Secret message to be decrypted

Next week, on Tuesday 5/11, we will discuss protocols for quantum cryptography. To prepare, it is useful to understand something about classical codes, and as a challenge to you I have prepared a secret message that you have to decrypt:

"Zvd zvpvyb lzvdc imcq dktat qkt pvmzysab ptqdttz qkt jhsccgjsh
szy omszqme yvesgz gc cgqmsqty. Evcq utth qksq trwtagetzqsh
cdgqjk ctqqgznc szy wvgzqta atsygznc sat vz qkgc cgyt. Pmq cvet
dvmhy qkgzl qkt pvmzysab ztsata, vqktac dvmhy qkgzl gq usaqkta,
szy eszb dvmhy watuta zvq qv qkgzl spvmq gq. Gz usjq, qkt esqqta
gc vu ftab hgqqht gewvaqszjt gz wasjqgjt. Qkgc gc ptjsmct vu qkt
geetzct yguutatzjt gz cjsht ptqdttz qkgznc uva dkgjk
omszqme-etjkszgjsh ytcjagwqgvz gc zmetagjshhb tcctzqgsh szy qkvct
vaygagzsaghb wtajtwqgpht pb kmesz ptgznc. Ztftaqkthtcc, qkt
evfspghgqb vu qkt pvmzysab gc vu vzhb swwavrgesqt fshgygqb;
ytevzcqasqgvzc vu gq ytwtzy vz ztnhtjqgzn zmeptac dkgjk sat
ceshh, pmq zvq xtav, dkgjk egnkq qtzy qv xtav uva gzugzgqthb
hsant cbcqtec, pmq sat vzhb ftab ceshh uva atsh ugzgqt cbcqtec.
S qktvab uvmzyty gz qkgc dsb vz sanmetzqc vu eszgutcqhb
swwavrgesqt jksasjqta, kvdtfta nvvy qkt swwavrgesqgvz, gc cmathb
vu wavfgcgvzsh zsqmat. Gq cttec htngqgesqt qv cwtjmhsqt vz kvd
qkt qktvab egnkq tfvhft.  Pmq vu jvmact zv vzt gc vphgnty qv ivgz
gz cmjk cwtjmhsqgvz.
                                                   Ivkz Pthh"