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# Frequently Asked Questions

### 0. Where can I read more about XML Signature and XML Encryption?

First of all, read the original specifications: XML Digital Signature and XML Encrytpion. Also there several books available that can help you get started.

### 1. License(s).

#### 1.1. Licensing Terms for xmlsec.

XML Security Library is released under the MIT License, see the file Copyright in the distribution for the precise wording.

#### 1.2. Can I use xmlsec with proprietary application or library? Can I use xmlsec with a GNU GPL application or library?

Probably, you will need to ask a lawyer. But not-a-lawyer answer can be found in the following table:

XML Security Library module | Dependencies | Dependencies License | Using with proprietary applications/libraries | Using with MIT/BSD applications/libraries | Using with GPL applications/libraries
xmlsec-core | LibXML2/LibXSLT | MIT License | Yes. | Yes. | Yes.
xmlsec-openssl (also requires xmlsec-core library) | OpenSSL | OpenSSL License | Yes. | Yes. | May be. OpenSSL FAQ states that OpenSSL library is covered by a special GPL exception thus it could be used in GPLed applications/libraries. However, some people think that this is not true (one and two).
xmlsec-gnutls (also requires xmlsec-core library) | GnuTLS | LGPL | May be, talk to a lawyer. | Yes. | Yes.
xmlsec-gcrypt (also requires xmlsec-core library) | LibGCrypt | LGPL | May be, talk to a lawyer. | Yes. | Yes.
xmlsec-nss (also requires xmlsec-core library) | NSS | Dual licensing: Mozilla Public License and GPL | Yes. | Yes. | Yes
xmlsec-mscrypto (also requires xmlsec-core library) | MSCrypto API | Microsoft licensing: The libraries are part of MS Windows, and are also distributed with Internet Explorer. | Unknown. | Unknown. | Unknown.

If you have questions about XML Security Library licensing then feel free to send these questions to the mailing list.

### 2. Installation.

#### 2.1. Where can I get xmlsec?

The original distribution comes from XML Security Library page.

#### 2.2. How to compile xmlsec?

On Unix just follow the "standard":

> `gunzip -c xmlsec-xxx.tar.gz | tar xvf -`
> `cd xmlsec-xxxx`
> `./configure --help`
> `./configure [possible options]`
> `make`
> `make check`
> `make install`

At that point you may have to rerun ldconfig or similar utility to update your list of installed shared libs.
On Windows the process is more complicated. Please check readme file in `xmlsec-xxxx/win32` folder.

#### 2.3. What other libraries are needed to compile/install xmlsec?

The XML Security Library requires:

- LibXML
- LibXSLT (optional)

- OpenSSL version 1.0.0 (or later).
- GnuTLS
- Libgcrypt
- NSS - Mozilla cryptographic library.

#### 2.4. Why does make check fail for some tests?

First of all, some tests must fail! Please read the messages printed before the tests.
If you have other failed tests then the next possible reason is that some features are not compiled into the library. Re-configure/re-compile xmlsec.
If this does not help then probably there is a bug in the xmlsec or in the xmlsec tests. Please submit the bug report and I'll try to fix it.

#### 2.5. I get the xmlsec sources from GitHub and there is no configure script. Where can I get it?

The configure (and other Makefiles) are generated. Use the `autogen.sh` script to regenerate the configure and Makefiles, like:

> `./autogen.sh --prefix=/usr`

#### 2.6. I do not need all these features supported by xmlsec. Can I disable some of them?

Yes, you can. Please run `./configure --help` for the list of possible configuration options.

#### 2.7. I am compiling XMLSec library on Windows and it does not compile (crashes right after the launch). Can you help me?

There are several possible reasons why you might have problems on Windows. All of them originated in the MS C compiler/linker and are specific to Windows. Thanks to Igor Zlatkovic for writing these long explanations.

1) Incorrect MS C runtime libraries.

Windows basically has two C runtimes. The one is called libc.lib and can only be linked to statically. The other is called msvcrt.dll and can only be linked to dynamically. The first one occurs in its single-threaded and multithreaded variant, which gives three different runtimes. These three then live in their debug and release incarnations, which results in six C runtimes. Worse, different versions of Microsoft Visual C/C++ have different runtimes (e.g. MSVC 6.0 runtime is not compatible with .NET 2003 runtime). The rule is simple: exactly the same runtime must be used throughout the application. Client code must use the same runtime as XMLSec, LibXML, LibXSLT, OpenSSL or any other library used.
If you downloaded XMLSec, LibXML, LibXSLT and OpenSSL binaries from Igor's page then all libraries are all linked to msvcrt.dll (Multithreaded DLL; /MD compiler switch). The click-next click-finish wizardry from Visual Studio chooses the single-threaded libc.lib as the default when you create a new project. And this causes great problems because you program crashes on first IO operation, first malloc/free from different runtimes or something even more trivial.
Do not forget that tf you need a different runtime for some reason, then you MUST recompile not only XMLSec, but LibXML, LibXSLT and OpenSSL as well.

2) Static linking without correct defines.

When people link statically to XMLSec, then they must `#define XMLSEC_STATIC` in their source files before including any XMLSec header. Almost none is doing that :) This macro has no effect on Unix, but it is vital on Windows.
This applies to LibXML and LibXSLT as well, no matter if these are used directly or not. If just XMLSec is used, but everything is linked statically, then there must be a

> `#define LIBXML_STATIC
> #define LIBXSLT_STATIC
> #define XMLSEC_STATIC
> `

before any xmlsec header is included. Even if the client code doesn't call into libxml at all, still this must be defined. XMLSec headers will include LibXML headers and they must have these definitions. Without them, every variable XMLSec includes from LibXML headers will have `__declspec(dllimport)` prepended and that will give headaches if static LibXML is used for linking.
This scheme makes it possible to have any combination of static and dynamic libraries in the resulting executable. Its cost is the need to `#define` apropriate macros. People would ideally define them by using the compiler's `/D` switch in projects that link statically.

