I have made a program to encrypt and decrypt a message using a vigenere cipher.

While encrypting or decrypting the text, some extra garbage values are printed in it.

It takes the input from a file named `input.txt` and outputs to `output.txt`, you have to write the message in the `input.txt` file and while running you have to give a key (a word with alphanumeric characters).

Why is this is happening?

The code is as follows:

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

int Encrypt(char key[])         // CODE FOR ENCRYPTION
{
    int sz = 0, i;
    FILE *ifp, *ofp;
    ifp = fopen("input.txt", "r");
    char *buffer;
    char outputFilename[] = "output.txt";

    if (ifp == NULL)
    {
        fprintf(stderr, "Cant open input file\n");
        exit(1);
    }

    fseek(ifp, 0, SEEK_END);
    sz = ftell(ifp);

    // printf("%d",sz);

    fseek(ifp, 0, SEEK_SET);

    /* allocate memory for entire content */
    buffer = (char *)malloc(sizeof(char) * sz);
    if (!buffer)
        fclose(ifp), fputs("memory alloc fails", stderr), exit(1);

    /* copy the file into the buffer */
    if (1 != fread(buffer, sz, 1, ifp))
        fclose(ifp), free(buffer), fputs("entire read fails", stderr), exit(1);

    ofp = fopen(outputFilename, "w");

    if (ofp == NULL)
    {
        fprintf(stderr, "Can't open output file !\n");
    }
    // fprintf(ofp,"%s",buffer);

    int j = 0;
    for (i = 0; i < strlen(buffer); i++)
    {
        if (j > strlen(key) - 1)
            j = 0;
        if (buffer[i] >= 65 && buffer[i] < 91)
        {

            int c = ((((buffer[i] - 65) + ((key[j] - 65) % 26))) % 26) + 65;
            fprintf(ofp, "%c", c);

        }
        else if (buffer[i] >= 97 && buffer[i] < 123)
        {
            int c = ((((buffer[i] - 97) + ((key[j] - 65) % 26))) % 26) + 97;

            fprintf(ofp, "%c", toupper(c));

        }
        else
        {
            fprintf(ofp, "%c", buffer[i]);
            continue;
        }
        j++;
    }
    printf("\n");

    fclose(ifp);
    fclose(ofp);

    return 0;
}

int Decrypt(char key[])         // CODE FOR DECRYPTION
{

    int sz = 0, i;
    FILE *ifp, *ofp;
    ifp = fopen("output.txt", "r");
    char *buffer;
    char outputFilename[] = "output2.txt";

    if (ifp == NULL)
    {
        fprintf(stderr, "Cant open input file\n");
        exit(1);
    }

    fseek(ifp, 0, SEEK_END);
    sz = ftell(ifp);

    // printf("%d",sz);

    fseek(ifp, 0, SEEK_SET);

    /* allocate memory for entire content */
    buffer = (char *)malloc(sizeof(char) * sz);
    if (!buffer)
        fclose(ifp), fputs("memory alloc fails", stderr), exit(1);

    /* copy the file into the buffer */
    if (1 != fread(buffer, sz, 1, ifp))
        fclose(ifp), free(buffer), fputs("entire read fails", stderr), exit(1);

    ofp = fopen(outputFilename, "w");

    if (ofp == NULL)
    {
        fprintf(stderr, "Can't open output file !\n");
    }
    // fprintf(ofp,"%s",buffer);

    int j = 0;
    for (i = 0; i < strlen(buffer); i++)
    {
        if (j > strlen(key) - 1)
            j = 0;
        if (buffer[i] >= 65 && buffer[i] < 91)
        {
            if (buffer[i] > key[j])
            {
                int c =
                    ((((buffer[i] - 65) - ((key[j] - 65) % 26))) % 26) + 65;
                fprintf(ofp, "%c", tolower(c));
            }
            else
            {
                int c = ((((buffer[i] - key[j]) + 26)) % 26) + 65;
                fprintf(ofp, "%c", tolower(c));
            }
        }
        else if (buffer[i] >= 97 && buffer[i] < 123)
        {
            int c = ((((buffer[i] - 97) - ((key[j] - 65) % 26))) % 26) + 97;

            fprintf(ofp, "%c", tolower(c));

        }
        else
        {
            fprintf(ofp, "%c", buffer[i]);
            continue;
        }
        j++;
    }
    printf("\n");

    fclose(ifp);
    fclose(ofp);

    return 0;
}

void main()
{
    int ch;
    char key[20];
  a:printf("0.Exit the Menu\n1.Encrypt\n2.Decrypt\n");
    printf("Enter your choice\n");
    scanf("%d", &ch);

    switch (ch)
    {

    case 0:
        printf("Goodbye\n");
        break;

    case 1:
        printf
            ("-----------------------------Welcome to the encryption zone---------------------\n");
        printf("Enter the key to be used\n");
        scanf("%s", key);

        Encrypt(key);
        break;

    case 2:
        printf
            ("-----------------------------Welcome to the decryption zone---------------------\n");
        printf("Enter the key to be used\n");
        scanf("%s", key);

        Decrypt(key);
        break;

    default:
        printf("Enter the correct choice\n");
        goto a;
        break;

    }
}

```