Secondary cipher challenge part 4

Here is the fourth part of a six-part challenge. Can you get to the end and crack the final message?

Age

11 to 16

Challenge level

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 fourth part of the Secondary Cipher Challenge.

Part 4

ctutithidrosaoiewblnssyulcceeseiheksnpherigefive

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 5 of the Cipher Challenge at http://nrich.maths.org/???

by replacing the ??? with the keyword.