Secondary Cipher Challenge Part 5

This is the fifth part of the Secondary Cipher Challenge.

Part 5

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.