Steam CD-key cache decryption

cdk_decrypt.cs

byte[] key = { 0xd6, 0x22, 0x4d, 0x40, 0x36, 0xef, 0x80, 0x0b, 0x9c, 0x10, 0x7f, 0x02, 0x64, 0x13, 0x8c, 0xf5, 0x08, 0xd3, 0x32, 0x70, 0x33, 0xd3, 0x65, 0xe7, 0xbb, 0x84, 0x09, 0x6c, 0x14, 0xe5, 0xa4, 0x00 };
string encryptedTicket = "cdk_* from localconfig.vdf";
byte[] encryptedTicketBytes = Convert.FromHexString(encryptedTicket);

using Aes aes = Aes.Create();
aes.Key = key;

// Decrypt IV
aes.Mode = CipherMode.ECB;
aes.Padding = PaddingMode.None;
using (var decryptor = aes.CreateDecryptor())
{
    var encryptedIv = encryptedTicketBytes.AsSpan().Slice(0, 16).ToArray();
    var decryptedIv = decryptor.TransformFinalBlock(encryptedIv, 0, encryptedIv.Length);
    aes.IV = decryptedIv;
}

// Decrypt data
aes.Mode = CipherMode.CBC;
aes.Padding = PaddingMode.PKCS7;
byte[] decryptedData;
using (var inMs = new MemoryStream(encryptedTicketBytes.AsSpan().Slice(16).ToArray()))
using (var cryptStream = new CryptoStream(inMs, aes.CreateDecryptor(), CryptoStreamMode.Read))
using (var outMs = new MemoryStream())
{
    cryptStream.CopyTo(outMs);
    decryptedData = outMs.ToArray();
}

// Extract CD-key, starting at offset 4, maximum 512 bytes
StringBuilder sb = new();
for (int i = 4; i < decryptedData.Length; ++i)
{
    if (decryptedData[i] != 0)
        sb.Append((char)decryptedData[i]);
    else
        break;
}

Console.WriteLine(sb.ToString());