Your application probably needs to communicate with a database of some kind. Naturally, that database isn’t open to the world – it needs to be protected and secured. The typical solution to this is to create a username and password combination (ideally, specific to each application or user that requires access) and configure the application with these credentials. In many cases, they’re simply stored in configuration, such as the section of web.config for an ASP.NET application. By default, such settings are stored in plaintext, and are checked into source control, which can have disastrous consequences (note: if you use GitHub and accidentally expose a secret publicly, you need to change it. Just deleting it isn’t enough). There are many different kinds of secrets an application might require, from database connection strings to API keys.

Including connection strings in the code is not a very good practice as your code can be de-compiled and it will be more prone to hijacking the website as well as database server.

To protect this, a good practice would be to encrypt the connection string and decrypt it while accessing the connection string in the code.

Use below code to encrypt and decrypt connection strings using key and hash.

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using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
 
namespace EncodingDecodingMain
{
    public static class EncDec
    {
        public static string Encrypt(string toEncrypt, string SecurityKey ,bool useHashing)
        {
            byte[] keyArray;
            byte[] toEncryptArray = UTF8Encoding.UTF8.GetBytes(toEncrypt);
 
            string key = SecurityKey;
 
            //If hashing use get hashcode regards to your key
            if (useHashing)
            {
                MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
                keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
                //Always release the resources and flush data
                // of the Cryptographic service provide. Best Practice
 
                hashmd5.Clear();
            }
            else
                keyArray = UTF8Encoding.UTF8.GetBytes(key);
 
            TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
            //set the secret key for the tripleDES algorithm
            tdes.Key = keyArray;
            //mode of operation. there are other 4 modes.
            //We choose ECB(Electronic code Book)
            tdes.Mode = CipherMode.ECB;
            //padding mode(if any extra byte added)
 
            tdes.Padding = PaddingMode.PKCS7;
 
            ICryptoTransform cTransform = tdes.CreateEncryptor();
            //transform the specified region of bytes array to resultArray
            byte[] resultArray =
              cTransform.TransformFinalBlock(toEncryptArray, 0,
              toEncryptArray.Length);
            //Release resources held by TripleDes Encryptor
            tdes.Clear();
            //Return the encrypted data into unreadable string format
            return Convert.ToBase64String(resultArray, 0, resultArray.Length);
        }
 
        public static string Decrypt(string cipherString, string SecurityKey, bool useHashing)
        {
            byte[] keyArray;
            //get the byte code of the string
 
            byte[] toEncryptArray = Convert.FromBase64String(cipherString);
 
            string key = SecurityKey;
 
            if (useHashing)
            {
                //if hashing was used get the hash code with regards to your key
                MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
                keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
                //release any resource held by the MD5CryptoServiceProvider
 
                hashmd5.Clear();
            }
            else
            {
                //if hashing was not implemented get the byte code of the key
                keyArray = UTF8Encoding.UTF8.GetBytes(key);
            }
 
            TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
            //set the secret key for the tripleDES algorithm
            tdes.Key = keyArray;
            //mode of operation. there are other 4 modes. 
            //We choose ECB(Electronic code Book)
 
            tdes.Mode = CipherMode.ECB;
            //padding mode(if any extra byte added)
            tdes.Padding = PaddingMode.PKCS7;
 
            ICryptoTransform cTransform = tdes.CreateDecryptor();
            byte[] resultArray = cTransform.TransformFinalBlock(
                                 toEncryptArray, 0, toEncryptArray.Length);
            //Release resources held by TripleDes Encryptor                
            tdes.Clear();
            //return the Clear decrypted TEXT
            return UTF8Encoding.UTF8.GetString(resultArray);
        }
 
    }
}

Make a separate tool that will encode and decode a string using above code, encode your connection strings and place in the web.config file the encrypted connection strings. This way even if someone views your code, he would not be able to easily reach out to your data source without the secret key which only you have access to.