How many bits?

Continuing my previous posting, here’s what I read from Rosing p.132:

For maximum security, we probably want 2.5 times as many bits as we are trying to hide. The factor 2 comes for the ability to crack elliptic curves using the square root of the order of the base point, and the factor of 0.5 comes in if you don’t know the order of a base point.

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