Secondary cipher challenge part 3

Here is the third 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 third part of the Secondary Cipher Challenge.

 

Part 3

Oden en oda odery MRETD tebdar tdgqqamca. Xaqq ysma ez usv'ia lgmgcay os nsqia eo. Smta gcgem, xa vnay g nvhnoeovoesm tebdar os amtsya oda lanngca, hvo oden oela xa fvlhqay vb oda qaooarn sz oda gqbdghao rgodar odgm fvno ndezoemc eo sr raiarnemc eo. Oden lgpan eo dgryar os trgtp odgm oda Tgangr ndezo sr oda Gohgnd. Em zgto, ez usv xsrp svo dsx lgmu yezzaramo xgun eo tsvqy ha ysma, usv xeqq zemy eo'n g qso dgryar os trgtp! Aiam ns, g tslbvoar tsvqy rgbeyqu tdatp tslhemgoesmn gmy trgtp nvtd g nvhnoeovoesm tebdar wveoa wvetpqu, gmy odara gra qson sz tqvan qepa zrawvamtu gmgqunen odgo tgm ha vnay os hragp oda tsya wvetpqu.

Zsr oda zsvrod tdgqqamca, usv xeqq ha zgtay xeod nslaodemc g qeooqa yezzaramo. Eo en g orgmnbsneoesm tebdar, xdetd lagmn odgo oda qaooarn sz oda lanngca dgia noguay oda ngla hvo odaer sryar dgn haam tdgmcay. Os nsqia oda bvjjqa, usv xeqq maay os ragrrgmca oda qaooarn em g crey, em g nbergq zsrlgoesm. Smta usv zemy oda recdo crey neja os vna, gmy oda recdo bqgta os hacem oda nbergq, oda lanngca ndsvqy alarca. Sd, gmy oda pauxsry en lvnetgq.

 

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

by replacing the ??? with the keyword.