Secondary Cipher Challenge Part 5

Here is the fifth part of a six-part challenge. Can you get to the end and crack the final message?
Age
11 to 16
Challenge level
filled star filled star empty star
Exploring and noticing Working systematically Conjecturing and generalising Visualising and representing Reasoning, convincing and proving
Being curious Being resourceful Being resilient Being collaborative

Problem

This is the fifth part of the Secondary Cipher Challenge.

 

Part 5

katjtjbtiaefbasllepmepnyuefrtqetktjysdeyhfedtrrtbnlkkhjhlqekaspkaeifeqthnjjnujk

tknkthpbtiaefjuebsnjekaefetjphinpbknskthphfbsitksllekkefjkhaelijhvelldhperhfjhlq

tpmtkbtiaefbasllepmepnyuefjtwtjmhtpmkhueeqepyhfedtrrtbnlkxhnytmakasqekh

lhhgniskxiehrbtiaefbslledqtmepefetrxhnasqephkaesfdhrtkuerhfesqtmepefebtiaefa

sjsgexvhfdspdrhfhnfjveasqebahjepkaepsyehrsyskaeysktbsljasiekaegexvhfdkhsbbe

jjbtiaefbasllepmejtwtjjiebtsl

 

If you want to work on a computer to solve the problem, you can download this cipher as a text file which doesn't contain any line breaks.

You can use the Cipher Challenge Toolkit to help you decipher it.  



Once you've decrypted this message, you should have a keyword.

You can access Part 6 of the Cipher Challenge at http://nrich.maths.org/???

by replacing the ??? with the keyword.