I am trying to make a substitution cipher for an assignment and we need a cipher.txt file that substitutes letters

My cipher.txt file

#include<stdio.h>

int main()
{
	//ask the user what they want to encrypt or decrypt a string
	printf("Do you want to encrypt or decrypt a string? (e/d): ");
	char choice;
	scanf("%c", &choice);

	//if the user wants to encrypt, then use cipher.txt to encrypt the string
	if (choice == 'e')
	{
		//ask the user for the string they want to encrypt
		printf("Enter the string you want to encrypt: ");
		char string[100];
		scanf("%s", string);

		//open cipher.txt
		FILE* cipher;
		cipher = fopen("cipher.txt", "r");

		//read the cipher.txt file
		char cipherText[26];
		fscanf(cipher, "%s", cipherText);

		//close cipher.txt
		fclose(cipher);

		//encrypt the string
		int i;
		for (i = 0; i < 100; i++)
		{
			if (string[i] == '
#include<stdio.h>
int main()
{
//ask the user what they want to encrypt or decrypt a string
printf("Do you want to encrypt or decrypt a string? (e/d): ");
char choice;
scanf("%c", &choice);
//if the user wants to encrypt, then use cipher.txt to encrypt the string
if (choice == 'e')
{
//ask the user for the string they want to encrypt
printf("Enter the string you want to encrypt: ");
char string[100];
scanf("%s", string);
//open cipher.txt
FILE* cipher;
cipher = fopen("cipher.txt", "r");
//read the cipher.txt file
char cipherText[26];
fscanf(cipher, "%s", cipherText);
//close cipher.txt
fclose(cipher);
//encrypt the string
int i;
for (i = 0; i < 100; i++)
{
if (string[i] == '\0')
{
break;
}
else if (string[i] >= 'a' && string[i] <= 'z')
{
string[i] = cipherText[string[i] - 'a'];
}
else if (string[i] >= 'A' && string[i] <= 'Z')
{
string[i] = cipherText[string[i] - 'A'] - 32;
}
}
//print the encrypted string
printf("The encrypted string is: %s: ", string);
}
//if the user chooses decryption, then decrypt the string
else if (choice == 'd')
{
//ask the user for the string they want to decrypt
printf("Enter the string you want to decrypt: ");
char string[100];
scanf("%s", string);
//open cipher.txt
FILE* cipher;
cipher = fopen("cipher.txt", "r");
//read the cipher.txt file
char cipherText[26];
fscanf(cipher, "%s", cipherText);
//close cipher.txt
fclose(cipher);
//decrypt the string
int i;
for (i = 0; i < 100; i++)
{
if (string[i] == '\0')
{
break;
}
else if (string[i] >= 'a' && string[i] <= 'z')
{
string[i] = cipherText[string[i] - 'a'];
}
else if (string[i] >= 'A' && string[i] <= 'Z')
{
string[i] = cipherText[string[i] - 'A'] - 32;
}
}
//print the decrypted string
printf("The decrypted string is: %s", string);
}
}
')
			{
				break;
			}
			else if (string[i] >= 'a' && string[i] <= 'z')
			{
				string[i] = cipherText[string[i] - 'a'];
			}
			else if (string[i] >= 'A' && string[i] <= 'Z')
			{
				string[i] = cipherText[string[i] - 'A'] - 32;
			}
		}

		//print the encrypted string
		printf("The encrypted string is: %s: ", string);



	}
	//if the user chooses decryption, then decrypt the string
	else if (choice == 'd')
	{
		//ask the user for the string they want to decrypt
		printf("Enter the string you want to decrypt: ");
		char string[100];
		scanf("%s", string);

		//open cipher.txt
		FILE* cipher;
		cipher = fopen("cipher.txt", "r");

		//read the cipher.txt file
		char cipherText[26];
		fscanf(cipher, "%s", cipherText);

		//close cipher.txt
		fclose(cipher);

