Point-to-Point Tunneling Protocol [PPTP] Team: Invincibles Deepak Tripathi Habibeh Deyhim Karthikeyan Gopal Satish Madiraju Tusshar RakeshNLN

Agenda Overview PPTP Connections PPTP Architecture PPTP Underlying Technology PPP Architechure PPTP Security

PPTP ? PPTP - enables secure data transfers between a remote client and an enterprise server by creating a VPN across an IP-based internetwork Success of PPTP The use of PSTNs (Public Switched Telephone Networks). Support to Non-IP protocols.

PPTP Connections For Remote Access: PPTP Client connects to the ISP using Dial Up Networking PPTP then creates a tunnel between the VPN client and VPN server. For LAN internetworking: It does not require the ISP connection phase so the tunnel could be directly created.

PPTP Architecture PPTP employs three processes to secure PPTP-based communication over unsecured media PPP-based connection establishment PPTP Connection control PPTP tunneling and data transfer

PPTP Connection Control

Common PPTP control messages NameDescription Start-Control-Connection-RequestRequest from the PPTP client to establish control connection. Start-Control-Connection-ReplyReply from the PPTP server to the client. Outgoing-Call-RequestRequest from the PPTP client to the server to establish a PPTP tunnel Outgoing-Call-ReplyResponse from the PPTP server to the client Echo-RequestKeep-alive mechanism from either server or client. Echo-ReplyResponse to the Echo-Request message. Stop-Control-Connection-RequestRequest Message from the PPTP client or server notifying the other end of the termination of control connection. Stop-Control-Connection-ReplyReply Response from the opposite end.

PPTP Data Tunneling and Processing Recipient end Sender end

Underlying Technology PPTP is based on PPP Operates at layer 2 of OSI Advantages: Can operate any DTE or DCE including EIA/TIA-232-C and ITUV.3 Does not restrict transmission rates Requirement: Availability of a duplex connection Synchronous Asynchronous

PPP architecture PPP standards-based protocol. PPP's frame format is based on the HDLC PPP can negotiate link options dynamically support multiple Layer 3 protocols, such as IP, IPX, and AppleTalk.

PPP architecture - LCP PPP defines the Link Control Protocol (LCP). The job of the LCP Establish, configure, and test the data-link connection. Callback Data compression Multilink PAP authentication CHAP authentication

LCP Authentication PAP vs. CHAP PAP(password authentication protocol) Remote host is in control of login requests. (Trial and error attack) Password is sent in clear text

LCP Authentication PAP vs. CHAP CHAP(challenge handshake authentication protocol) Access servers is in control of login attempts Password is not transmitted in clear text

CHAP Operation

PPP architecture – NCP Link partners exchange NCP packets to establish and configure different network-layer protocols including IP, IPX, and AppleTalk. Each Layer 3 protocol has its own NCP. The NCP can build up and tear down multiple Layer 3 protocol sessions over a single data link.

PPTP Security Data Encryption Data Authentication Packet Filtering Firewalls & Routers

Encryption Microsoft Point to Point Encryption RSA RC4 Algorithm with 40 or 128 Bit key XOR Attack Bit Flipping Attack

Authentication Methods Clear Text password LANMAN Hash NT Encryption Hash Challenge/Response MSCHAP

LAN Manager Hash Password Convert to uppercase Divide into two 7 character strings Encrypt a fixed constant with a string Merge both 8 byte strings 16 byte hashed string 14 Byte String

NT Encryption Hash Password Hash using MD4 16 Byte hash Unicode

MSCHAP Client requests login challenge Server sends 8 byte random challenge Client calculates LANMAN hash or NT hash Partitions the key into three keys Each key encrypts the challenge Three keys are merged and sent as response

P0P0 P1P1 P2P2 P3P3 P4P4 P5P5 P6P6 P7P7 P8P8 P9P9 P 10 P 11 P 12 P 13 H0H0 H1H1 H2H2 H3H3 H4H4 H5H5 H6H6 H7H7 H8H8 H9H9 H 10 H 11 H 12 H 13 H 15 H 14 K0K0 K1K1 K2K2 K3K3 K4K4 K5K5 K6K6 K7K7 K8K8 K9K9 K 10 K 11 K 12 K 13 R0R0 R1R1 R2R2 R3R3 R4R4 R5R5 R6R6 R7R7 K 15 K R8R8 R9R9 R 10 R 11 R 12 R 13 R 15 R 14 R 16 R 17 R 18 R 19 R 20 R 21 R 23 R 22 Secret Password: LM hash of the password: 3 DES keys derived: Challenge response: 3 DES encryptions of 8-byte challenge: DES MSCHAP…

Packet Filtering & Firewalls Packet filtering allows a server to route packets to only authenticated clients Firewalls filter the traffic on the basis of ACL ( Access Control List )

Cakewalk! AsLEAP

No Such Thing As Free Lunch! PPTP is weaker option, security wise, IPSec, L2TP are more secure PPTP is platform dependent Requires extensive configuration

References IPSec VPN DesignBy Vijay Bollapragada, ISBN-13: Vijay Bollapragada