How to sign a text file?

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C#

C#4.0

signing

I wanted to sign a plain text file.

The attempts to use the sn tool or the SignTool failed: they do support to sign assemblies or any exe/dll/... files, but no text files. If I try to nonetheless use those tools, they do complain that the file format is unknown.

I even tried to simply rename the test.txt file to test.dll but no success neither ;-).

I assumed this was a no-brainer, but I could not find any ready-to-use solution.
Maybe one of you could shade some light on this.

What I came up with is to go over XML signing which is directly supported by .NET Framework.
The followin pseudo code gives an idea on my approach:

public static string SignText(string unsignedText)
{
    var xmlDoc = WrapUnsignedTextIntoXml(unsignedText);

    var signedXmlDoc = SignXmlDoc(xmlDoc);

    var signatureXml = GetXmlTextFromXml(signedXmlDoc);
    var base64 = TextToBase64(signatureXml);
    var textSignature = WrapBase64IntoSignatureBlock(base64);

    var signedText = unsignedText + textSignature;

    return signedText;
}

public static bool VerifySignedText(string signedText)
{
    var {unsignedText, base64} = SplitTextAndSignature(signedText);
    var signatureXml = Base64ToText(base64);

    var signatureXmlNode = CreateXmlNodeFromSignature(signatureXml);
    var xmlDoc = WrapUnsignedTextIntoXml(unsignedText);
    var signedXmlDoc = CreateSignedXmlDoc(xmlDoc, signatureXmlNode);

    return VerifyXml(signedXmlDoc);
}

This approach bases on How to: Sign XML Documents with Digital Signatures[^] and How to: Verify the Digital Signatures of XML Documents[^].

The resulting signed text file would be something like:

This is a sample text with a signature that
verifies tha the integrity of the plain text is
not corrupted.

All whitespaces are relevant - no single character of the text must have changed.

