var data = Encoding.Unicode.GetBytes("123123"); // то, что подписываем на клиенте
var extSign = Convert.FromBase64String(sig); // sig == то, что получаем от SignedData.Sign(Signer, true, 0);
var contentInfo = new ContentInfo(data);
var signedCms = new SignedCms(contentInfo, true);
signedCms.Decode(encodedSignature);
signedCms.CheckSignature(true); // проверка

System.InvalidCastException was unhandled
  Message=[A]CryptoPro.Sharpei.Gost3410CryptoServiceProvider cannot be cast to [B]CryptoPro.Sharpei.Gost3410CryptoServiceProvider. Type A originates from 'CryptoPro.Sharpei.Base, Version=1.4.0.10, Culture=neutral, PublicKeyToken=473b8c5086e795f5' in the context 'Default' at location 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\CryptoPro.Sharpei.Base\v4.0_1.4.0.10__473b8c5086e795f5\CryptoPro.Sharpei.Base.dll'. Type B originates from 'CryptoPro.Sharpei.Base, Version=1.0.0.10, Culture=neutral, PublicKeyToken=473b8c5086e795f5' in the context 'Default' at location 'C:\Windows\assembly\GAC_MSIL\CryptoPro.Sharpei.Base\1.0.0.10__473b8c5086e795f5\CryptoPro.Sharpei.Base.dll'.
  Source=ConsoleApplication1
  StackTrace:
       at ConsoleApplication1.Program.SignMsg(Byte[] msg, X509Certificate2 signerCert) in c:\Program Files (x86)\Crypto Pro\Sharpei SDK\Examples\ConsoleApplication1\Program.cs:line 109
       at ConsoleApplication1.Program.Main(String[] args) in c:\Program Files (x86)\Crypto Pro\Sharpei SDK\Examples\ConsoleApplication1\Program.cs:line 59
       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.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: