Dennis Shasha/Codes, Puzzles, and Conspiracy/1/Coded Letter

Code

 * My dear Evangeline and Scarlet,


 * Fazr wxxkiqxn xa qocu zq if xrwoh mwek dkkb johotkbx,
 * xmaztm wccarjobt xa if cwqxar, hwztmwdhf obcaiqkxkbx. Qhkwnk
 * jab'x xwuk appkbnk, par mk on wb obxahkrwdhk nbad.


 * Dkcwznk mk on znobt ik xa nahek iwbf koppoczhk qzyyhkn w
 * jwf, mk toekn ik qroewcf wbj xoik xa xmobu. Xmk rknxhx on
 * xmwx O cwb lroxk xa faz wx if hkonzrk. O mwek if jazdxn xmwx
 * xmon hkxxkr lokh rkwcm faz, dzx O mwek w qhwb xa hkwek ox
 * ob w trackr'n nxacu raai.


 * Am, fkn, if cwqxar on Dkbgwiob Dwnukrmazbj, xmk iwxm
 * qrajotf lma dkcwik w qmohanaqmf qrapknnar wx Qrobckxab, bal
 * xzrbkj uojbwqqkr. Mk iwf kbxkr axmkr cwrkkrn ekrf naab. Loxm
 * zmk habt-lobjkj obxrajzcxoab azx ap xmk lwf, faz lohh pobj ox kwnf
 * xa jkcajk xmk rknx ob nqoxk ap xmk pwcx xmwx smk cajk on
 * raxwxobt df abk hkxxkr rotmx bal.


 * Exovlsnback xck npo oyxqq xsl dbcypcaxiig bc ynl jbfl. Nl
 * dbcysbio npo xqqxpso eg dliiaixs ylilrnbcl, aopcu rspfxyl oxyliipyl
 * dnxcclio. Nl qpcko slxkg mlidbjlo pc ixcko mnbol rlbril xkjpsl slelio.
 * Ml xsl cbm pc Asauaxg, x ixck qpiilk mpyn lwpilk Exotalo mnb nxk
 * qbauny xuxpcoy Qsxcdb pc Orxpc. Elqbsl ynpo, ml mlsl pc Nbrp
 * dbacysg mnlsl ynl ysxkpypbcxi oaorpdpbc ybmxsko bayopklso mxo
 * kporliilk mnlc Exovlsnback (xck P) nlirlk yb ibdxyl x upcxy xtapqls.
 * Ynl dbkd po sbyxypcu xuxpc. Nm pzyh qd y sjyem rcty lyh, y
 * nmmw, y kcdzo. Zomd nm ytm crr, bpbyjjh qd Fypwmtocbdl'p
 * JmytImzp.


 * Fypwmtocbdl qp pc ecdrqlmdz cr oqp yfqjqzh zc mgylm
 * lmzmezqcd zoyz om yjjcnp km zc sjyh zom zcbtqpz -- yeecksydqml fh
 * kh "fclhvbytlp," cr ecbtpm. Zom eclm qp tczyzqdv.


 * Kddx sdu ln re Cucwczi. R orkk aui ad srwcun dca dcu enya
 * mnqarezarde zem orkk knzhn odum orap z fpnnusck zem caanuki
 * ezrhn pduqn-gunnmnu de z mrua udzm enzu z hrkkzwn fzkknm
 * Tceaz Gzkknez. Zqx sdu apn truzan'q pdcqn zem idc ode'a gn szu
 * zozi. Opne idc srem apn pduqn-gunnmnu, zqx prl opnapnu pn rq
 * uzfrew zei kzln pduqnq aprq inzu. Fdmn fpzewnq zwzre. S psll feb
 * ho ahlo be dro bqsr geno axasf.


 * Bqo ofgensfx et bqo morraxo S loaio fozb bsmo psll vebabo
 * efgo oagq bsmo bqo morraxo gefbasfr bqo lobbov ... poll, jed psll
 * tsxdvo sb edb. Bqo bsmo atbov bqab, bqo geno psll ho bqo ramo
 * efo bqab bqsr lobbov hoxaf psbq, ozgoub bqab bqo morraxo psll ho
 * rulsb sfbe xvedur et osxqb gqavagborr. Bqoro xvedur psll ho
 * pvsbbof sf vafnem evnov, hdb remo et bqom eiovlau.


