# In a lattice, how can one define a good basis and a bad basis?

When it comes to lattice based cryptographic systems, all the literature talks about, good bases and bad bases.

How does one define what a good basis is and what a bad basis is?

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A good basis are the set of "almost" orthogonal set of vectors which have small norms. They allow us to solve certain variants of closest vector problem $\mathsf{CVP}$ very efficiently (for example, use LLL). Usually, the private keys are "good" basis.