IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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Tunnelling at Layer 2: PPTP and L2TP
The Layer 2 tunnel protocols can tunnel both the most commonly used
protocols, but differ as to over which underlying protocols tunnelling is
possible: PPTP can only be transmitted over an IP-based network, whereas
L2TP can also be transmitted over various WAN protocols and hence offers
greater flexibility. The chart below shows how packets in an application are
assembled by PPTP over a PPP connection. As can be seen from the table
above, several independent tunnels (e.g. with different levels of quality
assurance) can be generated with the more recent L2TP protocol. During user
identification and encryption the security mechanisms of the underlining PPP
connection make themselves felt under both protocols.
Application data
IP TCP/UDP Application data
GRE PPP IP TCP/UDP Application data
IP
UDP PPP IP TCP/UDP Application data
PPP IP UDP PPP IP TCP/UDP Application data
= Optionally encrypted and compressed
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IT-Baseline Protection Manual: Oktober 2000
Application
TCP/IP stack
PPTP driver
TCP/IP stack
PPP driver
Figure: Assembly of packets of application data with the PPTP protocol
Security mechanisms of the PPP protocol
1. User authentication
Most implementations of the PPP protocol support the following standard
procedures for user authentication (see also S 5.50 Authentication via
PAP/CHAP):
- Password Authentication Protocol (PAP). The PPP-Server requests
transmission of a user name and password by the PPP client. Both
items are transmitted here in plaintext. This mechanism is insecure
and cannot protect against "Replay" attacks, under which an
unauthorised person can re-send the data intercepted at a later time.
Use of the PAP authentication protocol is therefore not advised.
- Challenge-Handshake Authentication Protocol (CHAP). The PPP
sends a "challenge", consisting of a session ID and a random
sequence of letters, the "challenge string", to the PPP client. The
client sends back the user name in plaintext as the reply, together
with the MD5 hash value made out of a combination of session ID,
challenge string and user password. Here the password is not sent in
plaintext. The use of the random sequence of letters ensures that the
protocol protects against Replay attacks.
2. Data encryption and key management
In the initialisation phase of the PPP protocol, the procedures to be used for
data encryption (and compression) are negotiated between client and
server. In general any procedure may be used here as long as client and