“Chinese” Attacks on Hashes
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“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 ([email protected]). “Chinese” Attacks on Hashes. Background

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“Chinese” Attacks on Hashes

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Chinese attacks on hashes

“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 ([email protected])


Chinese attacks on hashes

“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 ([email protected])


Chinese attacks on hashes

“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 ([email protected])


Chinese attacks on hashes

“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 ([email protected])


Chinese attacks on hashes

“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 ([email protected])


Chinese attacks on hashes

“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 ([email protected])


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