ANALYSIS OF WIRED EQUIVALENT PRIVACY CS265, Spring 2003 Xunyan Yang

OVERVIEW Introduction Working Mechanism Attackable Entries Improvement In WEP Implementation Conclusion & Recommendations

INTRODUCTION What is WEP --- Wired Equivalent Privacy What is WEP used for --- Provides confidentiality for wireless LAN What’s problems with WEP --- Cryptographic errors

WORKING MECHANISM A string cipher using the RC4 encryption algorithm A message (plaintext M) A Integrity Check (checksum algorithm c ) A shared secret key (short key k ) A per packet Initialization Vector (IV) Ciphertext = (M·c(M)) XOR RC4(IV ·k)

WORKING MECHANISM (Contd.) Integrity Check 32-bit Cyclic Redundancy Check (CRC-32) checksum Confidentiality Initialization Vector (IV) A 24-bit field and appended to the cleartext part of a message

ATTACKABLE ENTRIES IV Collisions IV Reuse Modify Checksum

IV COLLISION & REUSE IV will be exhausted after about five hours 1500-byte packet at the speed of 11MbPS: 1500 * 8 / (11 * 10^6) / 60^2 * 2^24 A common wireless card resets the IV to 0 each time a card is initialized, and increments the IV by 1 with each packet.

MODIFY CHECKSUM CRC-32 is a linear algorithm Compute the bit difference of two CRCs based on the bit difference of the messages Flip arbitrary bits in an encrypted message and correctly adjust the checksum

IMPROVEMENT IN WEP IMPLEMENTATION Automatic WEP rotation Constant automatic key rotation Continual key replacement Unique key generation http://www.wavelink.com

CONCLUSTION & RECOMMENDATION WEP Provides Inadequate Security Assume that the link layer offers no security Don’t rely on WEP. Use higher-level security mechanisms Place all access points outside the firewall Assume that anyone within physical range can communicate on the network as a valid user Always turn on WEP in your access point

???...