XML Digital Signature
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You could sign the XML data just like any other data with OpenSSL. The easiest way is probably to invoke the necessary command line program openssl, see http://www.madboa.com/geek/openssl/#cert-self . Or you could use libssl-dev directly in your program.
Another possibility is to use libraries like the "XMLSecLib":http://www.aleksey.com/xmlsec/index.html, but they basically also wrap openssl. Don't confuse the XML-formatted-Signature with the signed data. There's nothing special about the data, you can sign any data you like, including XML.There might be a way to do this with the OpenSSL-Qt-Integration, but I've only come across using this with QSSLSockets (see QSslCertificate etc.).