		//decrypt the string
		int i;
		for (i = 0; i < 100; i++)
		{
			if (string[i] == '
#include<stdio.h>
int main()
{
//ask the user what they want to encrypt or decrypt a string
printf("Do you want to encrypt or decrypt a string? (e/d): ");
char choice;
scanf("%c", &choice);
//if the user wants to encrypt, then use cipher.txt to encrypt the string
if (choice == 'e')
{
//ask the user for the string they want to encrypt
printf("Enter the string you want to encrypt: ");
char string[100];
scanf("%s", string);
//open cipher.txt
FILE* cipher;
cipher = fopen("cipher.txt", "r");
//read the cipher.txt file
char cipherText[26];
fscanf(cipher, "%s", cipherText);
//close cipher.txt
fclose(cipher);
//encrypt the string
int i;
for (i = 0; i < 100; i++)
{
if (string[i] == '\0')
{
break;
}
else if (string[i] >= 'a' && string[i] <= 'z')
{
string[i] = cipherText[string[i] - 'a'];
}
else if (string[i] >= 'A' && string[i] <= 'Z')
{
string[i] = cipherText[string[i] - 'A'] - 32;
}
}
//print the encrypted string
printf("The encrypted string is: %s: ", string);
}
//if the user chooses decryption, then decrypt the string
else if (choice == 'd')
{
//ask the user for the string they want to decrypt
printf("Enter the string you want to decrypt: ");
char string[100];
scanf("%s", string);
//open cipher.txt
FILE* cipher;
cipher = fopen("cipher.txt", "r");
//read the cipher.txt file
char cipherText[26];
fscanf(cipher, "%s", cipherText);
//close cipher.txt
fclose(cipher);
//decrypt the string
int i;
for (i = 0; i < 100; i++)
{
if (string[i] == '\0')
{
break;
}
else if (string[i] >= 'a' && string[i] <= 'z')
{
string[i] = cipherText[string[i] - 'a'];
}
else if (string[i] >= 'A' && string[i] <= 'Z')
{
string[i] = cipherText[string[i] - 'A'] - 32;
}
}
//print the decrypted string
printf("The decrypted string is: %s", string);
}
}
')
			{
				break;
			}
			else if (string[i] >= 'a' && string[i] <= 'z')
			{
				string[i] = cipherText[string[i] - 'a'];
			}
			else if (string[i] >= 'A' && string[i] <= 'Z')
			{
				string[i] = cipherText[string[i] - 'A'] - 32;
			}
		}

		//print the decrypted string
		printf("The decrypted string is: %s", string);

	}
}

I tried to open and use the Cipher.txt file to swap the letters and encrypt simple words like hello and be able to decrypt them as well by inputting the cipher text. The cipher.txt file is written as so to swap the letters:

H,X
E,P
L,R
O,Q

And I cannot figure out how to get my program to read the file and take swap the letters with that given file

Code has at least these problems.

No limit on input

Code risks overflowing cipherText[]. Instead, use a width limit which is 1 less than the size.

    char string[100];
    // scanf("%s", string);
    scanf("%99s", string);

    char cipherText[26];
    // fscanf(cipher, "%s", cipherText);
    fscanf(cipher, "%25s", cipherText);

cipher.txt issues

It is unclear concerning the contents of cipher.txt, but it appears to have 25 letter mappings [A-Z], but not 'O'.

I would expect cipher.txt to be 26 uppercase characters: QWRST...A.

cipher.txt file is dubious as it 1) does not have 26 mappings and 2) maps some letters to non-letters.

#2 makes it problematic to decipher if the original message contained non-letters.

cipherText[] too small

To read a cipher from a text file with fscanf(cipher, "%s", cipherText); and use it to encode 26 letters, cipherText[] needs to be 27 to hold the 26 letter mappings and the null character.

    char cipherText[26+1];
    // fscanf(cipher, "%s", cipherText);
    fscanf(cipher, "%26s", cipherText);

Wrong case mapping

OP indicates the cipher has an uppercase mapping. Then encoding needs to change:

        else if (string[i] >= 'a' && string[i] <= 'z')
        {
            //string[i] = cipherText[string[i] - 'a'];
            string[i] = cipherText[string[i] - 'a'] - 'A' + 'a';
        }
        else if (string[i] >= 'A' && string[i] <= 'Z')
        {
            // string[i] = cipherText[string[i] - 'A'] - 32;
            string[i] = cipherText[string[i] - 'A'];
        }

Code decrypts incorrectly

Looks like a repeat of encrypt code using same "cipher.txt" file. Code needs to reverse the mapping, not simply apply it again.

Perhaps other issues too