To decrypt a cipher text encrypted using keyed substitution

Question

I'm looking for decryption of this cipher text which was encrypted using simple substitution using secret key. I have already spend a lot of time on this. However it should not be that tough. So I think my procedure may not be correct.

The question is something like this:

The secret key (a permutation of the English alphabet) was generated from a key letter and a keyword. The key word is an English word (names of cities and countries are allowed) having no repeated letters. The secret key is then derived by writing the key word beneath the key letter in the alphabet; followed by then writing the remaining letters of the alphabet in cyclic order after the key word.

For example,if the key letter is 'm' and the key word is LAND, then the secret key is:

a b c d e f g h i j k l m n o p q r s t u v w x y z
O P Q R S T U V W X Y Z L A N D B C E F G H I J K M

All punctuation and spaces were removed from the plaintext, which was then blocked into groups of 5 letters prior to encryption.

The cipher text is:

LUPFK LLVBB VSTBU PAPFK VBVST QEALE ZPBUV STEAL EZPME OHKSO LVSNP LVBBV STKSO
FKVBV STFKL BUPES YHBUV STBEO EWCLB BUPSL UPLKB KSOFK VBPOF VBUKY YUPAZ VTUBK
SOZKV SZKBB UPFPS NECSB PAPOB UPLBK BVESZ KLBPA YENXV STCRB UPBVN XPBEQ QVNPR
APRKA KBEAH BETEV STUEZ PQEAL CRRPA KSOKL XPOUV ZVQBU PQVDP BUVAB HBAKV SFECY
OLEES MPKYE STBUP QVDPB UVABH BAKVS UKLMP PSVSK SOTES PUKYQ KSUEC AKTEK SLFPA
POBUK BMAVL XEQQV NVKYM CBBUP APFKL KRKLL PSTPA OAERR POEQQ QEAHE CKYVB BYPTV
AYLUP LLVBB VSTEC BBUPA PESBU PLUVS TYPLV KLXPO UPABE TEVSB EBUPY KOVPL FKVBV
STAEE ZMCBL UPVSQ EAZPO ZPTAK DPYHB UKBLU PRAPQ PAAPO BELBK HECBL VOPBU PAPFK
LZEAP LNERP QEAVZ KTVSK BVESL UPLKV OLUPL KNKLP VLUEC YOLKH VZSEB PGRPN BVSTK
TVAYL KVOZK BBUPF MYKSX YHVBL KMEHV DPNEZ PQEAU PLUEC YOMPU PAPZA LKYPG KSOPA
LRPSN PAFKL BEMAV STUVZ EDPAQ AEZSE DKLNE BVKQE AZPBU PLBKB VESZK LBPAF UVLBY
POPBN

We know the following are the letters of the English alphabet, grouped by letters whose frequencies are approximately equal.

The letters in each group are listed in order of decreasing frequency:

Group 1:   e
Group 2:   t a o i n s h r
Group 3:   d l
Group 4:   c u m w f g y p b
Group 5:   v k j x q z

The most commonly occurring digrams in the English language, in decreasing order of frequency, are:

th  he  in  er  re  on  an  en  at  es  ed  te  or  ti st

So I started with frequency analysis and some guesses and could find:

T was encrypted as L
H was encrypted as U
E was encrypted as P

A was encrypted as V
N was encrypted as S
D was encrypted as T

R was encrypted as A
S was encrypted as B

Am I doing things correctly?

I seem to be stuck after doing this. Are there any techniques that I get try to get this solved?

