Hi fariba805,
To your request, the best place to start looking for a PhD in IT Security in Germany is the Center for Advanced Security Research Darmstadt (CASED):
http://www.cased.de/en/about.html and
http://www.cased.de/en.html .
They have a Graduate School specifically for training PhD students in IT Security (in English):
http://www.cased.de/en/further_education/graded/graded.html .
It is headed by Prof. Dr.-Ing. Mira Mezini . You can find her Bio etc and even contact her by the email address on her page:
http://www.stg.tu-darmstadt.de/staff/mira_mezini/index.en.jsp .
Do contact the coordinator of the Graduate School, Eric Bodden at the email address on the School's page to assist you.
They even have some scholarships available for PhD work posted at
http://www.cased.de/en/further_education/graded/stipendien_all.html and
http://www.cased.de/en/about/jobs.html
You may find the Research Groups at CASED at:
http://www.cased.de/en/research/groups_projects.html
Since your interest is in Network Security, you may want to look at the Secure Mobile Networking Group at:
http://www.seemoo.tu-darmstadt.de/home-vision/
CASED is a collaboration between the Technical University Darmstadt (TU Darmstadt), Fraunhofer Institute for Secure IT (Fraunhofer SIT) and Hochschule Darmstadt (University of Applied Sciences). It is based at TU Darmstadt, fed by the Computer Science and other related departments.
You can read about Fraunhofer SIT here: http://www.sit.fraunhofer.de/en/the-institute.html
and be free to contact the appropriate person from the Competency Group you are interested in, since they also have their own training program for PhDs which involve doing research work with them and being assigned an academic supervisor from a chosen university etc.
For your interest in Network Security, etc you may want to contact Dr. Jens Tölle (jens.toelle AT fkie.fraunhofer.de), at their Cyber Defence Lab:
http://www.fkie.fraunhofer.de/en.html ;
http://www.fkie.fraunhofer.de/en/research-areas/cyber-defense.html
The Lab do accept PhD candidates for training as described above and they typically work together with their local University:
www.uni-bonn.de.
Well, this is already a lot of information. There are other sources, but try these first and I believe you will get a positive feedback. Be sure to let me know when you finally get a place for a PhD in IT Security with any of these leads. Wish you all the best.