Mirror, Mirror on the wall

Question

Alice walked into the library where her friend Bob said he would meet her, but he was nowhere to be found. She waited a while, and very shortly grew bored. Looking around the room she saw there was a book lying open on a table by the wall on which a mirror was hung.

She looked through the book, but it had no pictures or conversation in it, and what is the use of a book without pictures or conversation? And furthermore, all the writing in it was backwards!

She sighed. "That's just like Bob" she thought to herself. "It's always ciphers with that guy. Why can't he ever leave me a normal note? Well the backwards part's easy enough," she thought. "I'll just hold it up to this mirror here, and then I can read it."

So she did, and this is what she read.

DJROK SDGER DJFJA DKTER KFVBL 
QOGIB VRIVK DCVBB JFGZQ QGIOW 
SFJKZ VWTYL HMIVK MTVQE DBEQS 
FKVLC EHTIP YDGML RBYQZ JRWZC 
IZUER GILQL RMLCU MDGIL VRIBW 
ORZKJ VGTKH QZXIV CJTUC XKSTK 
ZNDYW XXFFK SAMZM IJYST UTDYW 
XGIMG RMFAK QXHVY SAMBL WWRAI 
EIIVK NXZFP NKVOH YZZXL HSUUW 
GWLTY OUFWJ CSVFY HIIQP TXZAY 
OUIQV XEMYR RJVJR WRNSJ RGTEC 
MDWIJ DJVOL XDTSO OUZUZ FCSQC 
SZYJL NXZJO HUUYW XDTSO OUFKV 
LFFZU DNJPD MYZBX QZUYF MDGEG 
LLYGM MJYMT RGTZP COBIW KTTYW 
XDTSO EGTWQ IZUHU ORIQD OGNMA 
CFUCA MZYVI UKZCW KQLVV CKZCW 
KQLVV CKZCP URVZO HRTYQ CAXMP 
DWISB IXHTP RRIZD FJOMN EZRYW 
XHZIV DUEVK IOWZC SNIZK IRUIR 
RGCAD PEMEV GBSRD KRMIG RGCAP 
UTVLB VEIVK DCZBK BZFJL QDWZC 
XZLVL IYZIB XBUQM QMNYJ RBYAF 
QZOMA OZYRS QIBMZ DTYTC QTUYE 
HEUUW DCWIR CWYVB MDHIH HUOVK 
IZHIV CKZCX IZOEL UVMGK MJZQY 
OUZUZ FJNEW SAMYM SEWEZ DJROX 
CTLLY OUILZ UMHAU JCBCZ PKIGJ 
BZUVK QTVAO JJFJA DK

What message did Bob leave for Alice to read?

Answer 1

Our ciphertext is

Jabberwocky, reversed left-to-right, reversed alphabetically (ABC... -> ZYX...), reversed line-for-line, and Vigenere-encrypted with key "pjwxnmexna".

Actually,

probably a better description of the "physical" reversals is: each line is reversed left-to-right. Which is a fairly appropriate thing for a mirror-image book.

I found this by repeated transformations of the text and it's possible that I have reconstructed what I did in the wrong order :-). And

maybe if you do the Vigenere-encryption at a different point in the process the key turns out to be JABBERWOCK or something nice like that.

I am greatly indebted to Sconibulus, who pointed out in the Sphinx's Lair chat that

the text is about the same length as Jabberwocky (though he reckoned it was one character out).

In fact, Sconibulus was correct:

the version of the text used here has a minor error, saying "has thou slain the Jabberwock" where it should be "hast thou slain the Jabberwock".