Answer 1

When trying to decrypt any substitution cipher, my first step would be to count the number of times each letter occurs in the ciphertext, and plot the ranked letter frequencies. Doing that for your ciphertext yields the following plot:

P: 84 ##########################################
B: 73 #####################################
V: 60 ##############################
K: 59 ##############################
E: 56 ############################
L: 52 ##########################
S: 47 ########################
A: 45 #######################
U: 44 ######################
O: 26 #############
T: 24 ############
Y: 21 ###########
Z: 21 ###########
Q: 19 ##########
F: 15 ########
C: 13 #######
N: 13 #######
R: 12 ######
H: 11 ######
M: 10 #####
D:  6 ###
X:  6 ###
G:  2 #
W:  1 #
I:  0
J:  0

(If you suspect you might be dealing with a simple Caesar shift cipher, looking at the same plot sorted alphabetically can also be useful, since you may be able to spot some distinctive patterns like the A/E/I triple peak that can reveal the shift amount.)

Comparing this plot with typical English letter frequencies, the general shape looks very similar, suggesting that this is indeed an monoalphabetic substitution cipher. The next step, then, is simply to try a few substitutions and see if they make sense.

For example, we might guess that the two most common letters in the ciphertext, P and B, stand for the plaintext letters E and T respectively. This guess yields the following partially decrypted message (shown with the ciphertext in lowercase, and the corresponding candidate plaintext below it in uppercase):

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
__E__ ___TT ___T_ E_E__ _T___ _____ _ET__ _____ __E__ _____ ____E __TT_ _____
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
___T_ _____ T_E__ __T__ __T__ ____T T_E__ _E__T _____ _TE__ _T___ __E__ ___T_
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
_____ ___TT _E_E_ ____T E_E_T _E_T_ T____ __TE_ _____ ____T _ET__ _ET__ ___E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E___ _T___ T____ _____ E____ ___E_ _____ _E___ ___T_ E___E T___T _T___ _____
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
_____ _E___ __T_E ___ET ___T_ T____ ____E E____ _____ E____ _____ _____ ___E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_T__ T____ _____ _____ _TT_E _E___ _____ E__E_ _____ E____ _____ ____T T_E__
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
____E ___TT _____ TT_E_ E__T_ E____ __E__ ___E_ _E_T_ ____T _T_E_ ___E_ ___T_
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
_____ ___T_ _E___ ___E_ _E___ _E__T __T__ E__E_ E__E_ T__T_ ___T_ __ET_ E_E__
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
____E ____E _____ _____ T____ _E___ ___E_ ____E _____ _____ ____T E__E_ T____
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_____ _____ TT_E_ _____ ___T_ _____ _E___ E____ E____ ___E_ E_E__ ___E_ ___E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
__E__ E____ T____ _____ __E__ _____ _____ T____ __ET_ E_T_T _____ _TE__ ___T_
popbn
E_ET_

The first thing you should check is that there are no unlikely letter sequences like TTT that might indicate a wrong guess. In this case, there aren't any, so we can assume for now that our initial guess that P=E and B=T may be correct. (If we did find some weird-looking letter sequences, we might want to try swapping the guesses around, and seeing if P=T and B=E would yield more sensible-looking results. We could also try assigning either P or B to the plaintext letter A, which can also be pretty common, etc.)

Once we're reasonably confident about our initial guesses, we can see if we may be able to infer any other ciphertext/plaintext letter pairs from them, e.g. by spotting likely words in the partial plaintext. Conveniently, the most common word in the English language is the definite article "the", which just happens to contain both T and E. Looking for patterns of the form T_E in the partial plaintext above, we can see that several of them have U as the middle ciphertext letter. We may thus tentatively guess that U might stand for H in the plaintext, and try another partial decryption:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
_HE__ ___TT ___TH E_E__ _T___ _____ _ETH_ _____ __E__ _____ ____E __TT_ _____
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
___T_ _____ THE__ __TH_ __T__ ____T THE__ HE__T _____ _TE__ _TH__ _HE__ __HT_
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
_____ ___TT HE_E_ ____T E_E_T HE_T_ T____ __TE_ _____ ____T HET__ _ET__ ___E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E___ _T___ T____ __H__ E____ ___E_ _____ _E_H_ ___TH E___E TH__T _T___ _____
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
_____ _E___ __THE ___ET H__T_ T____ H___E E____ _____ EH___ __H__ _____ ___E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_TH_ T____ _____ _____ _TTHE _E___ _____ E__E_ _____ E____ _____ ____T T_E__
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
___HE ___TT _____ TTHE_ E__TH E_H__ __E__ ___E_ HE_T_ ____T _THE_ ___E_ ___T_
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
_____ ___T_ HE___ ___E_ _E___ _E__T H_T_H E__E_ E__E_ T__T_ ___T_ __ETH E_E__
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
____E ____E _____ _____ T____ HE___ __HE_ ____E __H__ _____ ____T E__E_ T____
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_____ _____ TTHE_ _____ ___T_ _____ _E___ E___H E_H__ ___EH E_E__ ___E_ ___E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
__E__ E____ T____ __H__ __E__ _____ _____ T____ __ETH E_T_T _____ _TE__ H__T_
popbn
E_ET_

