how to fix the error in Mono-alphabetic Cipher decryption?

Question

i have written a program on mono-alphabetic ciphers , my code is running perfect for encryption but it give me wrong values when i am doing decryption. here is my code

 using System;

class SubstitutionCipher
{
static void Main()
{
    string key = "zyxwvutsrqponmlkjihgfedcba";


    string plainText = "the quick brown fox jumps over the lazy dog";
    string cipherText = Encrypt(plainText, key);
    string decryptedText = Decrypt(cipherText, key);

    Console.WriteLine("Plain     : {0}", plainText);
    Console.WriteLine("Encrypted : {0}", cipherText);
    Console.WriteLine("Decrypted : {0}", decryptedText);
    Console.ReadKey();
}
static string Encrypt(string plainText, string key)
{
    char[] chars = new char[plainText.Length];
    for (int i = 0; i < plainText.Length; i++)
    {
        if (plainText[i] == ' ')
        {
            chars[i] = ' ';
        }
        else
        {
            int j = plainText[i] - 97;
            chars[i] = key[j];
        }
    }
    return new string(chars);
}
public string reverse(string cipherText)
{
    char[] charArray = cipherText.ToCharArray();
    Array.Reverse(charArray);

    return new string(charArray);
}
static string Decrypt(string cipherText, string key)
{
    char[] chars = new char[cipherText.Length];
    for (int i = 0; i < cipherText.Length; i++)
    {
        if (cipherText[i] == ' ')
        {
            chars[i] = ' ';
        }
        else
        {
            int j = key.IndexOf(cipherText[i]) - 97;
            chars[i] = (char)j;
        }
    }
    return new string(chars);
}
}

If key= zyxwvutsrqponmlkjihgfedcba

Outputs:

Plain : the quick brown fox jumps over the lazy dog

Encrypted : wyo xevks flnqa tnu hecdr nbol wyo pjzi gnm

Decrypted : ???????????????????????????????????????????

its decryption not working

Answer 1

Replace the line in Decrypt function:

int j = key.IndexOf(cipherText[i]) - 97;

with

int j= key.IndexOf(cipherText[i]) + 97;