 * Haryovqedfn psll ho wdsgylj apavo et afj makev uelsgo
 * euovabsef, re uloaro ho rdhblo. Lob remoefo yfep pqovo jed avo
 * xesfx sf garo jed nsrauuoav.


 * Jedvr,
 * Kageh

Solution

 * Initial Observations

The message is apparently written in a substitution cipher such that the punctuation and word structure are unchanged.

It can be seen that there are several occurrences of a single-letter word consisting of one capital letter.

There is only one such in the English language, that is: I.

But it is not as straightforward as that.

This single-letter word appears as O, P, R and S, as we progress through the message.

That means the substitution cipher is not constant throughout the entire message, but changes a number of times.

By the pattern of the supposed translation of I, the change may be assumed (unless we find a contraindication) that the change consists of moving the ciphertext one letter further on in the alphabet.

The omission of Q suggests that two of these changes happened without I appearing in the meantime.


 * Preliminary Signposts

The message is clearly a letter.

From background knowledge (that is, from reading the introduction to the puzzle) it is believed that it came from Jacob Ecco, who is believed to have been kidnapped by Benjamin Baskerhound.

The signoff fits the pattern: Yours, Jacob</tt> even to the repeated o</tt>.

Hence we can infer those letters in the final cipher, and, assuming $5$ ciphers in total, work back to the first.

Note that we still need to identify the points at which the cipher changes, which at this point is not obvious.

It can be noted that the word patterns are the same from the final S</tt> until the end of the letter, for example:
 * S psll</tt> at the end of the $6$th paragraph
 * jed psll</tt> in the $7$th paragraph
 * jed</tt> appearing twice in the $8$th paragraph.

which leads to the conclusion that the same cipher is used from at least the end of paragraph $6$ to the end of the letter.

Clearly jed</tt> is code for you</tt>.

However, let us see what we have so far.

Fazr wxxkiqxn xa qocu zq if xrwoh mwek dkkb johotkbx,

xmaztm wccarjobt xa if cwqxar, hwztmwdhf obcaiqkxkbx. Qhkwnk

jab'x xwuk appkbnk, par mk on wb obxahkrwdhk nbad.

Dkcwznk mk on znobt ik xa nahek iwbf koppoczhk qzyyhkn w

jwf, mk toekn ik qroewcf wbj xoik xa xmobu. Xmk rknxhx on

xmwx O cwb lroxk xa faz wx if hkonzrk. O mwek if jazdxn xmwx I                                I

xmon hkxxkr lokh rkwcm faz, dzx O mwek w qhwb xa hkwek ox                               I

ob w trackr'n nxacu raai.

Am, fkn, if cwqxar on Dkbgwiob Dwnukrmazbj, xmk iwxm

qrajotf lma dkcwik w qmohanaqmf qrapknnar wx Qrobckxab, bal

xzrbkj uojbwqqkr. Mk iwf kbxkr axmkr cwrkkrn ekrf naab. Loxm

zmk habt-lobjkj obxrajzcxoab azx ap xmk lwf, faz lohh pobj ox kwnf

xa jkcajk xmk rknx ob nqoxk ap xmk pwcx xmwx smk cajk on

raxwxobt df abk hkxxkr rotmx bal.

Exovlsnback xck npo oyxqq xsl dbcypcaxiig bc ynl jbfl. Nl

dbcysbio npo xqqxpso eg dliiaixs ylilrnbcl, aopcu rspfxyl oxyliipyl

dnxcclio. Nl qpcko slxkg mlidbjlo pc ixcko mnbol rlbril xkjpsl slelio.

Ml xsl cbm pc Asauaxg, x ixck qpiilk mpyn lwpilk Exotalo mnb nxk

qbauny xuxpcoy Qsxcdb pc Orxpc. Elqbsl ynpo, ml mlsl pc Nbrp

dbacysg mnlsl ynl ysxkpypbcxi oaorpdpbc ybmxsko bayopklso mxo

kporliilk mnlc Exovlsnback (xck P) nlirlk yb ibdxyl x upcxy xtapqls. (   I)

Ynl dbkd po sbyxypcu xuxpc. Nm pzyh qd y sjyem rcty lyh, y

nmmw, y kcdzo. Zomd nm ytm crr, bpbyjjh qd Fypwmtocbdl'p

JmytImzp.

