Can't Solve Monoalphabetic Cryptosystem

Question

I've stumbled across the following Monoalphabetic Cryptosystem question in Thomas Judson's book on Abstract Algebra (http://abstract.ups.edu/aata/exercises-crypt.html).

3. Assuming that monoalphabetic code was used to encode the following secret message, what was the original message?

APHUO EGEHP PEXOV FKEUH CKVUE CHKVE APHUO
EGEHU EXOVL EXDKT VGEFT EHFKE UHCKF TZEXO
VEZDT TVKUE XOVKV ENOHK ZFTEH TEHKQ LEROF
PVEHP PEXOV ERYKP GERYT GVKEG XDRTE RGAGA

What is the significance of this message in the history of cryptography?

Could somebody please attempt this question and confirm whether it is possible with the given encrypted message. The examples in the text promote the use of frequency analysis which suggests E=X (spaces and punctuation), the hint given in the solutions state V=E, E=X and K=R which don't have the same key (http://abstract.ups.edu/aata/solutions-1.html). I've gone through all 26 possible keys and the encoded message hasn't made any sense. Any help would be great.

Answer 1

Using the convention that capital letters are ciphertext and small letters are plaintext.

Making the replacements E→x (space), K → r, and V → e, as specified in the hints, gives:

APHUO G HPP XOeFr UHCreU CHre APHUO G HU XOeL XDrTeG FT HFr UHCrFTZ XOe ZDTTerU XOere NOHrZFT HT HrQL ROFPe HPP XOe RYrPG RYTGer GXDRT RGAGA

Note that XO appears in the onset of multiple words, including two examples of XOe. The most common English word ending in e is the. So assume X → t and O → h.

APHUh G HPP theFr UHCreU CHre APHUh G HU theL tDrTeG FT HFr UHCrFTZ the ZDTTerU there NhHrZFT HT HrQL RhFPe HPP the RYrPG RYTGer GtDRT RGAGA

theFr could be their, so try F → i.

APHUh G HPP their UHCreU CHre APHUh G HU theL tDrTeG iT Hir UHCriTZ the ZDTTerU there NhHrZiT HT HrQL RhiPe HPP the RYrPG RYTGer GtDRT RGAGA

And Hir = air? Seems more likely than fir. So, H → a.

APaUh G aPP their UaCreU Care APaUh G aU theL tDrTeG iT air UaCriTZ the ZDTTerU there NharZiT aT arQL RhiPe aPP the RYrPG RYTGer GtDRT RGAGA

The letter U can occur before "h", or at the end of the word, including a two-letter word "aU". Maybe U → s?

APash G aPP their saCres Care APash G as theL tDrTeG iT air saCriTZ the ZDTTers there NharZiT aT arQL RhiPe aPP the RYrPG RYTGer GtDRT RGAGA

aPP is a multiply-occurring word with a double letter. Maybe all? So P → l.

Alash G all their saCres Care Alash G as theL tDrTeG iT air saCriTZ the ZDTTers there NharZiT aT arQL Rhile all the RYrlG RYTGer GtDRT RGAGA

T occurs in two-letter words iT and aT. So it could be n or s, but I've already used s. So T → n.

Alash G all their saCres Care Alash G as theL tDrneG in air saCrinZ the ZDnners there NharZin an arQL Rhile all the RYrlG RYnGer GtDRn RGAGA

tDrneG could be turned, so try D → u and G → d.

Alash d all their saCres Care Alash d as theL turned in air saCrinZ the Zunners there NharZin an arQL Rhile all the RYrld RYnder dtuRn RdAdA

Rhile, RYrld, and RYnder could be while, world, and wonder, so R → w and Y → o.

Alash d all their saCres Care Alash d as theL turned in air saCrinZ the Zunners there NharZin an arQL while all the world wonder dtuwn wdAdA

Z appears in the suffix inZ and the word Zunners, so try Z → g.

Alash d all their saCres Care Alash d as theL turned in air saCring the gunners there Nhargin an arQL while all the world wonder dtuwn wdAdA

From here, I guess A → f, C → b, L → y, Q → m, and N → c.

flash d all their sabres bare
flash d as they turned in air
sabring the gunners there
chargin an army while all the world wonder dtuwn wdfdf

Which is a quote from "The Charge of the Light Brigade" by Alfred Tennyson.

Flash'd all their sabres bare,
Flash'd as they turned in air
Sabring the gunners there,
Chargin' an army, while all the world wonder'd.

But I can't figure out what the tuwnwdfdf at the end of the decoded text means. Probably just a red herring.

The complete set of letter substitutions is:

• A → f
• C → b
• D → u
• E → x (space or apostrophe)
• F → i
• G → d
• H → a
• K → r
• L → y
• N → c
• O → h
• P → l
• Q → m
• R → w
• T → n
• V → e
• X → t
• Y → o
• Z → g