aspnet core encrypting and decrypting a string

Question

I am trying to decrypt a string using some pretty standard algorithm.

public static string DecryptString(string cipherText)
{
    string keyString = string.Empty;

    // Check whether the environment variable exists.
    keyString = Environment.GetEnvironmentVariable("EncryptKey");

    if (keyString == null)
    {
        keyString = "E546C8DF278CD5931069B522E695D4F2";
    }

    var fullCipher = Convert.FromBase64String(cipherText);
    using (var aesAlg = Aes.Create())
    {
        byte[] iv = new byte[aesAlg.BlockSize / 8];
        var cipher = new byte[16];

        Buffer.BlockCopy(fullCipher, 0, iv, 0, iv.Length);
        Buffer.BlockCopy(fullCipher, iv.Length, cipher, 0, iv.Length);
        var key = Encoding.UTF8.GetBytes(keyString);

        string result;
        using (var decryptor = aesAlg.CreateDecryptor(key, iv))
        using (var msDecrypt = new MemoryStream(cipher))
        using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
        using (var srDecrypt = new StreamReader(csDecrypt))
        {
            result = srDecrypt.ReadToEnd();
        }
        return result;
    }
}

I keep getting the error :

System.Security.Cryptography.CryptographicException: Specified padding mode is not valid for this algorithm.

I have tried multiple ways like this

var iv = new byte[16];
var cipher = new byte[16];

Or

var iv = aesAlg.IV;

I still get an error at this point. What am I doing wrong ?

Answer 1

Two changes are required

var cipher = new byte[fullCipher.Length - iv.Length];

and

Buffer.BlockCopy(fullCipher, iv.Length, cipher, 0, fullCipher.Length - iv.Length);