Again, we should check that the partial decryption looks plausible. With the plaintext letters T, H and E tentatively assigned, one possible next step would be to look for plaintext patterns of the form TH_T, which might correspond to the (also fairly common) word "that". As it happens, we find two such instances, both with the ciphertext letter K in the middle. Looking at the letter frequency plot above, K is the fourth most common letter in the ciphertext, just slightly behind V in the third place, so it's entirely plausible that it might be the encryption of A, which is generally counted as the third most common letter in English on average. With that guess, the partial plaintext becomes:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
_HE_A ___TT ___TH E_E_A _T___ _____ _ETH_ _____ __E__ __A__ ____E __TT_ __A__
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
_A_T_ ___A_ THE__ __TH_ __T__ ____T THE__ HE_AT A___A _TE__ _THA_ _HE__ __HTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
___A_ __ATT HE_E_ ____T E_E_T HE_TA T____ A_TE_ _____ ____T HET__ _ET__ ___E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E_A_ AT___ T____ __H__ E____ ___E_ A__A_ _E_H_ ___TH E___E TH__T _T_A_ _____
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
_____ _EA__ __THE ___ET H__T_ T_A__ HA__E E___A _____ EHA__ A_H__ _A__A ___E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_THA T____ _____ __A__ _TTHE _E_A_ A_A__ E__E_ _____ E____ _____ _A__T T_E__
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
___HE ___TT _____ TTHE_ E__TH E_H__ __E__ A__E_ HE_T_ ____T _THE_ A__E_ _A_T_
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
_____ ___T_ HE___ ___E_ _E__A _E__T HAT_H E__E_ E__E_ T__TA ___T_ __ETH E_E_A
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
____E ____E _____ A___A T____ HE_A_ __HE_ A_A_E __H__ ___A_ ____T E__E_ T___A
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_____ A___A TTHE_ __A__ ___T_ A____ _E___ E___H E_H__ ___EH E_E__ _A_E_ A__E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
__E__ E__A_ T____ __H__ __E__ _____ _A___ T_A__ __ETH E_TAT ____A _TE__ H__T_
popbn
E_ET_

