JavaScript: Decript string that has been encrypted in C#

Question

I created this method to encryped string with a key:

 public static string EncryptString(string key, string plainText)
        {
            byte[] iv = new byte[16];
            byte[] array;

            using (Aes aes = Aes.Create())
            {
                aes.Key = Encoding.UTF8.GetBytes(key);
                aes.IV = iv;

                ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

                using (MemoryStream memoryStream = new MemoryStream())
                {
                    using (CryptoStream cryptoStream = new CryptoStream((Stream)memoryStream, encryptor, CryptoStreamMode.Write))
                    {
                        using (StreamWriter streamWriter = new StreamWriter((Stream)cryptoStream))
                        {
                            streamWriter.Write(plainText);
                        }

                        array = memoryStream.ToArray();
                    }
                }
            }

            return Convert.ToBase64String(array);
        }

But I also need this method in JS, to also decrypt the same string.

I am wondering about

AES, Encoding.UTF8.GetBytes, ICryptoTransform, Cryptostreammode, and MemoryStream.

Do these exist in JS?

IS this method even usable in JS?

AES encryption is of course possible in JavaScript. A 1:1 porting, however, is generally not feasible, i.e. you have to implement it using the respective JavaScript library's own constructs. — Topaco
– Topaco, Commented Feb 8, 2022 at 11:02
Where will this JavaScript run? In a browser? If so you don't want to decrypt it there because you would have to expose the key. — Crowcoder
– Crowcoder, Commented Feb 8, 2022 at 11:49

stacj · Accepted Answer · 2022-02-08 15:38:46Z

1

Using your C# code to encrypt a string via public static string EncryptString(string key, string plainText) I get following result:

EncryptString("xxxxxxxxxxxxxxxx", "Hello World") -> "yM377gXxX5Du71hgkPH+Fg=="

To reverse this you can check out this Javascript code:

const key = "xxxxxxxxxxxxxxxx";
const msg = "yM377gXxX5Du71hgkPH+Fg==";

const decrypted = CryptoJS.AES.decrypt(
  msg, 
  CryptoJS.enc.Utf8.parse(key),
  { mode: CryptoJS.mode.ECB }
);

console.log(decrypted.toString(CryptoJS.enc.Utf8));

<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/4.1.1/crypto-js.min.js"></script>

answered Feb 8, 2022 at 15:38

stacj

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1 Comment

innom Over a year ago

We decided to run this logic in our c# backend now, but your logic seems totally fine. Awesome!

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