What are "birthday attacks"?

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Dan Goodwin  KD1XN Waltham,MA
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On Tue, 7 Jan 1997, John Payson wrote:

> a system immune to both.  The one difficulty is that federal government reg-
> ulations make it hard to use any secure code over 40 bits long, and codes
> of that length may be subject to "birthday" attacks.  I could nonetheless
>
>