Next, we could try to look for more partial words, or we could simply guess a few more common letters. The most common ciphertext letters that we haven't assigned to any plaintext letter yet are V and E; both seem to be occurring in places where one might expect a vowel, so we might tentatively guess that they stand for I and O in either order. Of the two choices, V=I and E=O yields a somewhat more likely-looking partial plaintext:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
_HE_A __ITT I__TH E_E_A ITI__ _O__O _ETHI __O__ O_E_O __A__ _I__E _ITTI __A__
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
_AITI ___A_ THEO_ __THI __TO_ O___T THE__ HE_AT A___A ITE__ ITHA_ _HE__ I_HTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
___AI __ATT HE_E_ _O__T E_E_T HE_TA TIO__ A_TE_ _O__I ____T HETI_ _ETO_ _I_E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E_A_ ATO__ TO_OI __HO_ E_O__ ___E_ A__A_ _E_HI _I_TH E_I_E THI_T _T_AI __O__
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
__OO_ _EA_O __THE _I_ET HI_T_ T_AI_ HA__E E_I_A ___O_ EHA__ A_HO_ _A_OA ___E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_THA T__I_ _O__I _IA__ _TTHE _E_A_ A_A__ E__E_ __O__ E_O__ _O__O _A_IT T_E_I
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
___HE __ITT I__O_ TTHE_ EO_TH E_HI_ __E_I A__E_ HE_TO _OI_T OTHE_ A_IE_ _AITI
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
___OO ___T_ HEI__ O__E_ _E__A _E__T HAT_H E__E_ E__E_ TO_TA _O_T_ I_ETH E_E_A
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
__O_E __O_E _O_I_ A_I_A TIO__ HE_AI __HE_ A_A_E I_HO_ ___A_ I__OT E__E_ TI__A
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_I___ AI__A TTHE_ __A__ __IT_ A_O_I _E_O_ E_O_H E_HO_ ___EH E_E__ _A_E_ A__E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
__E__ E__A_ TO__I __HI_ O_E__ _O__O _A__O TIA_O __ETH E_TAT IO__A _TE__ HI_T_
popbn
E_ET_

Now we can spot the repeating pattern ATIO_, where the following ciphertext letter is always S. The suffix "-ation" is quite common in English words of Latin origin, so we may guess that S=N, giving:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
_HE_A __ITT IN_TH E_E_A ITIN_ _O__O _ETHI N_O__ O_E_O __AN_ _IN_E _ITTI N_AN_
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
_AITI N__A_ THEON __THI N_TO_ O___T THEN_ HE_AT AN__A ITE__ ITHA_ _HE__ I_HTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
N__AI N_ATT HE_EN _O_NT E_E_T HE_TA TION_ A_TE_ _O__I N___T HETI_ _ETO_ _I_E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E_A_ ATO__ TO_OI N_HO_ E_O__ ___E_ AN_A_ _E_HI _I_TH E_I_E THI_T _T_AI N_O__
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
__OON _EA_O N_THE _I_ET HI_T_ T_AIN HA__E ENINA N__ON EHA__ ANHO_ _A_OA N__E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_THA T__I_ _O__I _IA__ _TTHE _E_A_ A_A__ EN_E_ __O__ E_O__ _O__O _A_IT T_E_I
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
___HE __ITT IN_O_ TTHE_ EONTH E_HIN __E_I A__E_ HE_TO _OINT OTHE_ A_IE_ _AITI
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
N__OO ___T_ HEIN_ O__E_ _E__A _E__T HAT_H E__E_ E__E_ TO_TA _O_T_ I_ETH E_E_A
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
__O_E __O_E _O_I_ A_INA TION_ HE_AI __HE_ A_A_E I_HO_ ___A_ I_NOT E__E_ TIN_A
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_I___ AI__A TTHE_ __AN_ __IT_ A_O_I _E_O_ E_O_H E_HO_ ___EH E_E__ _A_E_ AN_E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
__EN_ E__A_ TO__I N_HI_ O_E__ _O_NO _A__O TIA_O __ETH E_TAT ION_A _TE__ HI_T_
popbn
E_ET_

