## Abstract

Standard signature schemes are usually designed only to achieve weak unforgeability - i.e. preventing forgery of signatures on new messages not previously signed. However, most signature schemes are randomised and allow many possible signatures for a single message. In this case, it may be possible to produce a new signature on a previously signed message. Some applications require that this type of forgery also be prevented - this requirement is called strong unforgeability.

At PKC2006, Boneh Shen and Waters presented an efficient transform based on any randomised trapdoor hash function which converts a weakly unforgeable signature into a strongly unforgeable signature and applied it to construct a strongly unforgeable signature based on the CDH problem. However, the transform of Boneh et al only applies to a class of so-called partitioned signatures. Although many schemes fall in this class, some do not, for example the DSA signature. Hence it is natural to ask whether one can obtain a truly generic efficient transform based on any randomised trapdoor hash function which converts any weakly unforgeable signature into a strongly unforgeable one. We answer this question in the positive by presenting a simple modification of the Boneh-Shen-Waters transform. Our modified transform uses two randomised trapdoor hash functions.

Original language | English |
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Title of host publication | Topics in Cryptology |

Subtitle of host publication | CT-RSA 2007 - The Cryptographers Track at the RSA Conference 2007, Proceedings |

Editors | Masayuki Abe |

Place of Publication | Berlin; Heidelberg |

Publisher | Springer, Springer Nature |

Pages | 357-371 |

Number of pages | 15 |

ISBN (Print) | 9783540693277 |

Publication status | Published - 2007 |

Event | Cryptographers Track at the RSA Conference, CT-RSA 2007 - San Francisco, United States Duration: 5 Feb 2007 → 9 Feb 2007 |

### Publication series

Name | Lecture Notes in Computer Science |
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Publisher | Springer-Verlag Berlin |

Volume | 4377 |

ISSN (Print) | 0302-9743 |

### Other

Other | Cryptographers Track at the RSA Conference, CT-RSA 2007 |
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Country | United States |

City | San Francisco |

Period | 5/02/07 → 9/02/07 |

## Keywords

- digital signature
- strong unforgeability
- trapdoor hash function
- provable security
- transform