Notable achievements include:

• 1996 Discovered a flaw in the implementation of SSL in Netscape Navigator (with Ian Goldberg);
• 1997 Cryptanalyzed the CMEA algorithm used in many US cellphones (with Bruce Schneier);
• 1998 Development of Twofish block cipher as a submission for NIST's AES competition (with Bruce Schneier, John Kelsey, Doug Whiting, Chris Hall, and Niels Ferguson);
• 1999 Invention of the slide attack, a new form of cryptanalysis (with Alex Biryukov);
• 1999 Cryptanalysis of Microsoft's PPTP tunnelling protocol (with Bruce Schneier and "Mudge");
• 2000 Cryptanalysis of the A5/1 stream cipher used in GSM cellphones (with Alex Biryukov and Adi Shamir);
• 2001 Cryptanalysis of WEP, the stream cipher used in 802.11 "WiFi" networks (with Nikita Borisov and Ian Goldberg).