Now we're starting to have enough decoded letters that, with a bit of imagination, we can start making out more and more full words in the plaintext. For example, near the end of the plaintext we have the sequence THE_TATION, with the undecrypted letter L in the middle. Just about the only plausible English phrase that this could stand for is "the station", suggesting that L=S. Again checking that the letter frequencies look plausible, we can try this and get:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
SHE_A SSITT IN_TH E_E_A ITIN_ _O_SO _ETHI N_O_S O_E_O __AN_ SIN_E SITTI N_AN_
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
_AITI N__AS THEON __THI N_TO_ O__ST THENS HESAT AN__A ITE__ ITHA_ _HE__ I_HTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
N__AI N_ATT HE_EN _O_NT E_E_T HESTA TION_ ASTE_ _O__I N___T HETI_ _ETO_ _I_E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
_E_A_ ATO__ TO_OI N_HO_ E_O_S ___E_ AN_AS _E_HI _I_TH E_I_E THI_T _T_AI N_O__
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
_SOON _EA_O N_THE _I_ET HI_T_ T_AIN HAS_E ENINA N__ON EHA__ ANHO_ _A_OA NS_E_
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_THA T__IS _O__I _IA__ _TTHE _E_AS A_ASS EN_E_ __O__ E_O__ _O__O _A_IT T_E_I
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
__SHE SSITT IN_O_ TTHE_ EONTH ESHIN __ESI AS_E_ HE_TO _OINT OTHE_ A_IES _AITI
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
N__OO ___TS HEIN_ O__E_ _E__A _E__T HATSH E__E_ E__E_ TOSTA _O_TS I_ETH E_E_A
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
S_O_E S_O_E _O_I_ A_INA TIONS HESAI _SHES A_ASE ISHO_ __SA_ I_NOT E__E_ TIN_A
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
_I__S AI__A TTHE_ __AN_ __ITS A_O_I _E_O_ E_O_H ESHO_ ___EH E_E__ SA_E_ AN_E_
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
S_EN_ E__AS TO__I N_HI_ O_E__ _O_NO _AS_O TIA_O __ETH ESTAT ION_A STE__ HIST_
popbn
E_ET_

Now, we can take a guess that the beginning of the plaintext, SHE_A SSITT IN_TH E_E_A ITIN_ _O_SO _ETHI N_ might read "she was sitting there waiting for something", giving us F=W, T=G, A=R, Q=F and Z=M. Plugging those guesses in, we get:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
SHEWA SSITT INGTH EREWA ITING FORSO METHI NGORS OME_O __AN_ SIN_E SITTI NGAN_
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
WAITI NGWAS THEON __THI NGTO_ O__ST THENS HESAT AN_WA ITE_W ITHA_ _HERM IGHTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
N_MAI NMATT HEWEN _O_NT ERE_T HESTA TIONM ASTER _O__I NG__T HETI_ _ETOF FI_E_
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
RE_AR ATOR_ TOGOI NGHOM EFORS ___ER AN_AS _E_HI MIFTH EFI_E THIRT _TRAI NWO__
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
_SOON _EA_O NGTHE FI_ET HIRT_ TRAIN HAS_E ENINA N_GON EHA_F ANHO_ RAGOA NSWER
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
E_THA T_RIS _OFFI _IA__ _TTHE REWAS A_ASS ENGER _RO__ E_OFF FOR_O _A_IT T_EGI
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
R_SHE SSITT INGO_ TTHER EONTH ESHIN G_ESI AS_E_ HERTO GOINT OTHE_ A_IES WAITI
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
NGROO M__TS HEINF ORME_ MEGRA _E__T HATSH E_REF ERRE_ TOSTA _O_TS I_ETH EREWA
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
SMORE S_O_E FORIM AGINA TIONS HESAI _SHES A_ASE ISHO_ __SA_ IMNOT E__E_ TINGA
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
GIR_S AI_MA TTHEW __AN_ __ITS A_O_I _E_OM EFORH ESHO_ ___EH EREMR SA_E_ AN_ER
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
S_EN_ ERWAS TO_RI NGHIM O_ERF ROMNO _AS_O TIAFO RMETH ESTAT IONMA STERW HIST_
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E_ET_

