Vlastimil Klíma (born 19 February 1957 in Benešov) is a cryptographer, computer security expert, white hat hacker and court expert in the field of computer technology.

Klíma received his doctorate from the Faculty of Mathematics and Physics at Charles University in 1984.

As an academic, he taught Applied Cryptography at the Faculty of Mathematics and Physics at Charles University in Prague.

Klíma is the former Head of the research and development group at the Federal Ministry of Defense of the Czech Republic and the Czech National Security Authority.

In 2002, Vlastimil Klíma and Tomáš Rosa identified a significant vulnerability in the OpenPGP format regarding the protection of private signature keys. Their research showed that a lack of integrity protection on the private key structure could be exploited by an attacker. This eventually led to improvements in how PGP-compatible programs handle private key storage.

In 2003, ethical hackers Klíma, Pokorný, and Rosa presented a sophisticated side-channel attack on SSL/TLS, often referred to as the "KPR attack". This had a major impact on strengthening the security of Internet communications.

In 2005 Klíma showed how to find collisions in the popular MD5 hash function on a laptop .

In 2006, Vlastimil Klíma introduced the "Tunnels" method, which reduced the time required to find collisions in the MD5 hash function to under one minute on a standard notebook computer. It accelerated the design of the new hashing standard SHA-3.