Point-to-Point Protocol (PPP) Security

1
Point-to-Point Protocol (PPP) Security

• Connecting to remote access servers (RASs)
• PPP authentication
• PPP confidentiality
• Point-to-Point Tunneling Protocol (PPTP)

2
PPP

• Point-to-Point Protocol (PPP)
• Data link layer protocol
• Created for dialing into a networks remote
  access server (RAS)
• Then get access to internal resources
• Also used for dialing into an ISP

RAS
PPP Connection
3
PPP

• Authentication
• Optional in PPP
• If done, done during authentication phase of
  PPPs initial negotiation process

I am X
RAS
PPP Connection
4
PPP

• PPP offers several authentication options
• Password Authentication Protocol (PAP)
• Challenge-Response Handshake Protocol (CHAP)
• MS-CHAPMicrosoft version of CHAP
• Extensible Authentication Protocol (EAP)
• Not equally strong

5
PPP

• Password Authentication Protocol (PAP)
• Applicant sends verifier one or more PAP
  authentication request messages giving
  applicants user name and password
• Stops sending when verifier sends an
  authentication-ACK message or sends a termination
  message

RAS
PAP Auth RQ
PAP Auth RQ
PAP Auth ACK
6
PPP

• Password Authentication Protocol (PAP)
• Password is sent in the clear (without
  confidentiality), so PAP is dangerous

Contains Users Unencrypted Password
RAS
PAP Auth RQ
7
PPP

• Password Authentication Protocol (PAP)
• Authentication is done only once, at the
  beginning of the session
• If session is taken over by an impostor, no check
  of authentication

8
PPP

• (CHAP) Challenge-Response Handshake Protocol
• Verifier (RAS) sends CHAP request-authentication
  message
• Applicant must respond with a response message

RAS
CHAP ARQ message
CHAP Resp message
9
PPP

• CHAP
• This may be done several times per session for
  ongoing authentication to ensure that the session
  has not been hijacked (taken over by an imposter)

10
PPP

• CHAP
• The applicant and verifier have a shared secret
• Applicant adds shared secret to the request
  message, then hashes the combination to produce
  the response message

CHAP Authentication Request Message
CHAP Authentication Response Message
Shared Secret
Hash
11
PPP
Transmitted Authentication Response Message

• CHAP
• Verifier adds the shared secret to its request
  message, then hashes the combination
• If this matches the transmitted response message,
  applicant knows the shared secret and so is
  authenticated

Original Authentication Request Message
Computed Authentication Response Message
Shared Secret
Hash
12
PPP
RAS

• MS-CHAP
• Microsoft version of CHAP
• The shared secret is the users password for the
  remote access server (RAS)

MS-CHAP Authentication Request Message
MS-CHAP Authentication Response Message
RAS Password
Hash
13
PPP

• MS-CHAP
• Realistic in terms of how RASs usually work
• Only as strong as the password, which often is
  very weak
• Must enforce strong passwords

MS-CHAP Authentication Request Message
MS-CHAP Authentication Response Message
RAS Password
Hash
14
PPP

• Extensible Authentication Protocol (EAP)
• During authentication phase of initial PPP
  negotiations, merely assert that EAP will be used
• After the negotiation phase, which is very
  limited, EAP does further negotiation on how
  authentication will be done

RAS
Agree to Use EAP Negotiate more later
15
PPP

• PPP Confidentiality
• Optional (not mandatory)
• Negotiated using the PPP encryption control
  protocol during the initial negotiation phase

RAS
Confidential Message
16
PPP

• PPP Confidentiality
• Current options are DES-CBC and 3DES-CBC
• Cipher block chaining (CBC) is discussed under
  IPsec in this chapter

RAS
Confidential Message
17
PPP

• PPP Confidentiality Encapsulation
• Encrypt the PPP frame with DES-CBC or 3DES-CBC
• Put encrypted frame in the data field of a new
  PPP frame
• Send frame to RAS

New PPP Header
New PPP Trailer
Encrypted PPP Frame In Data Field