1. Jesse Walker, Intel Corporation Russ Housley, Vigil Security
March 2003
EAP Archie
Jesse Walker, Intel Corporation
Russ Housley, Vigil Security
Walker & Housley

2. Background Fact: IETF EAP WG has asked IEEE 802.11 TGi for guidance
March 2003
Background
Fact: IETF EAP WG has asked IEEE TGi for guidance
Observation: The mandatory-to-implement EAP authentication method (MD5 Challenge) is unsuitable for  This increases market confusion around security!
Question: Is there a suitable replacement algorithm?
Answer: Not really
Walker & Housley

3. Goals Design a candidate for a better EAP default:
March 2003
Goals
Design a candidate for a better EAP default:
Based on a long-lived pre-shared secret
Light weight; no public key operations
Mutual authentication
Prevent man-in-the-middle attacks
Protect against dictionary attacks if key is not weak
Guarantee a fresh session key
Walker & Housley

4. EAP Archie Key Hierarchy
March 2003
EAP Archie Key Hierarchy
PSK = Long-lived Pre-Shared Key = KCK | KEK | KDK
SK = Session Key = Archie-PRF(PSK, “Archie session key” | ASNonce | PeerNonce)  TLS Master Key
Pairwise Key = Archie-PRF(SK, “Archie pairwise key” | BSSID | STA MAC Addr)  PMK
Walker & Housley

5. Archie PRF Algorithm Archie-PRF
Input: Key K, Label L, Nonce N, Output Length OL
Output: OL-octet string Out
Out = “”
for i = 1 to (OL+15)/16 do
Out = Out | AES-CBC-MAC(K, L | N | i | OL)
return first OL octets out of Out
Walker & Housley

6. The Archie Exchange STA AS March 2003 PSK PSK Request: AS-ID, ARandom
Response: Hash1, STA-ID, SNonce, Binding, MIC1
Confirm: Hash2, Counter2, ANonce, Binding, MIC2
Finish: Hash3, MIC3
Walker & Housley

7. Archie Features Hash1 = SHA1(Request) Hash2 = SHA1(Response)
March 2003
Archie Features
Hash1 = SHA1(Request)
Hash2 = SHA1(Response)
Hash3 = SHA1(Confirm)
Defeats replay and reflection attacks
Each hash is a simple transaction id
Each hash ties msg to all prior msgs of this session
Explicit binding protects STA from man-in-the-middle attacks in the infrastructure
Derived SK is always fresh
Walker & Housley

8. March 2003
Status
Draft-jwalker-eap-archie-00.txt to be posted after San Fransisco IETF.
Attempt to stimulate discussion on what TGi needs from EAP WG.
Walker & Housley

9. March 2003
Feedback?
Walker & Housley