2012-02-19/A.Gieriet

# BEGIN-SIGNATURE
# //48AFMAaQBnAG4AYQB0AHUAcgBlACAAeABtAGwAbgBzAD0AIgBoAHQAdABwADoALwAvAHcAdwB3
# AC4AdwAzAC4AbwByAGcALwAyADAAMAAwAC8AMAA5AC8AeABtAGwAZABzAGkAZwAjACIAPgA8AFMA
# aQBnAG4AZQBkAEkAbgBmAG8APgA8AEMAYQBuAG8AbgBpAGMAYQBsAGkAegBhAHQAaQBvAG4ATQBl
# AHQAaABvAGQAIABBAGwAZwBvAHIAaQB0AGgAbQA9ACIAaAB0AHQAcAA6AC8ALwB3AHcAdwAuAHcA
# MwAuAG8AcgBnAC8AVABSAC8AMgAwADAAMQAvAFIARQBDAC0AeABtAGwALQBjADEANABuAC0AMgAw
# ADAAMQAwADMAMQA1ACIAIAAvAD4APABTAGkAZwBuAGEAdAB1AHIAZQBNAGUAdABoAG8AZAAgAEEA
# bABnAG8AcgBpAHQAaABtAD0AIgBoAHQAdABwADoALwAvAHcAdwB3AC4AdwAzAC4AbwByAGcALwAy
# ADAAMAAwAC8AMAA5AC8AeABtAGwAZABzAGkAZwAjAHIAcwBhAC0AcwBoAGEAMQAiACAALwA+ADwA
# UgBlAGYAZQByAGUAbgBjAGUAIABVAFIASQA9ACIAIgA+ADwAVAByAGEAbgBzAGYAbwByAG0AcwA+
# ADwAVAByAGEAbgBzAGYAbwByAG0AIABBAGwAZwBvAHIAaQB0AGgAbQA9ACIAaAB0AHQAcAA6AC8A
# LwB3AHcAdwAuAHcAMwAuAG8AcgBnAC8AMgAwADAAMAAvADAAOQAvAHgAbQBsAGQAcwBpAGcAIwBl
# AG4AdgBlAGwAbwBwAGUAZAAtAHMAaQBnAG4AYQB0AHUAcgBlACIAIAAvAD4APAAvAFQAcgBhAG4A
# cwBmAG8AcgBtAHMAPgA8AEQAaQBnAGUAcwB0AE0AZQB0AGgAbwBkACAAQQBsAGcAbwByAGkAdABo
# AG0APQAiAGgAdAB0AHAAOgAvAC8AdwB3AHcALgB3ADMALgBvAHIAZwAvADIAMAAwADAALwAwADkA
# LwB4AG0AbABkAHMAaQBnACMAcwBoAGEAMQAiACAALwA+ADwARABpAGcAZQBzAHQAVgBhAGwAdQBl
# AD4ATABaAGoAbABSAHIAVQBVAHEAVQBrAGEARgBzAHIAdwBUAGYAeABNAGYAZABoAGYAMQBHAEEA
# PQA8AC8ARABpAGcAZQBzAHQAVgBhAGwAdQBlAD4APAAvAFIAZQBmAGUAcgBlAG4AYwBlAD4APAAv
# AFMAaQBnAG4AZQBkAEkAbgBmAG8APgA8AFMAaQBnAG4AYQB0AHUAcgBlAFYAYQBsAHUAZQA+AEkA
# KwB5AHAAaQBHADAAdgBhAGoAbABMADMATABjAHUAZgBFAHMANQBqAFkAUABRAHAARgA2AHMANQAv
# AEwALwBGAHkARABhAFMAaAA0ADkATAAxAEQATgBLAFQAWAB6AEQASABpAG4AdgBLAFcAKwBnAG0A
# MwA1AGkAcgBUAHYASAAvAEsASgBHAGYASwBZADUAOQBhAGoAZQBrAGEAUwBDAEwANQBUAGoATAAw
# ADIASwBCAGcASABGAEwAQgBmADEAaQBzAG0AaQA3AFEAdwBYAEYARgByAHMAQgB4ADIATABhADQA
# eQBGAEEAZwBPAHEAUwBuAGkAVQAwAEsAQgBaAE8AKwBWAGwAUgB3AEkAQgBJADcAWgBOADEAVwBR
# AEQASgBKADkASABTAEUAdwA3AEEATgB6AHEAagBsAE8AagBqAEMAcgBSAEQAdABVAGoAawA4AD0A
# PAAvAFMAaQBnAG4AYQB0AHUAcgBlAFYAYQBsAHUAZQA+ADwALwBTAGkAZwBuAGEAdAB1AHIAZQA+
# AA==
# END-SIGNATURE

Or similary for a C# file:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;
using System.Text;
using System.Text.RegularExpressions;
using System.Xml;

namespace SigningText
{
    /// <summary>
    /// <seealso href="http://msdn.microsoft.com/en-us/library/ms229745.aspx"/>
    /// <seealso href="http://msdn.microsoft.com/en-us/library/ms229950.aspx"/>
    /// </summary>
    public class SignText
    {
        private static readonly AsymmetricAlgorithm RSAKEY = 
            new RSACryptoServiceProvider(new CspParameters() { KeyContainerName = "XML_DSIG_RSA_KEY" });
        private static readonly string NL = Environment.NewLine;
        private static readonly string BS = "BEGIN-SIGNATURE";
        private static readonly string ES = "END-SIGNATURE";
...
}

