UK National Cipher Challenge: No idea where to begin

Question

A teacher gave this cipher. It is originally from UK National Cipher Challenge Week 5 Part B. I have tried most substitution ciphers I know of. Brute forcing with frequency analysis also yielded no results.

Here is the code in full; please don't solve it I just need some idea of where to begin.

JKYRMGHJ WDMDI YNZ GLZ HE KJLX KD RCYHTR YR WZGKYZI ZK NZA JNZYV HYZPYR SD CYR HIDMGHJ WDMDI HAK ZKYD PGDLFYH SZ YZDJJDXXZI JDA CHCLWIYZI PWKYHIHG JRA HA GDRSPRWB YZGRE XZGS HIDMCR GZS VJR ZK YRWB D JDAK YZ CYR YZDJLSYZI NZJ ZK HE KJLX PTHKRGKJ GLZ JPRNWR JR HXZIKLZ HAK HIYHLWSYD ZK YRWB R CHHY D CYR JDJDGI HAK ZK HJYZBJHG HAK HYDXGHKHC ZK YZZJ KCRKJAIDHG YD TYDKHHX HE WWDN JPZMYH JGZGHBXH HAKPWWLSHGRI CRHGK KJLX HN CYR HBZGLH YGHAKGZY YD HIYHLWSYD JKD SZ AKCRHGE HAK YHMDT HGDBXH HAK ZK KRHGAK XGHK GHTYZW R KYHJHGB PRX SWHJKD HLTRHW HAK CYRA GHAKZ HAK YZYZDJDM PGRYZDJYRBOH JRDJJLG YD YDHG ZK XDA YZ PWHG YRI HN VYDAK D ZJ HTRKYRMCR JDA ZK KZY JD RDJJLG CHWCDGEYL YR KRAK CHIYDMYZI HE ZK CHHY KZY WWDN HA CYR GRN GHAKGLS CDZMR ZK CYR YZDTHG HAK YD GHNZB SZ HIYRWRE R YDRKYDRX ZK YHHV PWGRWLIDKGRB JD VIGRXJDEHJLRI GLZ ZK JGZGHBXH HHGAK HAK SZ HLTRHW HAK HTRTYH ZK HWEDJJZB HE PRX KD JKATDWKZZS HAK SZ HGRWT HAK XZGS PRNR PRWB HAK KIHGDC ZK CYR JGZKIR HAK HIYHLWSYD ZK PGK ZK YZDKHGIJDC AKDN CHHIZGB ZK JL NZWWR PRX JDAK KYHCLGB KHP CYR CHTRTYH PWWRGZX HXDK HXRJ HAK KR HGDBXH HAK SZ JHIRS ZNK HAK CWGZN HAK ZK JHKRGKJYZXHC JJHGB HAK CYR KYHXRDWGRB YD CHKILCYZI HKLBJDC GDHAK PKDYLKGZBBZ YR JKYHJHGB DWHRGJDC CYR HYZKJCRWT YHHNKHE KYHXHHGTRJDC IDWELB HAK KRAK VYDAK D YZDKIHWSHG YZ KLE JXDR GLZ KLZER YZDJLSYZI ZK CRHW CWLZI JDAK KRAK HLGK JD KD CYR JIDKIRK GZ PTHKRGKJ PIDWZB YZ HHGTR ZK HWERYL HGR JGHCRHW WRIDKDWZB GLZ KRAK HKRYLKGZSYL GRHBBR PRX KD CRZGER CYR HXZA KR JLZWWRI CYR VRHN TYDGRHBBR VJDG HA YHAK YZDKIRYD JWHJYLZI HA SD HIYHGHSGHKYD YRDJJLG YXHCYZI ZK KWLIDSSDC HE WWDN KD YHAK YZDKIR JKGZBBLJ HA SD VGZS JYZKGZX AKDN CHIRS JD HA GHKJDYDX HXDGB HAK GZS KWLIDSSDC JTYDAK HVRX CYR YZDKYHMGHKYD GZS KGZBBLJ ZK CRHW CWLZI AIDAN YZDKIR GZS HKDKHBBR IDWELB R CHGGDKJ JRA KJRH HAK SZ YZDKJHLF HAK CYR JGZGGZA YRDGRTWLE KHWABXRB JHYZKJCRWT HXZA KRKRHGAK GHCYL HE WWDN RDCYD ZK JHKLZG TYDCRGK GLZ YHAK HWBZYDKYRKJYZI HVRK CYR KIR PHAK SD CYR JYRXZKKZ HAK HTRTYH ZK JVHHJ KD HJLIOH HAK JR JDAK HVRK WWDN RDJJLG KRAK VJDG R JD HGHAK HBZGLH SZ JHDGHWWHIYRAI HAK YD HTRGKLZ CHJLRI JRA JVGLK HAK PE YZDJJHGBBLJ WRKLGE JKD CYR RDGRTWLE YD TYDJDGBL WDGBR HAK

Answer 1

A few observations to begin with:

• There are exactly two one-letter words repeated many times in the ciphertext: R and D. Since there are exactly two one-letter words in English (discounting extremely uncommon ones), this means that:
  1. This is most likely a straight substitution cipher (since the same two letters encode one-letter words every time)
  2. D and R likely decode to i and a (in some order)
• There are a lot of two-letter words ending in both R and D, and combined with several other letters. This seems strange, because you would expect the most-common ending letters to be t or s or maybe o.

