# Re: Determining the inverse function operation from a function definition

## nmm1@cus.cam.ac.uk (Nick Maclaren)

30 Apr 2005 10:56:55 -0400

*From comp.compilers*

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**From: ** | nmm1@cus.cam.ac.uk (Nick Maclaren) |

**Newsgroups: ** | comp.compilers |

**Date: ** | 30 Apr 2005 10:56:55 -0400 |

**Organization: ** | University of Cambridge, England |

**References: ** | 05-04-067 05-04-086 |

**Keywords: ** | theory |

**Posted-Date: ** | 30 Apr 2005 10:56:55 EDT |

SM Ryan <wyrmwif@tsoft.org> wrote:

*>*

*>Some function inversions are thought to be inherently difficult;*

*>cryptography is based on this assumption. For example DES is an*

*>invertible function that is intentionally hard to invert.*

Yes. One then gets onto the interesting difference of whether a

function has an efficient inverse and whether there is an efficient

method of find one of its better inverses. And, in both cases, the

state can be yes, no or unknown.

Regards,

Nick Maclaren.

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