// BEGIN-SIGNATURE
// //48AFMAaQBnAG4AYQB0AHUAcgBlACAAeABtAGwAbgBzAD0AIgBoAHQAdABwADoALwAvAHcAdwB3
// AC4AdwAzAC4AbwByAGcALwAyADAAMAAwAC8AMAA5AC8AeABtAGwAZABzAGkAZwAjACIAPgA8AFMA
// aQBnAG4AZQBkAEkAbgBmAG8APgA8AEMAYQBuAG8AbgBpAGMAYQBsAGkAegBhAHQAaQBvAG4ATQBl
// AHQAaABvAGQAIABBAGwAZwBvAHIAaQB0AGgAbQA9ACIAaAB0AHQAcAA6AC8ALwB3AHcAdwAuAHcA
// MwAuAG8AcgBnAC8AVABSAC8AMgAwADAAMQAvAFIARQBDAC0AeABtAGwALQBjADEANABuAC0AMgAw
// ADAAMQAwADMAMQA1ACIAIAAvAD4APABTAGkAZwBuAGEAdAB1AHIAZQBNAGUAdABoAG8AZAAgAEEA
// bABnAG8AcgBpAHQAaABtAD0AIgBoAHQAdABwADoALwAvAHcAdwB3AC4AdwAzAC4AbwByAGcALwAy
// ADAAMAAwAC8AMAA5AC8AeABtAGwAZABzAGkAZwAjAHIAcwBhAC0AcwBoAGEAMQAiACAALwA+ADwA
// UgBlAGYAZQByAGUAbgBjAGUAIABVAFIASQA9ACIAIgA+ADwAVAByAGEAbgBzAGYAbwByAG0AcwA+
// ADwAVAByAGEAbgBzAGYAbwByAG0AIABBAGwAZwBvAHIAaQB0AGgAbQA9ACIAaAB0AHQAcAA6AC8A
// LwB3AHcAdwAuAHcAMwAuAG8AcgBnAC8AMgAwADAAMAAvADAAOQAvAHgAbQBsAGQAcwBpAGcAIwBl
// AG4AdgBlAGwAbwBwAGUAZAAtAHMAaQBnAG4AYQB0AHUAcgBlACIAIAAvAD4APAAvAFQAcgBhAG4A
// cwBmAG8AcgBtAHMAPgA8AEQAaQBnAGUAcwB0AE0AZQB0AGgAbwBkACAAQQBsAGcAbwByAGkAdABo
// AG0APQAiAGgAdAB0AHAAOgAvAC8AdwB3AHcALgB3ADMALgBvAHIAZwAvADIAMAAwADAALwAwADkA
// LwB4AG0AbABkAHMAaQBnACMAcwBoAGEAMQAiACAALwA+ADwARABpAGcAZQBzAHQAVgBhAGwAdQBl
// AD4AbwBiAGwAVwBDAGUAbABrAEwAdABPAFEARgBVAGkATwBzAEkATQBwAEEARABhACsAUwA1AGcA
// PQA8AC8ARABpAGcAZQBzAHQAVgBhAGwAdQBlAD4APAAvAFIAZQBmAGUAcgBlAG4AYwBlAD4APAAv
// AFMAaQBnAG4AZQBkAEkAbgBmAG8APgA8AFMAaQBnAG4AYQB0AHUAcgBlAFYAYQBsAHUAZQA+AFoA
// SgBhADIAVwBWADYAYgB5AE0AbQAxAGwAYgBwAE0AbgBwAHYAYwBGAG0ATgB1AFcAVgBCAHUANABp
// AHQAeQBHAGgAcgBJAFAAdgBqADMAaQBWAG0AWgArADUAbwA4AGwAYwBIAEIAVQA0AE0AbgB4AGcA
// ZwBXAGUAagBUAEsATwBRAG0AdQBoAGgAUwBwAHgATABJADAANABuAE0AKwBoAEcAeABmAFoARABN
// AGMANAAzAGoARQBDAEkAOQBOAGEAQQAxAFAAegBqACsAMABUADMAaQBtAGYAZgBUAEMATwBVAEYA
// VwArAFEAWQB0AHkAegBuAFQASQBBAE4ANgBoAEsAQwBmADEAYwBQAHQAbQBiAGEAbQBiAEEAbwBt
// AGoAQwBaAGoAMgAvAE8ASgBCAC8AYQA1AGkARwAyAE0ATwBsADUAZAAzAFUASwBwAHIAZQBFAD0A
// PAAvAFMAaQBnAG4AYQB0AHUAcgBlAFYAYQBsAHUAZQA+ADwALwBTAGkAZwBuAGEAdAB1AHIAZQA+
// AA==
// END-SIGNATURE

I still assume that there is some simpler and more straight forward way to solve this under the requirement that I want to append the signature to the plain text file as shown in the examples above.

Any hint is very much appreciated.

Thanks

Andi