using System;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Xml;
using System.Xml;

namespace EncryptedXmlTest
{
	class Program
	{
		static void Main()
		{
			var document = new XmlDocument();
			document.LoadXml("<data>Some data.</data>");

			X509Certificate2 certificate; // Получение сертификата, например, из хранилища машины

			Encrypt(document, certificate);
			Console.WriteLine("Encrypted XML:");
			Console.WriteLine(document.OuterXml);

			Decrypt(document);
			Console.WriteLine("Decrypted XML:");
			Console.WriteLine(document.OuterXml);

			Console.ReadLine();
		}

		private static void Encrypt(XmlDocument document, X509Certificate2 certificate)
		{
			var encryptedXml = new EncryptedXml();
			var encryptedElement = encryptedXml.Encrypt(document.DocumentElement, certificate);
			EncryptedXml.ReplaceElement(document.DocumentElement, encryptedElement, false);
		}

		private static void Decrypt(XmlDocument document)
		{
			var encryptedXml = new EncryptedXml(document);
			encryptedXml.DecryptDocument(); // System.Security.Cryptography.CryptographicException, CryptoPro.Sharpei.Base.dll, Additional information: The parameter is incorrect.
		}
	}
}

<EncryptedData Type="http://www.w3.org/2001/04/xmlenc#Element" xmlns="http://www.w3.org/2001/04/xmlenc#"><EncryptionMethod Algorithm="urn:ietf:params:xml:ns:cpxmlsec:algorithms:gost28147" /><KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"><EncryptedKey xmlns="http://www.w3.org/2001/04/xmlenc#"><EncryptionMethod Algorithm="urn:ietf:params:xml:ns:cpxmlsec:algorithms:transport-gost2001" /><KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"><X509Data><X509Certificate>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</X509Certificate></X509Data></KeyInfo><CipherData><CipherValue>MIGkMCgEIHF9Nam4Mqp0gJpKJZl0W91cAIst8oxEUUYB9l2HIlUQBATmyxYaoHgGByqFAwICHwGgYzAcBgYqhQMCAhMwEgYHKoUDAgIkAAYHKoUDAgIeAQNDAARAm1Z7IU6Iv21qvvsExhkuJ8XqU+iXfckcopdFWWOt6SfepdXhwEMroGNvvgkX6DEn/ulf4rhQTid7EQxNPFNefAQIqCcPiv1eaZQ=</CipherValue></CipherData></EncryptedKey></KeyInfo><CipherData><CipherValue>eXrnQpz06NhNTRNNJ7x3t8OSXdn0xjdH19qzcP6oOQw=</CipherValue></CipherData></EncryptedData>

   at CryptoPro.Sharpei.COMCryptography.SetKeyParamDw(SafeKeyHandleCP hKey, Int32 param, Int32 dwValue)
   at CryptoPro.Sharpei.CPCryptoAPITransform..ctor(Int32 cArgs, Int32[] rgArgIds, Object[] rgArgValues, SafeKeyHandleCP hKey, PaddingMode padding, CipherMode cipherChainingMode, Int32 blockSize, CPCryptoAPITransformMode encDecMode)
   at CryptoPro.Sharpei.Gost28147CryptoServiceProvider._NewEncryptor(SafeKeyHandleCP hKey, CipherMode mode, Byte[] rgbIV, Int32 feedbackSize, CPCryptoAPITransformMode encryptMode)
   at CryptoPro.Sharpei.Gost28147CryptoServiceProvider.CreateDecryptor()
   at System.Security.Cryptography.Xml.EncryptedXml.DecryptData(EncryptedData encryptedData, SymmetricAlgorithm symmetricAlgorithm)
   at System.Security.Cryptography.Xml.EncryptedXml.DecryptDocument()
   at EncryptedXmlTest.Program.Decrypt(XmlDocument document)
   at EncryptedXmlTest.Program.Main()
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

// И на нем расшифровываем данные.
byte[] decryptedData = exml.DecryptData(encryptedData, decryptionKey);