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Chinese Attacks on Hash Functions: Understanding, Implications, and Security Measures

Learn about Chinese collision attacks, results for MD4 and MD5 attacks, and what steps to take to enhance digital security in a world of evolving cryptographic threats.

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Chinese Attacks on Hash Functions: Understanding, Implications, and Security Measures

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  1. “Chinese” Attacks on Hashes Topic • Background • “Chinese” collision attacks • Results for MD4 and MD5 attacks • What does it mean and what to do about it? • Conclusion March 11, 2006, Bing Wu (bwu@cs.washington.edu)

  2. “Chinese” Attacks on Hashes Background • Two sides of a coin: developing new hash functions and breaking them. • MD4 (1990) family hash functions and attacks on them. • Breakthroughs by “Chinese” attacks in 2004 and 2005: MD4, MD5, HAVAL, RIPEMD, SHA-0, SHA-1. • Best results: MD4: 2^8 MD4 operations. MD5: 2^39 MD5 operations for first blocks and 2^32 for second blocks. SHA-1: 2^63 SHA-1 operations. March 11, 2006, Bing Wu (bwu@cs.washington.edu)

  3. “Chinese” Attacks on Hashes “Chinese” collision attacks • Find a “low-Hamming-weight differential” Δ (a vector of almost all zeros) such that for messages M, the probability that h(MΔ) = h(M) is larger than it should be. • Basically, the attacks are involved with three steps: • Find a collision differential for which M and M’ probably produce a collision. • Derive a set of sufficient conditions which ensure the collision differential to hold. • Make some modification to M such that almost all the sufficient conditions hold. This is done by two types of message modification techniques, which are termed as “single-step modification” and “multi-step modification”. This greatly improves the probability that M and M’ may produce a collision. March 11, 2006, Bing Wu (bwu@cs.washington.edu)

  4. “Chinese” Attacks on Hashes Results for MD4 and MD5 attacks • Computational resource: My PC, Pentium4, 3.40G, WinXP. • C programs on Unix/Linux (Cygwin on Windows). • Results for “Chinese” attacks on MD4 and MD5. • MD4: about 5 seconds to produce a collision. • MD5: about 1 hour to produce a collision. March 11, 2006, Bing Wu (bwu@cs.washington.edu)

  5. “Chinese” Attacks on Hashes What does it mean and what to do about it? • Hash functions such as MD5 are no longer useful as digital signature hashes. • No panic. Attacks are collision resistance attacks, not pre-image attacks. Applications that use hashes, such as HMAC-MD5 protocols are still fine. • Don’t use MD4, MD5, HAVAL, RIPEMD, SHA-0, and avoid SHA-1 if possible. • Upgrade to stronger ones, such as SHA-2. • VSH is about the best generally published hash function, but needs more review. • Alternative approaches: 1) Protocols without requiring that the hash function be collision resistant, such as adding randomness to hash functions. 2) Message pre-processing to convert plaintext messages into a form that makes all existing collision attacks inapplicable. March 11, 2006, Bing Wu (bwu@cs.washington.edu)

  6. “Chinese” Attacks on Hashes Conclusion • “Chinese” attacks on hashes are remarkable in the cryptographic area. • Makes people upgrade their systems to employ better hash functions as well as develop new and more collision-resistant hash functions. • Greatly help us achieve a more secure digital world. March 11, 2006, Bing Wu (bwu@cs.washington.edu)

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