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# Frequently Asked Questions

### 0. Why have you wrote xmlsec?

Very simple: when I decided to understand the XML Digital Signature and XML Encrytpion specs there were no open source C/C++ implementation available. After spending a couple days trying to install Java implementation (Apache XML Security Suite) I gave up and decided to implement these specs by myself.

### 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 embed xmlsec in a proprietary application ?

Yes. The MIT License allows you to also keep proprietary the changes you made to xmlsec, but it would be graceful to provide back bug fixes and improvements as patches for possible incorporation in the main development tree. Probably you should also check OpenSSL and LibXML2 licenses as well.

#### 1.3. Can I use xmlsec with a GNU GPL library?

Yes. MIT license is compatible with GNU GPL library. However, xmlsec is based on OpenSSL and OpenSSL license is not compatible with GNU GPL. In most cases, this should not cause any problems because of a special exception in the GPL (also check what OpenSSL FAQ says about this).

### 2. Installation.

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

The original distribution comes from XML Security Library page. Also xmlsec is available from rpmfind.net miror.

#### 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 install`
> `make check`

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 0.9.7 (prefered) or version 0.9.6c

#### 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 you use OpenSSL 0.9.6 and some xmlsec features are disabled in this case. Please try to upgrade to OpenSSL 0.9.7 and 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 CVS 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`

#### 2.5. 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.

### 3. Programming with XMLSec.

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

No! Function xmlSecDSigValidate() 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 xmlSecDSigValidate() function returns 0 and the `result` member of the returned `xmlSecDSigResult` structure is equal to `xmlSecTransformStatusOk`.

#### 3.2. 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. For example, the following Reference element:

> `<?xml version="1.0" encoding="UTF-8">
> <Root>
> <Data Id="1234">
> The data I want to sign
> </Data>
> <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
> ...
> <Reference URI="#1234">
> ...
> </Reference>
> ...
> </Signature>
> </Root>
> `

always results in an empty nodes set (and an empty string signed!) unless you have a DTD that declares `Id` attribute of the `Data` element to be an ID attribute:

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

If you are using XML Security Library in your application (not the xmlsec command line utility!) then you can do a "hack" and tell LibXML2 (and XMLSec) which attributes are ID attributes without providing a DTD by calling `xmlAddID` function. However, this might make you signature non-interoperable with other XMLDSig implementations.

#### 3.3. 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.
When you have design ready the implementation is simple:

- Enable the custom expiration time check in the simple keys manager:
  <blockqoute>`
  xmlSecKeysMngrPtr keysMngr = xmlSecSimpleKeysMngrCreate();
  ...
  xmlSecSimpleKeysMngrSetCertsFlags(keysMngr, X509_V_FLAG_USE_CHECK_TIME);
  ` </blockqoute>
- Extract the signature timestamp from the XML document.
- Assign the extracted timestamp to the `certsVerificationTime` member of the `xmlSecDSigCtx` or `xmlEncCtx` structures.
- Call `xmlSecDSigValidate` or `xmlSecDecrypt` function as usual.

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).

Aleksey Sanin