Second Preimages on n-Bit Hash Functions for Much Less than 2n Work

  • John Kelsey
  • Bruce Schneier
Part of the Lecture Notes in Computer Science book series (LNCS, volume 3494)


We expand a previous result of Dean [Dea99] to provide a second preimage attack on all n-bit iterated hash functions with Damgård-Merkle strengthening and n-bit intermediate states, allowing a second preimage to be found for a 2k-message-block message with about k × 2n/2 + 1 + 2n − k + 1 work. Using RIPEMD-160 as an example, our attack can find a second preimage for a 260 byte message in about 2106 work, rather than the previously expected 2160 work. We also provide slightly cheaper ways to find multicollisions than the method of Joux [Jou04]. Both of these results are based on expandable messages–patterns for producing messages of varying length, which all collide on the intermediate hash result immediately after processing the message. We provide an algorithm for finding expandable messages for any n-bit hash function built using the Damgård-Merkle construction, which requires only a small multiple of the work done to find a single collision in the hash function.

Copyright information

© Springer-Verlag Berlin Heidelberg 2005

Authors and Affiliations

  • John Kelsey
    • 1
  • Bruce Schneier
    • 2
  1. 1.National Institute of Standards and Technology 
  2. 2.Counterpane Internet Security, Inc 

Personalised recommendations