### Abstract

The most common way of constructing a hash function (e.g., SHA-1) is to iterate a compression function on the input message, The compression function is usually designed from scratch or made out of a block-cipher. In this paper, we introduce a new security notion for hash-functions, stronger than collision-resistance. Under this notion, the arbitrary length hash function H must behave as a random oracle when the fixed-length building block is viewed as a random oracle or an ideal block-cipher. The key property is that if a particular construction meets this definition, then any cryptosystem proven secure assuming H is a random oracle remains secure if one plugs in this construction (still assuming that the underlying fixed-length primitive is ideal). In this paper, we show that the current design principle behind hash functions such as SHA-1 and MD5 - the (strengthened) Merkle-Damgård transformation - does not satisfy this security notion. We provide several constructions that provably satisfy this notion; those new constructions introduce minimal changes to the plain Merkle-Damgård construction and are easily implementable in practice.

Original language | English (US) |
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Title of host publication | Advances in Cryptology - CRYPTO 2005 - 25th Annual International Cryptology Conference, Proceedings |

Pages | 430-448 |

Number of pages | 19 |

State | Published - 2006 |

Event | 25th Annual International Cryptology Conference, CRYPTO 2005 - Santa Barbara, CA, United States Duration: Aug 14 2005 → Aug 18 2005 |

### Publication series

Name | Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) |
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Volume | 3621 LNCS |

ISSN (Print) | 0302-9743 |

ISSN (Electronic) | 1611-3349 |

### Other

Other | 25th Annual International Cryptology Conference, CRYPTO 2005 |
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Country | United States |

City | Santa Barbara, CA |

Period | 8/14/05 → 8/18/05 |

### ASJC Scopus subject areas

- Theoretical Computer Science
- Computer Science(all)

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## Cite this

*Advances in Cryptology - CRYPTO 2005 - 25th Annual International Cryptology Conference, Proceedings*(pp. 430-448). (Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics); Vol. 3621 LNCS).