PPTP

From Wikipedia, the free encyclopedia

The

Point-to-Point Tunneling Protocol

(PPTP) is a method for implementing virtual private networks. PPTP uses a control channel over TCP and a GRE tunnel operating to encapsulate PPP packets.

The PPTP specification does not describe encryption or authentication features and relies on the PPP protocol being tunneled to implement security functionality. However the most common PPTP implementation, shipping with the Microsoft Windows product families, implements various levels of authentication and encryption natively as standard features of the Windows PPTP stack. The intended use of this protocol is to provide similar levels of security and remote access as typical ××× products.

PPTP specification

A specification for PPTP was published as RFC 2637[1] and was developed by a vendor consortium formed by Microsoft, Ascend Communications (today part of Alcatel-Lucent), 3Com, and others. PPTP has not been proposed nor ratified as a standard by the IETF.

A PPTP tunnel is instantiated by communication to the peer on TCP port 1723. This TCP connection is then used to initiate and manage a second GRE tunnel to the same peer.

The PPTP GRE packet format is non standard, including an additional acknowledgement field replacing the typical routing field in the GRE header.

The GRE tunnel is used to carry encapsulated PPP packets, allowing the tunnelling of any protocols that can be carried within PPP, including IP, NetBEUI and IPX.

In the Microsoft implementation, the tunneled PPP traffic can be authenticated with PAP, CHAP, Microsoft CHAP V1/V2 or EAP-TLS. The PPP payload is encrypted using Microsoft Point-to-Point Encryption (MPPE) when using MSCHAPv1/v2 or EAP-TLS. MPPE is described by RFC 3078.

[edit] PPTP implementations

PPTP was the first ××× protocol that was supported by Microsoft Dial-up Networking. All releases of Microsoft Windows since Windows 95 OSR2 are bundled with a PPTP client, although they are limited to only 2 concurrent outbound connections. The Routing And Remote Access Service for Microsoft Windows contains a PPTP server.

Microsoft Windows Mobile 2003 and higher also support the PPTP protocol.

Windows Vista and later support the use of PEAP with PPTP. The authentication mechanisms supported are PEAPv0/EAP-MSCHAPv2 (passwords) and PEAP-TLS (smartcards and certificates). Windows Vista removed support for using the MSCHAP-v1 protocol to authenticate remote access connections. [2]

Until recently, Linux distributions lacked full PPTP support because MPPE was believed to be patent encumbered. Full MPPE support was added to the Linux version 2.6.14 on October 28, 2005. SuSE Linux 10 was the first Linux distribution to provide a complete working PPTP client.

[edit] Security of the PPTP protocol

• MSCHAP-v1 is fundamentally insecure. Tools exist to trivially extract the NT Password hashes from a captured MSCHAP-v1 exchange. [3]
• MSCHAP-v2 is vulnerable to dictionary attack on the captured challenge response packets. Tools exist to perform this process rapidly. [1]
• When using MSCHAP-v1, MPPE uses the same RC4 session key for encryption in both directions of the communication flow. This can be cryptanalysed with standard methods by XORing the streams from each direction together. [4]
• MPPE uses RC4 stream cipher for encryption. There is no method for authentication of the ciphertext stream and therefore the ciphertext is vulnerable to a bit-flipping attack. An attacker could modify the stream in transit and adjust single bits to change the output stream without possibility of detection. These bit flips may be detected by the protocols themselves through checksums or other means. [5]