### 3. Developing with XMLSec.

#### 3.1. xmlSecDSigCtxValidate() function returned 0. Does this mean that the signature is valid?

No!

Function xmlSecDSigCtxValidate() returns 0 when there were no processing errors during signature validation (i.e. the document has correct syntax, all keys were found, etc.). The signature is valid if and only if the xmlSecDSigCtxValidate() function returns 0 and the `status` member of the `xmlSecDSigCtx` structure is equal to `xmlSecDSigStatusSucceeded`.

#### 3.2. I am trying to sign use a part of XML document using an "Id" attribute but it does not work. Do you support "Id" attributes at all?

Yes. LibXML2 and XMLSec libraries do support ID attributes. However, you have to tell LibXML2/XMLSec what is the name of your ID attribute. XML specification does not require ID attribute to have name "Id" or "id". It can be anything you want!

`Id` `Data`

> `<?xml version="1.0" encoding="UTF-8">
> <Root>
> <Data Id="1234">
> The data I want to sign
> </Data>
> </Root>
> `

One can use a simple DTD:

> `<!DOCTYPE test [
> <!ATTLIST Data Id ID #IMPLIED>
> ]>
> `

The DTD might be directly included in the XML file or located in a standalone file. In the second case, you might load the DTD in xmlsec command line utility with "--dtd-file" option.

2) Use xml:id. This is a new W3C Working Draft and not all XML parsers support it now (LibXML2 does!).

3) Application can directly declare ID attribute to LibXML2/XMLSec. If you are using xmlsec command line utility see "--id-attr" option. If you are writing a C/C++ application yourself, call`xmlAddID` function. However, this approach might make you signature non-interoperable with other XMLDSig implementations.

#### 3.3. I am trying to sign an XML document and I have a warning about "empty nodes set". Should I worry about this?

Most likely yes. When it's not an error from specification point of view, I can hardly imagine a real world case that requires signing an empty nodes set (i.e. signing an empty string). Most likely, you have this error because you are trying to use ID attribute and you do not provide a DTD for the document (see section 3.2 about ID attributes).

####

#### 3.4. I am trying to sign/validate a document but xmlXPtrEval function can't evaluate "xpointer(id('XXXXXXX'))" expression. What's wrong?

First of all, read section 3.2 about ID attributes. If you have tried to declare required ID attribute in DTD and you still have problems then I would guess that you are playing with Visa 3D protocol. This protocol tries to reference to an "id" attribute defined as CDATA instead of ID in the DTD (it is impossible in XML as described in section 3.2). Even worse, the value of this Visa 3D "id" attribute may start from number or contain "+" or "/" and this breakes XML specification again. Based on this, I have to say that Visa 3D protocol does not use XML or XMLDSig specifications. And if you can then you should probably let Visa guys know about this problem (thought it was already done several times).

The only good solution for this problem is changing Visa 3D protocol. However, it might take time. As a short term solution you can use a special "Visa 3D hack" in xmlsec. Please note, that nobody (including me) knows what else might be broken in your application if you decide to use this hack. You are on your own here because this hack makes your application to work with non-XML and non-XMLDSig but some "Visa 3D" files.

In order to process "Visa 3D" documents, you need to do two things:

- Register ID attributes manually (`xmlAddID` function or `--id-attr` option for xmlsec command line utility).
- Enable Visa 3D hack in XML DSig context (`dsigCtx->flags |= XMLSEC_DSIG_FLAGS_USE_VISA3D_HACK` or `--enable-visa3d-hack` option for xmlsec command line utility).

This is a hack. You are warned!

UPDATE: It appears that recent version (Novemeber, 2005) of Visa3D DTD does have this problem corrected and now "id" attribute is declared as ID. Just get the new DTD and everything should work without this hack.

#### 3.5. I have a document signed with a certificate that is now expired. Can I verify this signature?

Yes, you can. However, you need to be carefull. Most likely you do want to make sure that the certificate was not expired when the document was signed. The XML Digital Signature specification does not have a standard way to include the signature timestamp. Which means that you need to define where to put timestamp by yourself. Please note, that the timestamp must be signed along with the other data.
Finaly set the desired verification time in `certsVerificationTime` member of the `xmlSecKeyInfoCtx` structure.

If you are using xmlsec command line utility then you can use `--verification-time <time>` option (where `<time>` is the local system time in the "`YYYY-MM-DD HH:MM:SS`" format).

#### 3.6. I really like the XMLSec library but it is based on OpenSSL and I have to use another crypto library in my application. Can you write code to support my crypto library?

The XMLSec library has a very modular structure and there should be no problem with using another crypto library. For example, XMLSec already supports NSS, MSCrypto API and GnuTLS. Check the latest release and/or the mailing list and you might find that your library is already supported or someone working on it.
If you are not so lucky, then you can either write some code by yourself or contact me in private email to discuss possible options.

#### 3.7. I really like the XMLSec library but it does not have cipher or transform that I need. Can you write code for me?

The XMLSec library has a very modular structure and there should be easy to add any cipher or other transform. Again, you can either write some code by yourself or try to talk to me in private email.