What is the Trifid cipher?

Félix Delastelle invented Trifid in 1902 as a three-dimensional cousin of his Bifid cipher. The 26 letters plus one extra symbol, usually a period, fill a 3 by 3 by 3 cube, so each letter gets three coordinates: layer, row and column. The coordinates of a block of letters are written out, mixed, and read back as new letters.

Enter the keyword used to fill the cube, or all 27 symbols in order, and the period, the size of the blocks. A period of 5 is common.

Example

SUEFECPHSEGYYJIXIMFOFOCEJLBSP with the alphabet EPSDUCVWYM.ZLKXNBTFGORIJHAQ and period 5 decodes to DEFENDTHEEASTWALLOFTHECASTLE.