At this point, it should be pretty straightforward to fill in the missing bits of the plaintext, giving us:

lupfk llvbb vstbu papfk vbvst qeale zpbuv steal ezpme ohkso lvsnp lvbbv stkso
SHEWA SSITT INGTH EREWA ITING FORSO METHI NGORS OMEBO DYAND SINCE SITTI NGAND
fkvbv stfkl bupes yhbuv stbeo ewclb bupsl uplkb ksofk vbpof vbuky yupaz vtubk
WAITI NGWAS THEON LYTHI NGTOD OJUST THENS HESAT ANDWA ITEDW ITHAL LHERM IGHTA
sozkv szkbb upfps necsb papob uplbk bvesz klbpa yenxv stcrb upbvn xpbeq qvnpr
NDMAI NMATT HEWEN COUNT EREDT HESTA TIONM ASTER LOCKI NGUPT HETIC KETOF FICEP
aprka kbeah betev stuez pqeal crrpa ksokl xpouv zvqbu pqvdp buvab hbakv sfecy
REPAR ATORY TOGOI NGHOM EFORS UPPER ANDAS KEDHI MIFTH EFIVE THIRT YTRAI NWOUL
olees mpkye stbup qvdpb uvabh bakvs uklmp psvsk sotes pukyq ksuec aktek slfpa
DSOON BEALO NGTHE FIVET HIRTY TRAIN HASBE ENINA NDGON EHALF ANHOU RAGOA NSWER
pobuk bmavl xeqqv nvkym cbbup apfkl krkll pstpa oaerr poeqq qeahe ckyvb byptv
EDTHA TBRIS KOFFI CIALB UTTHE REWAS APASS ENGER DROPP EDOFF FORYO UALIT TLEGI
aylup llvbb vstec bbupa pesbu pluvs typlv klxpo upabe tevsb ebupy kovpl fkvbv
RLSHE SSITT INGOU TTHER EONTH ESHIN GLESI ASKED HERTO GOINT OTHEL ADIES WAITI
staee zmcbl upvsq eazpo zptak dpyhb ukblu prapq paapo belbk hecbl vopbu papfk
NGROO MBUTS HEINF ORMED MEGRA VELYT HATSH EPREF ERRED TOSTA YOUTS IDETH EREWA
lzeap lnerp qeavz ktvsk bvesl uplkv olupl knklp vluec yolkh vzseb pgrpn bvstk
SMORE SCOPE FORIM AGINA TIONS HESAI DSHES ACASE ISHOU LDSAY IMNOT EXPEC TINGA
tvayl kvozk bbupf myksx yhvbl kmehv dpnez pqeau pluec yompu papza lkypg ksopa
GIRLS AIDMA TTHEW BLANK LYITS ABOYI VECOM EFORH ESHOU LDBEH EREMR SALEX ANDER
lrpsn pafkl bemav stuvz edpaq aezse dklne bvkqe azpbu plbkb veszk lbpaf uvlby
SPENC ERWAS TOBRI NGHIM OVERF ROMNO VASCO TIAFO RMETH ESTAT IONMA STERW HISTL
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EDETC

(If you're curious about the source of the plaintext, a quick Google search will reveal that it comes from chapter 2 of the book Anne of Green Gables by Lucy Maud Montgomery, published in 1908.)

Oh, and the keyword? Well, we didn't really need it to decode the message, but since we now have (almost) the whole ciphertext alphabet, we can write it down sorted by plaintext letter to get:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
kmnopqtuvwxyzser?albcdfgh?

where the two question marks stand for the letters I and J that don't occur in the ciphertext, as they correspond to the rare plaintext letters Q and Z. Guessing that I=Q and J=Z, we can see that the keyword is "serial", and the key letter is N. (Of course, the keyword "serjal" would also work in this particular case, but as far as I know there is no such word in English.)

Answer 2

Ignore the secret key. This is an algorithm for mapping the substituted letters.

Letter, digram and trigram frequency will help. The frequency changes based on language and whether the plaintext is sentences.

Also note the Caesar Shift effect, a=O, b=P... This applies to all letters except those substituted by the keyword, and are further shifted for each letter following a letter used in the keyword, q=B, r=C, s=E (extra shift due to D in the keyword).