function  CryptVerifyMessageSignature(pVerifyPara :PCRYPT_VERIFY_MESSAGE_PARA;
                                     dwSignerIndex :DWORD;
                                const  pbSignedBlob :PBYTE;
                                     cbSignedBlob :DWORD;
                                     pbDecoded :PBYTE;
                                     pcbDecoded :DWORD;
                                     ppSignerCert :PCCERT_CONTEXT
                                     ):BOOL ; stdcall;

 function  CryptVerifyMessageSignature( pVerifyPara : PCRYPT_VERIFY_MESSAGE_PARA;
                                      dwSignerIndex : DWORD;
                                 const  pbSignedBlob : PBYTE;
                                      cbSignedBlob : DWORD;
                                      pbDecoded : PBYTE;
                                  var   pcbDecoded : DWORD;
                                      ppSignerCert : PCCERT_CONTEXT
                                     ) : BOOL; stdcall;                                     

BOOL WINAPI CryptVerifyMessageSignature(
  __in       PCRYPT_VERIFY_MESSAGE_PARA pVerifyPara,
  __in       DWORD dwSignerIndex,
  __in       const BYTE* pbSignedBlob,
  __in       DWORD cbSignedBlob,
  __out      BYTE* pbDecoded,
  __inout    DWORD* pcbDecoded,
  __out_opt  PCCERT_CONTEXT* ppSignerCert
);

 function  CompareCertificateInfo(  const  Base, Comp: PCERT_INFO ) : Boolean;
 var  iSize : DWORD;
 begin 
  iSize := Base.SerialNumber.cbData;
   if  iSize > Comp.SerialNumber.cbData  then 
    iSize := Comp.SerialNumber.cbData;
  Result := CompareMem( Base.SerialNumber.pbData, Comp.SerialNumber.pbData, iSize );
 end ;

 function  GetSignerCertificate( pvGetArg: PVOID; dwCertEncodingType: DWORD; pSignerId: PCERT_INFO;
          hMsgCertStore: HCERTSTORE  ) : PCCERT_CONTEXT; stdcall;
 begin 
  Result :=  nil ;
   repeat 
    Result := CertEnumCertificatesInStore( hMsgCertStore, Result );
     if  Assigned( Result )  then 
       if  CompareCertificateInfo( pSignerId, Result.pCertInfo )  then 
        Break;
   until   not  assigned( Result );
 end ;

...
 var 
  Certificate : PCCERT_CONTEXT;
  VerifyParams : CRYPT_VERIFY_MESSAGE_PARA;
  pStreamIn, pStreamOut : TMemoryStream;
  cbDecodedMessageBlob : DWORD;
 begin 
  Memo.Clear;
  pStreamIn := TMemoryStream.Create;
  pStreamOut := TMemoryStream.Create;
   try 
    pStreamIn.LoadFromFile( 'c:\test.sig' );
    FillChar( VerifyParams, SizeOf( CRYPT_VERIFY_MESSAGE_PARA ), # 0  );
    VerifyParams.cbSize := SizeOf( CRYPT_VERIFY_MESSAGE_PARA );
    VerifyParams.dwMsgAndCertEncodingType := MY_ENCODING_TYPE;
    VerifyParams.pfnGetSignerCertificate := @GetSignerCertificate;
     if   not  ( CryptVerifyMessageSignature( VerifyParams,  0 , pStreamIn.Memory, pStreamIn.Size,
                                           nil , cbDecodedMessageBlob,  nil  ))  then 
      Err( 'Не удалось узнать размер буфера' );
    pStreamOut.Size := cbDecodedMessageBlob;
    Memo.Lines.Add( IntToStr( cbDecodedMessageBlob ));
     if   not  ( CryptVerifyMessageSignature( VerifyParams,  0 , pStreamIn.Memory, pStreamIn.Size,
                                          pStreamOut.Memory, cbDecodedMessageBlob, @Certificate ))  then 
      Err( 'Не удалось проверить подпись' );
    pStreamOut.Size := cbDecodedMessageBlob;
    pStreamOut.SaveToFile( 'c:\test.res' );
    Memo.Lines.Add( 'Подпись верна' );
     { тут можно делать с контекстом серитфиката все, что вашей душе угодно } 
     if   not  ( CertFreeCertificateContext( Certificate ))  then 
      Err( 'Не удалось удалить контекс сертификата' );
   finally 
    FreeAndNil( pStreamIn );
    FreeAndNil( pStreamOut );
   end ;
 end ;

 function  CompareBlob(  const  Base, Comp: PCRYPTOAPI_BLOB ) : Boolean;
 begin 
  Result := ( Base.cbData = Comp.cbData );
   if  Result  then 
    Result := CompareMem( Base.pbData, Comp.pbData, Base.cbData );
 end ;

 function  CompareCertificateInfo(  const  Base, Comp: PCERT_INFO ) : Boolean;
 begin 
  Result := CompareBlob( @Base.Issuer, @Comp.Issuer )  and 
            CompareBlob( @Base.SerialNumber, @Comp.SerialNumber );
 end ;

Win32Check( CertVerifyTimeValidity(  nil , Certificate.pCertInfo ) =  0  );

 function  DecodeDate( FileTime : TFileTime ) : TDateTime;
 var 
  SystemTime : TSystemTime;
  ModifiedTime : TFileTime;
 begin 
  FileTimeToLocalFileTime( FileTime, ModifiedTime );
  FileTimeToSystemTime( ModifiedTime, SystemTime );
  Result := SystemTimeToDateTime( SystemTime );
 end ;

  ...
  DateTimeToStr( DecodeDate( Certificate.pCertInfo.NotBefore ))
  DateTimeToStr( DecodeDate( Certificate.pCertInfo.NotAfter ))
  ...