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Doc.: IEEE 802.11-00/253 Submission May 2001 Bernard Aboba, MicrosoftSlide 1 WEP2 Security Analysis Bernard Aboba Microsoft.

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Presentation on theme: "Doc.: IEEE 802.11-00/253 Submission May 2001 Bernard Aboba, MicrosoftSlide 1 WEP2 Security Analysis Bernard Aboba Microsoft."— Presentation transcript:

1 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 1 WEP2 Security Analysis Bernard Aboba Microsoft

2 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 2 Goals To (briefly) summarize security weaknesses discovered in WEP v1.0 To analyze security vulnerabilities of WEP2 To recommend potential improvements

3 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 3 Classes of Attacks Against WEP v1.0 IV (key) reuse [Walker, Berkeley team, Arbaugh] –Made possible by small IV space in WEPv1.0, lack of IV replay protection –Enables statistical attack against ciphertexts w/replayed IVs Known plaintext attack [Walker, Berkeley team, Arbaugh] –Lots of known plaintext in IP traffic: ICMP, ARP, TCP ACK, etc. –Can send pings from Internet through AP to snooping attacker –Enables recovery of key stream of length N for a given IV –Can forge packets of size N by reusing IV in absence of a keyed MIC Partial known plaintext [Berkeley team, Arbaugh] –May only know a portion of the plaintext (e.g. IP header) –Possible to recover M octets of the keystream, M < N –Via repeated probing, can extend keystream from M to N [Arbaugh] –Possible to flip bits in realtime, adjust CRC32, divert traffic to attacker Enabled by linearity of CRC32, absence of keyed MIC

4 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 4 Classes of Attacks (cont’d) Authentication forging [Berkeley team] –WEP v1.0 encrypts challenge using IV chosen by client –Recovery of key stream for a given IV enables re-use of that IV for forging WEP v1.0 authentication –Does not provide key, so can’t join LAN Denial of service –Disassociate, reassociate messages not authenticated Dictionary attack –Possible where WEP keys derived from passwords Realtime decryption [Berkeley team, Arbaugh] –Repeated IV reuse, probing enables building of a dictionary of IVs, key streams –Enables decryption of traffic in realtime –Possible to store dictionary due to small IV space Need 1500 octets of key stream for each IV 2^24 * 1500 octets = 24 GB

5 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 5 WEP2 Increases size of IV space to 128 bits Key may be changed periodically via IEEE 802.1X re- authentication to avoid staleness No keyed MIC No authentication for reassociate, disassociate No IV replay protection Use of Kerberos for authentication within IEEE 802.1X

6 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 6 WEP2 Security Analysis IV (key) reuse –Larger IV, re-key support makes unintentional reuse much less likely –Without IV replay protection, intentional reuse still possible Known/Partial plaintext attacks –Not affected by larger IV –Probing, key stream extension still possible in absence of keyed MIC –Still possible to recover key streams via ping from Internet –Can still forge packets by reusing IV, key stream –Can still divert traffic in absence of non-linear, keyed MIC Authentication forging attack –Not affected by larger IV, since intentional IV replay still possible Dictionary attack –New vulnerabilities introduced by mandatory KerberosV authentication Realtime decryption –Much more difficult due to larger IV 2^128 * 1500 octets = 5.1E32 GB

7 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 7 KerberosV Dictionary Attack Vulnerabilities References –Bellovin & Meritt “Limitations of the Kerberos authentication system”, USENIX 1991 –Wu, T. “A Real-World Analysis of Kerberos Password Security”, Scenario –Attacker snoops AS_REQ/AS_REP exchange, recovers passwords offline –In popular networks (“hot spots”), may be possible to collect many such exchanges in a single attempt Vulnerabilities –PADATA or TGT encrypted with client Key derived from password via STRING-TO-KEY(P) Results [Wu, 1998] –Password checkers not successful in significantly increasing password entropy –Structure of TGT (service name = krbtgt) enables verification of key guess by decrypting only 14 octets; similar issues with PADATA –Use of DES to encrypt TGT enables use of parallel DES cracking techniques –Of 25,000 sample TGTs, 2045 could be decrypted in two weeks using a cluster of 3 UltraSPARC-2 (200 Mhz) and 5 UltraSPARC-1 (167 Mhz) machines –Today, 15 off-the-shelf PCs could accomplish the same thing in 1 day at a cost of < $15K

8 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 8 Solutions Machine versus user authentication –Machine keys typically have full entropy Use of alterative ciphers in Kerberos –Draft-raeburn-krb-gssapi-krb5-3des-01.txt –Draft-raeburn-krb-rijndael-krb-00.txt Revision to Kerberos [Wu] –SRP used for Kerberos pre-authentication –Derived key used to encrypt TGT

9 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 9 Reassociate, Disassociate & Beacon Security Currently, reassociate, disassociate messages are not secure –Enables denial of service attacks Proposal –Add an authenticator to reassociate and disassociate messages –Replay counter, HMAC-SHA1 (replay counter || SourceMAC || destMAC || transmit key) –On disassociate: ignore if HMAC is not valid –On reassociate: validate authenticator via move-request to old AP; if invalid, old AP ignores move-request Beacon security –Currently, beacon messages are not authenticated –Enables station to roam to a rogue AP –Proposal: validate beacon before reassociating Replay Counter, HMAC-SHA1 (replay counter || sourceMAC || multicast key) Any station can forge this, but better than nothing

10 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 10 Summary – Vulnerabilities Thwarted

11 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 11 Conclusions WEP2 not significantly more secure than WEPv1.0 –Small IV only part of the problem; absence of a keyed MIC remains a major deficiency –Denial of service attacks not addressed –WEP2 should not be treated as a significant security enhancement (should state this explicitly in security considerations section) Kerberos V vulnerable to dictionary attack –Most important in “Hot spot” scenarios where many exchanges could be recovered Expect at least 10 percent of passwords to be crackable in 24 hours Downside greater than WEP v1.0 vulnerabilities: not only can traffic be decrypted, but attacker can assume user identity and access other services! –Protocol modifications required to address the vulnerability Support for 3DES, AES ciphers Support for SRP pre-authentication Backward compatibility issues –AP with built-in KDC AP would need software upgrade to support new ciphers, pre-auth types –AP in “pass-through” mode (IAKERB or RADIUS) AP does not need to understand AS_REQ/AS_REP, so no issue

12 doc.: IEEE /253 Submission May 2001 Bernard Aboba, MicrosoftSlide 12 Recommendations Examine feasibility of adding keyed MIC to WEP2 Without keyed MIC, downplay security value of WEP2 –Make this clear up front Choose a mandatory-to-implement authentication method resistant to dictionary attack –Example: SRP: RFC 2945 –EAP-SRP: draft-ietf-pppext-eap-srp-01.txt


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