What is the Hill cipher?

Lester Hill published this cipher in 1929. It turns pairs of letters into pairs of numbers and multiplies them by a 2 by 2 grid of numbers, the key. Decoding uses the inverse of that grid. Not every grid has an inverse, so some keys can never be used, and CacheCrack tells you if yours is one of them.

Enter the key as four numbers read across the rows, or as a four letter word where A is 0 and Z is 25.

Example

APADJTFTWLFJ with the key HILL decodes to SHORTEXAMPLE.