S 2.374 Scope of VoIP encryption

Initiation responsibility: Head of IT, IT Security Officer

Implementation responsibility: IT Security Officer, Head of IT

If an attacker is able to gain access to an internal network at a suitable location, he/she can log all network communication in the LAN. If the VoIP data is not encrypted, the attacker will be able to read all information. For example, he/she could determine who has called whom and for how long by evaluating the signalling information. An attacker could also analyse the messages exchanged using the media transport protocol and listen in on the telephone calls this way. For this reason, consideration should be given to encrypting the VoIP user data. However, encryption must be supported by all telecommunication systems involved in this case.

When considering whether to encrypt communication over VoIP, it often makes sense to distinguish between internal and external communication.

For VoIP telephone calls in the LAN, you may consider doing without encryption. In this case, it must be ensured that an attacker from outside is not able to access this information using an insecure section of the network such as a WLAN. It may make sense, though, to use encryption to protect internal telephone calls from being accessed by insiders. In this case, possible solutions include operating the VoIP terminal devices as VPN end points or using an encrypted media transport protocol such as SRTP.

If all VoIP devices used support encrypted signalling protocols, it is recommended to use encryption. Through the use of encryption, an attacker can be prevented from reading passwords and logging in to the SIP registrar as a different user, for example.

If packets containing VoIP information leave the secure LAN, they must be protected using an appropriate method. One or more of the following methods can be selected to protect the VoIP communication:

• the use of encrypting media transport protocols such as SRTP (Secure Real-time Transport Protocol)
• encryption of the signalling protocols, for example using TLS (Transport Layer Security)
• Virtual private networks (VPNs):
  
  Using VPN gateways, it is possible to transmit information in encrypted form between remote LANs. Individual devices can be operated as VPN end points. The additional advantage of this method is that an insider is not able to gain access to the information either. In this manner, protocol-independent encryption can be used without requiring direct support from encrypting signalisation and media transport protocols.
  
  If several VoIP switching units (middleware) are needed to communicate between various sites and properties, for example, they should also be collected in a VPN if no other encryption mechanisms can be activated. If the connection between several middleware components located at different sites, for example, is not adequately protected, an attacker could listen in on all telephone calls made between the sites under some circumstances. If the middleware is operated on an IT system, it is usually not a problem to install VoIP-protocol-independent VPN support later on.
• Encryption of the wireless network:
  
  An insecure wireless network in an organisation could also be used by someone located off-site to access the network. If the people taking part in the VoIP telephone call are connected to each other using a WLAN, qualified protection must be used for the WLAN, for example WPA2 (see also module S 4.6 WLAN). Since this encryption is limited to the wireless network, it should be noted that the information in the rest of the LAN would be transmitted without such protection. If the VoIP information does leave the LAN over any other route, the same conditions apply to qualified encryption as for internal communication (i.e. encryption may not be necessary under some circumstances).

If a call to a telephone subscriber is made using a public telephone network, the connection between the VoIP terminal device and the gateway used between the IP network and the public line-switching network can be protected, when necessary, using VPNs or encrypting signalisation and media transport protocols. Since only a few telephones designed for line switching networks provide such protection mechanisms and the ability to use these mechanisms depends on who is being called, the use of encryption between the VoIP gateway and the person being called is often unrealistic.

When encrypted communication, for example with external communication partners, is impossible, the users must be informed of this and made aware of the risks involved. Confidential conversations should not be conducted on the telephone when encryption is not used.

When purchasing VoIP components, it must be ensured that these components support encrypting signalisation and media transport protocols such as TLS and SRTP, for example (see S 2.375 Selection of suitable VoIP systems).

Review questions:

• Are VoIP data packets leaving the secure LAN protected by suitable security mechanisms?
• Are VoIP connections between middleware components in different properties protected according to the level of protection?
• Are the users informed about threats concerning VoIP communication and is their awareness in this regard raised?
• VoIP in the WLAN: Is qualified protection of the WLAN guaranteed?