Fypwmtocbdl qp pc ecdrqlmdz cr oqp yfqjqzh zc mgylm

lmzmezqcd zoyz om yjjcnp km zc sjyh zom zcbtqpz -- yeecksydqml fh

kh "fclhvbytlp," cr ecbtpm. Zom eclm qp tczyzqdv.

Kddx sdu ln re Cucwczi. R orkk aui ad srwcun dca dcu enya I

mnqarezarde zem orkk knzhn odum orap z fpnnusck zem caanuki

ezrhn pduqn-gunnmnu de z mrua udzm enzu z hrkkzwn fzkknm

Tceaz Gzkknez. Zqx sdu apn truzan'q pdcqn zem idc ode'a gn szu

zozi. Opne idc srem apn pduqn-gunnmnu, zqx prl opnapnu pn rq

uzfrew zei kzln pduqnq aprq inzu. Fdmn fpzewnq zwzre. S psll feb I i    o

ho ahlo be dro bqsr geno axasf. b ab    o us    is co   a ai

Bqo ofgensfx et bqo morraxo S loaio fozb bsmo psll vebabo co i  o        ssa   I   a         i    i   ro a

efgo oagq bsmo bqo morraxo gefbasfr bqo lobbov ... poll, jed psll o c  ac   i         ssa   co  ai s          r           you  i

tsxdvo sb edb. Bqo bsmo atbov bqab, bqo geno psll ho bqo ramo i ur i  ou. i  a   r   a       co    i   b      sa

efo bqab bqsr lobbov hoxaf psbq, ozgoub bqab bqo morraxo psll ho o    a    is      r b  a   i      c      a        ssa    i   b

rulsb sfbe xvedur et osxqb gqavagbovr. Bqoro xvedur psll ho s i  i  o  rou s o   i    c arac  rs. s  rou s  i   b

pvsbbof sf vafnem evnov, hdb remo et bqom eiovlau. ri    i  ra  o  or  r  bu  so   o       o  r a

Haryovqedfn psll ho wdsgylj apavo et afj makev uelsgo Bas r ou    i   b   uic  y a ar  o  a y  ajor  o ic

euovabsef, re uloaro ho rdhblo. Lob remoefo yfep pqovo jed avo o ra io   so    as  b  sub. so o     o     r  you ar

xesfx sf garo jed nsrauuoav. oi  i  cas  you  isa   ar

Jedvr, Yours,

Kageh Jacob

It is now just about possible to restore the plaintext of the last few paragraphs directly by inspection.

We note that Haryovqedfn</tt> is most probably Baskerhound</tt> (see above), and replace its letters with their plaintext whenever they appear from S psll</tt> onwards:

S psll feb I i   no

ho ahlo be dro bqsr geno axasf. be ab e o use  his code a ain.

Bqo ofgensfx et bqo morraxo S loaio fozb bsmo psll vebabo he encodin o   he  essa e I  ea e ne    i e  i   ro a e

efgo oagq bsmo bqo morraxo gefbasfr bqo lobbov ... poll, jed psll once each i e  he  essa e con ains  he  e  er      e , you  i

tsxdvo sb edb. Bqo bsmo atbov bqab, bqo geno psll ho bqo ramo i ure i ou. he i e a  er  ha    he code  i   be  he sa e

efo bqab bqsr lobbov hoxaf psbq, ozgoub bqab bqo morraxo psll ho one ha   his  e  er be an  i h  e ce    ha   he  essa e  i   be

rulsb sfbe xvedur et osxqb gqavagbovr. Bqoro xvedur psll ho s i  in o  rou s o  ei h  charac ers. hese rou s  i   be

pvsbbof sf vafnem evnov, hdb remo et bqom eiovlau. ri en in rando  order  bu  so e o   he  o er a

Haryovqedfn psll ho wdsgylj apavo et afj makev uelsgo Baskerhound i   be  uick y a are o  any  ajor  o ice

euovabsef, re uloaro ho rdhblo. Lob remoefo yfep pqovo jed avo o era ion so   ease be sub  e.  e  so eone kno   here you are

xesfx sf garo jed nsrauuoav. oin in case you disa  ear

This part of the message falls into place, and we can establish that the keys for the cipher are:

Initial ciphertext:


