The Insecurity of Tunnelled Authentication Protocols N. ASOKAN, VALTTERI NIEMI, KAISA NYBERG

1. The Insecurity of Tunnelled Authentication Protocols N. ASOKAN, VALTTERI NIEMI, KAISA NYBERG Nokia Research Center

2. Remote Authentication Methods • Two network access scenarios • Subscription based – there is a home network • Alternative access based – there is no home network In both cases the local authentication agent (e.g., AAAL) contacts some back-end authentication server to verify authenticity of mobile client

3. Remote Authentication Methods • Two cryptographic scenarios • Public key based • Secret key based In both cases authenticity of mobile client is based on some secrets it has

4. Remote Authentication Methods • At least two session key scenarios • Session credentials for mobile client – goal is service level session security, or session connection security with a different party • Session connection security, e.g., communication security in link, transport and/or network layer • … In all cases session keys are derived as a result of successful authentication between mobile client and an authentication agent

5. Remote Authentication Methods - EAP • Extensible Authentication Protocol (EAPblack boxeneral protocol framework that supports • multiple authentication mechanisms • allows a back-end server to implement the actual mechanism • authenticator simply passes authentication signaling through • EAP was initially designed for use with PPP network access • But has been adapted by for many types of access authentication • WLAN (IEEE 802.1X), Bluetooth, … • And even other applications • charging, authorization • EAP consists of • several Request/Response pairs; Requests are sent by network • starts with EAP-Request/Identity sent by network • ends with EAP-Success or EAP-Failure sent by network • But drawbacks of EAP prompted attempts to secure it

6. Privacy requirements • Confidentiality of the identity of the mobile client on the air interface • Prevention of linking between pairs of authentication messages involving the same mobile client • Confidentiality against radio interface eavesdropping for data exchanged during the authentication protocol Existing EAP based authentication methods fail…

7. Different session key derivation methods • Many legacy protocols for mobile client authentication • Encapsulated in EAP types • EAP does not provide a standard way for deriving session keys that can be used for message authentication or encryption • Examples: • One-time passwords – totally insecure if not protected. Typically tunnelled through TLS. Session keys derived from TLS (proprietary to PEAP or TTLS). • EAP/SIM – proprietary protection methods - network authentication, session key derivation A consistent method of session key derivation is desirable

8. Protecting EAP- the PEAP approach • Designed to protect any EAP method for client authentication. • Provides client anonymity. • Backend server authenticated to client based on public key of server. • Designed to provide mutual authentication. • EAP protocol runs within a protected TLS tunnel. • Designed to provide unified method for session key derivation. • Session keys derived from TLS: e.g., protection of subsequent session is based on the same secrets as the TLS tunnel.

9. Protecting EAP – the PEAP approach

10. Protecting EAP – the PIC approach • Bootstraps IKE (JFK etc) from any EAP protocol – intended for remote access to VPN gateways • Designed to protect any EAP method for client authentication • especially password-based authentication • Provides client anonymity • Server authenticated to client based on public key of server. • Provides unified method for credential transport • Tunnel protocol: simplified unilateral version of ISAKMP (Layer 3) • Session credentials for IPSec SA created by Back-end server transported to client through the protected tunnel • A main design goal is not to require changes to legacy protocols

11. Protecting EAP – the PIC approach

12. PIC and PEAP - Open issues • If it can be done, at what cost and under what assumptions on the use of PK? • DoS attacks on access network? • DoS attacks on radio interface? • Additional roundtrip necessary? • How to obtain network’s public key and link it to network’s identity? • How can user verify network’s certificate? • What about revocation?