With so many words ending in i and a, I thought something else must be going on. Then I came to the realization that

there are a great many two-letter words that start with either i or a.

Armed with that knowledge, I quickly

reversed the text

After that, it was trivial to solve using

quipqiup to solve as a simple substitution cipher.

The plaintext is (punctuation added):

THE APRIL UPRISING IN BULGARIA AND ITS BRUTAL SUPPRESSION BY THE TURKS HAS CAUSED OUTRAGE IN THE CHANCELLERIES OF EUROPE. THERE IS A RISK THAT RUSSIA WILL TAKE THIS AS THE EXCUSE IT SEEKS TO ENGAGE THE OTTOMANS, AND IF THEY ACT AND TAKE CONSTANTINOPLE THEN OUR TRADING ROUTES TO INDIA WILL BE UNDER THREAT. AT HOME, GLADSTONE'S PAMPHLET "BULGARIAN HORRORS AND THE QUESTION OF THE EAST" HAS STIRRED A PUBLIC APPETITE FOR ACTION WHICH COULD LEAD TO SUPPORT FOR INTERVENTION AND MAKE THINGS DIFFICULT FOR THE PRIME MINISTER. HE IS FACED WITH MORTON'S FORK: IF HE SUPPORTS ACTION THEN IT WILL BE DIFFICULT TO CONDEMN RUSSIAN INTERFERENCE; IF HE COUNSELS INACTION THEN HE RISK APPEARING WEAK AND CALLOUS AT HOME AND ABROAD. IT MAY APPEAR UNFORTUNATE THAT OUR POLITICAL LEADERS ARE UNABLE TO AGREE ON POLICY, STRATEGY, OR TACTICS AND IT IS TRUE THAT THIS COULD LEAD TO CONFUSION ABOUT OUR AIMS, BUT ON REFLECTION I THINK THAT THE PUBLIC DISAGREEMENT BETWEEN GLADSTONE AND DISRAELI PRESENTS AN OPPORTUNITY. THEIR DISPUTE CONDUCTED IN PARLIAMENT AND THE PRESS DEMONSTRATES TO THE WORLD THE TWO FACES OF THE EMPIRE AT THE SAME TIME MORALLY ENGAGED AND YET PRUDENT. THIS MAY ALLOW US TO PROCEED WITH DISCRETION TO TRY TO INFLUENCE THE ACTORS AND TO DIRECT THE PLAY AWAY FROM THE GLARE OF THE FOOTLIGHTS. IT MAY BE POSSIBLE TO ENGAGE THE LEAGUE OF THE THREE EMPERORS TO OUR CAUSE. BISMARCK IS PARTICULARLY KEEN TO MAINTAIN A BALANCE OF POWER IN THE REGION AND TO AVOID FURTHER WAR AND HE WILL NOT NEED TO BE CONVINCED THAT AN UNBRIDLED RUSSIA IS NOT TO HIS ADVANTAGE, SO I THINK WE CAN RELY ON HIM TO REIN IN RUSSIA'S EXPANSIONARY VISION. ON THE OTHER HAND THE LEAGUE ITSELF MAY PRESENT A LONGER TERM THREAT TO THE EMPIRE GIVEN THE BREADTH OF ITS INFLUENCE IN NORTHERN EUROPE, AND WE MUST TREAD CAREFULLY. THE EMPEROR'S ENVOYS WILL BE MEETING IN REICHSTADT SOON TO DETERMINE THE RESPONSE TO THE CRISIS AND I NEED A PLAN TO INFLUENCE THE OUTCOME. AS ALWAYS, OUR STRATEGY MUST BE TO SOW CONFUSION AND ON THIS I PLAN TO ASK FOR ADVICE FROM BARON PLAYFAIR. HE HAS RECENTLY CONCLUDED HIS COMMISSION OF ENQUIRY INTO THE CIVIL SERVICE, AND IF ANYONE KNOWS HOW TO CONTROL AN AGENDA, IT MUST BE OUR OWN CIVIL SERVANTS.

Answer 2

Looking at 1-letter words, only

R D

are used (9 words in total).

Looking at 2-letter words, only

GZ HA HE HN JD JL JR KD KR PE SD SZ YD YR YZ ZJ

are used (85 words in total).

This suggests a substitution code with a word key only known to encoder and decoder.

Answer 3

Rather than the solution to this particular cipher, you seem to be more interested in ciphers and code breaking in general.

It's a bit of a lengthy read, but we have a pretty decent guide to cipher puzzles on this site, written by user Beastly Gerbil. You are probably most interested in the "Cryptanalysis" section of that text, and the references to outside sources it gives.

(To find that guide the next time without visiting this answer, you can click "Tags" near the top of any page here, and then find the cipher tag. After clicking it, you should see the "Learn more..." link, which brings you to the tag wiki. Under the tag wiki's "A Guide" heading, there's a link to the question whose accepted answer the guide is.)