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Voice Over IP Security Voice over IP security Studienarbeit: Studienarbeit Voice over IP Security Hochschule Rapperswil SS 05, 21.03.2005 –04.07.2005 Betreuer: Prof. Dr. Andreas Steffen Projektteam: Christian Höhn Silvan Geser Dokumentname: Voice Over IP Security.Doc Druckdatum: 16.01.2007 Studienarbeit Voice over IP Security Abstract 2 / 68 Voice over IP security I Abstract Im Rahmen dieser Studienarbeit galt es, den populären Voice over IP Client „KPhone“ für Linux „sicher“ zu machen. Konkret musste KPhone so erweitert werden, dass die Voice Daten, effi- zient verschlüsselt über das Netzwerk übertragen werden können. Um dies zu realisieren, wurde auf ein bereits vorhandenes Softwarekonstrukt, die Verschlüsse- lungs-Bibliothek ’libsrtp’, zurückgegriffen. KPhone wurde so erweitert, dass die von der Library zur Verfügung gestellten Funktionen zur Verschlüsselung verwendet werden können. KPhone ist nun mit dieser Erweiterung in der Lage, alle versendeten Voice Pakete symmetrisch, mit dem modernen AES Algorithmus zu verschlüsseln. Zusätzlich wird mittels einer verschlüssel- ten Prüfsumme für jedes einzelne Paket sichergestellt, dass es nicht verändert worden ist und der Sender tatsächlich im Besitzt des richtigen Schlüssel ist. Die beiden Kommunikationspartner müssen sich vorgängig auf anderem Wege (Telefon, Email etc.) auf einen gemeinsamen Schlüssel einigen. Dieser Schlüssel kann nun vom Benutzer in der grafischen Oberfläche von KPhone eingegeben werden. Dort kann die Verschlüsselung zudem an- und abgestellt werden. Dank der übersichtlichen grafischen Oberfläche ist die Bedienung von KPhone einfach und für Jedermann gut verständlich. Die Benutzung von KPhone ist auf allen gängigen Linux Distributionen möglich. Die verwendete Verschlüsselungs-Bibliothek, sowie der Voice over IP Client, sind beide im Internet kostenlos erhältlich. Nach dem Abschluss dieser Studienarbeit, wird diese SRTP Erweiterung höchstwahrscheinlich in das KPhone Projekt aufgenommen werden. Das Projekt wird als Diplomarbeit weitergeführt. Dort soll ein Key Management Protokoll einge- führt werden. Mittels eines Schlüssel Austausch Verfahrens (nach Diffie-Hellman) oder sogar mit elektronischen Zertifikaten, sollen so die schwachen Pre-Shared Schlüssel und die dadurch not- wendige Absprache abgelöst werden. Dies wird die Sicherheit massgeblich verbessern. Team: Christian Höhn, Silvan Geser Letzte Änderung: Betreuer: Prof. Dr. Andreas Steffen 16.01.2007 Studienarbeit Voice over IP Security Inhaltsverzeichnis 3 / 68 Voice over IP security II Inhaltsverzeichnis I Abstract....................................................................................................................2 II Inhaltsverzeichnis...................................................................................................3 III Aufgabenstellung....................................................................................................6 IV Management Summary.........................................................................................7 1 Ausgangslage ..................................................................................................................................... 7 2 Vorgehen ............................................................................................................................................. 7 3 Ergebnisse........................................................................................................................................... 7 3.1 Evaluation ......................................................................................................................................... 7 3.2 Design................................................................................................................................................ 8 3.3 Sicherheit.......................................................................................................................................... 8 3.4 Integration in KPhone..................................................................................................................... 8 3.5 Features ............................................................................................................................................ 9 4 Ausblick............................................................................................................................................... 9 V SRTP...................................................................................................................... 10 1 Allgemein..........................................................................................................................................10 1.1 Einleitung ........................................................................................................................................10 2 Funktionsweise.................................................................................................................................10 2.1 Integration in RTP..........................................................................................................................10 2.2 SRTP Paketaufbau ........................................................................................................................10 2.3 Cryptographic Context ..................................................................................................................11 2.4 SRTP Keymanagement.................................................................................................................13 2.5 Verschlüsselungsalgorithmus......................................................................................................14 2.6 Integritätssicherung ......................................................................................................................14 2.7 Ablauf ..............................................................................................................................................14 3 Secure RTCP .....................................................................................................................................15 4 Pre-Defined Cryptographic Transforms .........................................................................................15 4.1 Encryption.......................................................................................................................................15 4.2 Message Authentication...............................................................................................................16 4.3 Vertraulichkeit des RTP Inhalts...................................................................................................17 4.4 Vertraulichkeit des RTP Headers ................................................................................................18 5 Key Management.............................................................................................................................18 5.1 Allgemein........................................................................................................................................18 5.2 SRTP Policy.....................................................................................................................................18 5.3 Re-Keying........................................................................................................................................19 5.4 SSRC Collision/Two Time Pad .....................................................................................................20 6 SRTP Erweiterungen ........................................................................................................................20 VI Anforderungsspezifikation ................................................................................. 21 1 Einleitung ..........................................................................................................................................21 1.1 Projekt Beschreibung....................................................................................................................21 2 Funktionale Anforderungen............................................................................................................21 3 Nichtfunktionale Anforderungen....................................................................................................21 3.1 Usability...........................................................................................................................................21 3.2 Reliability ........................................................................................................................................21 3.3 Performance...................................................................................................................................21 3.4 Security ...........................................................................................................................................21 3.5 Supportability .................................................................................................................................22 3.6 Implementation .............................................................................................................................22 4 Use-Cases..........................................................................................................................................23 4.1 Use-Cases Priorität 1 ....................................................................................................................23
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