13. PEAP/AKA- How it works

14. PIC EAP/AKA- How it works

15. PEAP/AKA- How it can fail

16. PIC EAP/AKA- How it can fail

17. Analysis of the problem • Outer protocol (e.g., TLS) and inner protocol (e.g., EAP AKA) are both secure! It is the composition that is insecure. • Inner protocol is a legacy remote client authentication protocol (EAP/SIM, EAP/AKA) –typically used also without TLS tunnelling, also without ANY tunnelling • MitM can initiate untunnelled authentication with the client: e.g., set up a false cellular base station to ask for IMSI and then for RES. • Even if inner protocol is used exclusively in tunnelled mode, authentication of tunnel relies solely upon client. • E.g., user may accept an unknown certificate! This is not acceptable to network operators. • Session keys are derived from tunnel protocol only (e.g., TLS Master Key generated using tunnel protocol; same key as used to create tunnel). • Keys derived in inner protocol (e.g., AKA Master Keys) are not used.

18. Impacts of failure • Under passive (eavesdropping) attacks: • Tunnelling provides some protection of user identity – however link-layer addresses are revealed anyway! • Under active (man-in-the-middle) attacks: Tunnelled authentication protocols • fail to protect user identity (e.g., IMSI in EAP AKA or EAP SIM) • allow attacker to masquerade as the victim (e.g., and hijack her WLAN link) • risk link confidentiality • with EAP SIM as auth. protocol, are weaker than plain EAP SIM • with EAP AKA as auth. protocol, are much weaker than plain EAP/AKA

19. Conditions for failure • A tunnelled authentication protocol is insecureunless • if the outer protocol does perform mutual authentication • not true for PEAP in server-authenticated mode,or PIC. • if the keys used for a particular subscription are not usedin the legacy untunnelled mode (even if other subscriptionsmay be used in this mode) • not true for integrated terminals (e.g., GPRS/WLAN) • not true when the same general purpose smartcard (SIM/UICC) is used withseparate single-purpose terminals (e.g., WLAN, GPRS)

20. General model of tunnelled authentication Authentication Agent Authentication Server Front-end authenticator Client Tunneling protocol Server authenticated secure tunnel establishment secure tunnel Authentication protocol Client authentication • Terminology • tunnel endpoint is authentication ”agent” • authentication protocol endpoint is authentication ”server” • ”front-end” authenticator is a pass-through server • Agent and Server may be co-located

21. Proposed solution • Create cryptographic binding between tunnelling protocol and authentication protocol: METHOD 1: Use a one-way function to compute session keys from tunnel secrets (e.g.TLS master key) and auth. protocol secrets (e.g. IK,CK). METHOD 2: Compute a MAC over client-specific text (e.g, challenge, PIC credential request, …), using a MAC key derived as session key in Method 1. MAC is verified by agent or server. Now tunnel is secure for handling of session keys or credentials. • In both methods, auth. protocol secrets must be sent from server to agent (or tunnel secrets must be sent from agent to server) • Both methods rely on the authentication protocol producing a session key as well (under some assumptions, also possible to use a long-term key)

22. Why is cryptographic binding needed? • To secure weakinner authentication protocols that use a weak key • they MUST be used within a server authenticated tunnel, and • they MUST NOT be used outside such a tunnel • Assumption #2 drastically reduces use of legacy auth. protocols • it MUST NOT be imposed on protocols that use strong keys • Tunneling protocols (PEAP, POTLS, PIC etc.) address issue #1 • But they treat the inner protocol as a blackbox (any EAP type) • Therefore tunnelling protocols SHOULD allow optional cryptographicbinding of the outer and inner protocols • This allows tunnelling protocols to • generic: handle both weak and strong authentication protocols • secure: avoid MitM attack • non-invasive: NOT have to impose unnecessary restrictions on good protocols

23. Conclusions • Composing two secure protocols may result in an insecure protocol • Using tunnelling to “improve” a remote authentication protocol is very common • Known vulnerable combinations: • HTTP Digest authentication and TLS • PEAP and any EAP subtype • PIC and any EAP subtype • POTLS (v0.0) and any EAP subtype • … (obviously not limited to the examples in these slides) • The proposed solutions can be used to fix the problem • the exact fix needs to be tailored to the specific protocols.