_decrypt(STRING_REF str, char key, int encrypt){
	int str_length, itr_str, next_key,current_key, remainder;
	str_length = strlen(VAL(str));
	for(itr_str = 0 ; itr_str < str_length ; ++itr_str){
		if(isalpha(VAL(str)[itr_str])){
			VAL(str)[itr_str] = toupper(VAL(str)[itr_str]);
		}
	}
	next_key = toupper(key)-'A';
	for(itr_str = 0 ; itr_str < str_length ; ++itr_str){
		if(isalpha(VAL(str)[itr_str])){
			current_key = next_key;
			if(encrypt){
				next_key = VAL(str)[itr_str]-'A';
				VAL(str)[itr_str] = (VAL(str)[itr_str]-'A'+current_key)%26+'A';
			}
			else{
				remainder = (VAL(str)[itr_str]-'A'-current_key)%26+'A';
				VAL(str)[itr_str] = remainder < 'A' ? remainder + 26 : remainder ;
				next_key = VAL(str)[itr_str]-'A';
			}
		}
	}
}

void flush_stdin()
{
	while(fgetc(stdin) != '\n');
}

int main(int argc, char **argv)
{
	int length;
	char key;
	char *str;
	printf("Enter the key to be used for encryption: ");
	scanf("%c",&key);
	printf("Enter the length of the string: ");
	scanf("%d",&length);
	str = (char*)malloc((length+1)*sizeof(char));
	flush_stdin();
	printf("Input the string you want to encrypt: ");
	fgets(str,length+1,stdin);
	flush_stdin();
	autokey_encrypt_decrypt(&str,key,1);
	printf("\nThe text after encrypting is: %s",str);
	autokey_encrypt_decrypt(&str,key,0);
	printf("\nAfter decrypting the same encrypted text: %s",str);
	printf("\n");
	free(str);
	return 0;
}

I tried running this code on my laptop using codeblocks, however, I'm not getting output, that is, the text after encrypting and after decryption sections are not getting displayed even though the input sections of the code are working

I think you should might have to link the Math library in addition to just running the command. I'm not sure about the solution for this, I wrote this code longback!

Okay thank you for your feedback sir

Sir, I have noticed that the code on works when the Plain Text is one complete word say AMALGAMATION but not for sentences such as LETS HEAD THERE. Seems like the space between words leads to this issue, in your provided example I can see you used the plain text ATTACK IS TODAY and you got output

TBH I, honestly have no time to fix this. Feel free to fix it if you want to.