Suppose that you have some XML document you wish to sign. It turns out it's very easy to do so because there is
xmlsec library, and in particular
xmlsec1 command line tool that's standard part of Fedora Linux distribution. The only problem is that its very picky and not very informative when it comes to error logging, finally, there are a lot of small details that can catch you. Since I had to sign a document I spent some time trying to figure out how to do that. In the end, I managed to do it and I'll write here how to for a future reference. Before I continue you'll need certificate and a key to be used for verification and signing. They are not the topic of this post so I'll just give you them:
private key,
certificate, and
CA certificate.
Ok, let's assume that you have the following XML document you wish to sign:
<?xml version="1.0" encoding="UTF-8"?>
<document>
<firstelement attr1="attr1">
Content of first element.
<secondelement attr2="attr2">
Content of the second element.
<thirdelement attr3="attr3">
And the content of the third element.
</thirdelement>
</secondelement>
</firstelement>
</document>
Basically, you can take any XML document you wish. I'll suppose that this XML document is stored in the file
tosign.xml. If you typed yourself XML document, or if you just want to be sure, you can check if XML is well formed. There is
xmllint tool that surves that purpose. Just run it like this:
$ xmllint tosign.xml
And if you don't get any error messages, or warnings, that the XML document is well formed. You can also check if the document is valid by providing schema, or DTD, via appropriate command line switches.
In order to sign this document you have to add XML Signature fragment to the XML file. That fragment defines how the document will be signed, what will be signed, and, where the signature, along with certificate, will be placed. The fragment has the following form:
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
<Reference>
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<DigestValue />
</Reference>
</SignedInfo>
<SignatureValue />
<KeyInfo>
<X509Data />
</KeyInfo>
</Signature>
Note that this (quite verbose) fragment has to be placed somewhere within the root element. Now, lets sign this, newly created document. To do so invoke
xmlsec1 command like this (this is one line in case it is broken into two due to the formatting):
xmlsec1 --sign --privkey-pem privkey.pem,cert.pem --output signed.xml tosign.xml
After this, the signed XML document will be in the file named
signed.xml. Take a look into it, the placeholders within signature fragment are filled up with signature data, and with a certificate who's private key was used to sign the XML document.
Note that signature itself is generated using private key (
privkey.pem) which, as its name suggest, has to be private for a signer. Otherwise, anyone can falsify signature.
Now, to verify signed XML document you have to specify trusted CA that will be used to verify signature. It has to be certificate of the certificate authority (CA) that issued signer's certificate. In my case that's cacert.pem, i.e.:
$ xmlsec1 --verify --trusted-pem cacert.pem signed.xml
OK
SignedInfo References (ok/all): 1/1
Manifests References (ok/all): 0/0
As you can see, the signature was verified OK. You can try now to change something in the XML document and see if the verification passes or not.
I'll mentioned one more thing before concluding this post. Namely, in the previous example the whole XML document was signed. But, you can sign only a part. To do so, you have to do two things. The first one is to mark the element that you wish to sign (its content will also be signed) and the second is to tell xmlsec1 to sign only that element.
The first step is accomplished by adding attribute to the element that should be signed. Let's assume that in our case we only want
secondelement to be signed. Modify the appropriate opening tag to have the following form:
<secondelement attr2="attr2" id="signonlythis">
Note that I added attribute
id, but basically any name can be used (unless you use some predefined schema or DTD).
The second step is to tell xmlsec1 that only this element should be signed. This is accomplished by modifying
Reference element to have the following form:
<Reference URI="#signonlythis">
If you now try to sign this modified XML document using command I gave above, you'll receive an error message:
$ xmlsec1 --sign --privkey-pem cert.key,cert.pem --output test_signed.xml tosign.xml
func=xmlSecXPathDataExecute:file=xpath.c:line=273:obj=unknown:subj=xmlXPtrEval:error=5:libxml2 library function failed:expr=xpointer(id('signonlythiselement'))
func=xmlSecXPathDataListExecute:file=xpath.c:line=356:obj=unknown:subj=xmlSecXPathDataExecute:error=1:xmlsec library function failed:
func=xmlSecTransformXPathExecute:file=xpath.c:line=466:obj=xpointer:subj=xmlSecXPathDataExecute:error=1:xmlsec library function failed:
func=xmlSecTransformDefaultPushXml:file=transforms.c:line=2405:obj=xpointer:subj=xmlSecTransformExecute:error=1:xmlsec library function failed:
func=xmlSecTransformCtxXmlExecute:file=transforms.c:line=1236:obj=unknown:subj=xmlSecTransformPushXml:error=1:xmlsec library function failed:transform=xpointer
func=xmlSecTransformCtxExecute:file=transforms.c:line=1296:obj=unknown:subj=xmlSecTransformCtxXmlExecute:error=1:xmlsec library function failed:
func=xmlSecDSigReferenceCtxProcessNode:file=xmldsig.c:line=1571:obj=unknown:subj=xmlSecTransformCtxExecute:error=1:xmlsec library function failed:
func=xmlSecDSigCtxProcessSignedInfoNode:file=xmldsig.c:line=804:obj=unknown:subj=xmlSecDSigReferenceCtxProcessNode:error=1:xmlsec library function failed:node=Reference
func=xmlSecDSigCtxProcessSignatureNode:file=xmldsig.c:line=547:obj=unknown:subj=xmlSecDSigCtxProcessSignedInfoNode:error=1:xmlsec library function failed:
func=xmlSecDSigCtxSign:file=xmldsig.c:line=303:obj=unknown:subj=xmlSecDSigCtxSigantureProcessNode:error=1:xmlsec library function failed:
Error: signature failed
Error: failed to sign file "tosign.xml"
The problem is that URI attribute references ID attribute of an element. But, ID element isn't recognized by name but has to be specified in DTD or in schema, depending what you have. In our case there is neither schema nor DTD and thus ID isn't recognized by xmlsec1. So, we have to tell it what is the name of the ID attribute, and that can be done in two ways. The first one is by using command line switch
--id-attr, and so the command to sign this document is:
xmlsec1 --sign --privkey-pem privkey.pem,cert.pem --id-attr:id secondelement --output signed.xml tosign.xml
The name after the column is the attribute name that is ID. Default value is "id", but can be anything else. If it is "id", then it can be omitted. The argument to
--id-attr is element whose attribute should be treated as an id. You should also be careful of namespaces. If they are used then the namespace of the element has to be specified too, and not shorthand but the full namespace name. Finally, note that XML is case sensitive!
The other possibility is to create DTD file and to give it as an argument to xmlsec1. In this case, DTD should look like this (I'll assume that this is the content of a file
tosign.dtd):
<!ATTLIST secondelement id ID #IMPLIED>
And you would invoke xmlsec1 like this:
xmlsec1 --sign --privkey-pem privkey.pem,cert.pem --dtd-file tosign.dtd --output signed.xml tosign.xml
Note that you'll receive a lot of warnings (DTD is incomplete) but the file will be signed. To check the signature, you again have to specify either
--dtd-file or
--id-attr options, e.g.
xmlsec1 --verify --trusted-pem cacert.pem --id-attr:id secondelement signed.xml
Now, you can experiment to check that really only
secondelement was signed and nothing else.
Final note. You have to put XML signature fragment in XML file you are signing. What can confuse you (and confused me) is that there is an option
sign-tmpl that adds this fragment, but it is very specific and used only for
testing purposes.
