Write encrypted data to xml

Question

In my application I have the following code for the encryption of some data as string:

private string EncryptPermission(User user)
{
    Encoding encoding = Encoding.ASCII;
    Cryptography cryptography = CreateCryptography(user);
    byte[] val = encoding.GetBytes(user.Permission.ToString());
    byte[] encrypted = cryptography.Encrypt(val);
    string result = encoding.GetString(encrypted);
    return result;
}

That works like I want. Now I want to write that data to an xml-file like:

internal void WriteUser(User user)
{
    XElement userElement = new XElement("user",
        new XAttribute("id", user.UserId.ToString("N")),
        new XElement("name", user.Username),
        new XElement("password", user.Password),
        new XElement("permission", EncryptPermission(user)));

    XDocument document = GetDocument();
    if (document.Root != null)
    {
        document.Root.Add(userElement);
        SaveDocument(document);
    }
}

At the moment I call SaveDocument I get an Exception like:

A first chance exception of type 'System.ArgumentException' occurred in System.Xml.dll

Additional information: ' ', hexadecimal value 0x06, is a invalid char.

(Translated from german)

How can I solve this? I've already tried to use Converter.ToBase64String(..) and Converter.FromBase64String(..) but there I get an exception that the data is too long.

Have a look here: http://stackoverflow.com/questions/8170739/dealing-with-invalid-xml-hexadecimal-characters — JaggenSWE, May 25 '16 at 12:00
`return Convert.ToBase64String(cryptography.Encrypt(val))` in your encoding method would return a value safe for XML. — Alex K., May 25 '16 at 12:16
https://msdn.microsoft.com/en-us/library/sb7w85t6(v=vs.110).aspx — Chris Cooper, May 25 '16 at 12:23

rossum · Answer 1 · 2016-05-25T13:40:00.893

1

Encryption produces bytes, and not all bytes are valid characters, as expected by XML. Your 0x06 is just one such. One possible solution is to convert the bytes into Base64 before writing to XML. Base64 uses only valid characters, at the cost of some increase in size: it uses four characters to write three bytes.

Have a look at the Convert class for the C# Base64 conversion methods.

edited May 25 '16 at 13:40

answered May 25 '16 at 12:16

rossum

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score 0 · Answer 2 · edited May 23 '17 at 11:52

0

It mostly likely down to a character within the string, has broken the xml.

Add the encrypted string in a CDATA[] tags, within the nodes and you should be fine

What does <![CDATA[]]> in XML mean?

edited May 23 '17 at 11:52

Community

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answered May 25 '16 at 12:11

Chris Cooper

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You cannot dump binary data into a cdata section, 0x0 for example is not legal. – Alex K. May 25 '16 at 12:16
I thought, if its an encrypted string. It would be fine with CDATA. My bad. – Chris Cooper May 25 '16 at 12:24
A page on msdn shows, how you could do it: https://msdn.microsoft.com/en-us/library/sb7w85t6(v=vs.110).aspx – Chris Cooper May 25 '16 at 12:25

Write encrypted data to xml

2 Answers2