cs50 pset2 substitution problem

substitution.c

#include<stdio.h>
#include"functions.c"

int check_key_validity(char argv[]); // checks whether the key given by the user is valid or not according to constraints

int main(int argc, char* argv[]){
	if(argc != 2){
		printf("Usage : ./program_name key");
		return 1;
	}
	int key_validity = check_key_validity(argv[1]);
	if(key_validity != 0){
		printf("INVALID KEY\n");
		return 1;
	}
	char plaintext[1000];
	char ciphertext[1000];
	printf("plaintext: ");
	fgets(plaintext, sizeof(plaintext), stdin);
	
	//encrypting with user key
	int i = 0;
	while(plaintext[i] != '\0'){
		if(is_alphabet_char(plaintext[i]) == 1){
			ciphertext[i] = plaintext[i];
			i++;
			continue;
		}	

		int plaintext_casecheck = alphabet_case_check(plaintext[i]);
		if(plaintext_casecheck == 0)
			ciphertext[i] = argv[1][(int)(plaintext[i] - 'a')]; // same concept as repeat letter function have to cast (plaintext[i] - 'A') to int because difference of 2 characters is an  integer
		else
			ciphertext[i] = argv[1][(int)(plaintext[i] - 'A')]; 

		int ciphertext_casecheck = alphabet_case_check(ciphertext[i]);
		if (plaintext_casecheck != ciphertext_casecheck){
			if(plaintext_casecheck == 0){
				ciphertext[i] = ciphertext[i] + ('a'-'A');
			}
			else if(plaintext_casecheck == 1){
				ciphertext[i] = ciphertext[i] - ('a'-'A');
			}
		}
		i++;
	}
	printf("ciphertext: %s", ciphertext);
	return 0;
}

int check_key_validity(char argv[]){
	return (calculate_key_size(argv) != 26) ||  (is_alphabet_arr(argv) == 1) || (repeat_letter_check(argv) == 1);
}