 * $\begin {array} {c|c} \text {Plaintext} & \texttt A & \texttt B & \texttt C & \texttt D & \texttt E & \texttt F & \texttt G & \texttt H & \texttt I & \texttt J & \texttt K & \texttt L & \texttt M & \texttt N & \texttt O & \texttt P & \texttt Q & \texttt R & \texttt S & \texttt T & \texttt U & \texttt V & \texttt W & \texttt X & \texttt Y & \texttt Z \\ \hline \text {Ciphertext} & \texttt W & \texttt D & \texttt C & \texttt J & \texttt K & \texttt P & \texttt T & \texttt M & \texttt O & \texttt G & \texttt U & \texttt H & \texttt I & \texttt B & \texttt A & \texttt Q & \texttt S & \texttt R & \texttt N & \texttt X & \texttt Z & \texttt E & \texttt L & \texttt V & \texttt F & \texttt Y \end {array}$

First change:


 * $\begin {array} {c|c} \text {Plaintext} & \texttt A & \texttt B & \texttt C & \texttt D & \texttt E & \texttt F & \texttt G & \texttt H & \texttt I & \texttt J & \texttt K & \texttt L & \texttt M & \texttt N & \texttt O & \texttt P & \texttt Q & \texttt R & \texttt S & \texttt T & \texttt U & \texttt V & \texttt W & \texttt X & \texttt Y & \texttt Z \\ \hline \text {Ciphertext} & \texttt X & \texttt E & \texttt D & \texttt K & \texttt L & \texttt Q & \texttt U & \texttt N & \texttt P & \texttt H & \texttt V & \texttt I & \texttt J & \texttt C & \texttt B & \texttt R & \texttt T & \texttt S & \texttt O & \texttt Y & \texttt A & \texttt F & \texttt M & \texttt W & \texttt G & \texttt Z \end {array}$

Second change:


 * $\begin {array} {c|c} \text {Plaintext} & \texttt A & \texttt B & \texttt C & \texttt D & \texttt E & \texttt F & \texttt G & \texttt H & \texttt I & \texttt J & \texttt K & \texttt L & \texttt M & \texttt N & \texttt O & \texttt P & \texttt Q & \texttt R & \texttt S & \texttt T & \texttt U & \texttt V & \texttt W & \texttt X & \texttt Y & \texttt Z \\ \hline \text {Ciphertext} & \texttt Y & \texttt F & \texttt E & \texttt L & \texttt M & \texttt R & \texttt V & \texttt O & \texttt Q & \texttt I & \texttt W & \texttt J & \texttt K & \texttt D & \texttt C & \texttt S & \texttt U & \texttt T & \texttt P & \texttt Z & \texttt B & \texttt G & \texttt N & \texttt X & \texttt H & \texttt A \end {array}$

Third change:


 * $\begin {array} {c|c} \text {Plaintext} & \texttt A & \texttt B & \texttt C & \texttt D & \texttt E & \texttt F & \texttt G & \texttt H & \texttt I & \texttt J & \texttt K & \texttt L & \texttt M & \texttt N & \texttt O & \texttt P & \texttt Q & \texttt R & \texttt S & \texttt T & \texttt U & \texttt V & \texttt W & \texttt X & \texttt Y & \texttt Z \\ \hline \text {Ciphertext} & \texttt Z & \texttt G & \texttt F & \texttt M & \texttt N & \texttt S & \texttt W & \texttt P & \texttt R & \texttt J & \texttt X & \texttt K & \texttt L & \texttt E & \texttt D & \texttt T & \texttt V & \texttt U & \texttt Q & \texttt A & \texttt C & \texttt H & \texttt O & \texttt Y & \texttt I & \texttt B \end {array}$

Fourth change:


 * $\begin {array} {c|c} \text {Plaintext} & \texttt A & \texttt B & \texttt C & \texttt D & \texttt E & \texttt F & \texttt G & \texttt H & \texttt I & \texttt J & \texttt K & \texttt L & \texttt M & \texttt N & \texttt O & \texttt P & \texttt Q & \texttt R & \texttt S & \texttt T & \texttt U & \texttt V & \texttt W & \texttt X & \texttt Y & \texttt Z \\ \hline \text {Ciphertext} & \texttt A & \texttt H & \texttt G & \texttt N & \texttt O & \texttt T & \texttt X & \texttt Q & \texttt S & \texttt K & \texttt Y & \texttt L & \texttt M & \texttt F & \texttt E & \texttt U & \texttt W & \texttt V & \texttt R & \texttt B & \texttt D & \texttt I & \texttt P & \texttt Z & \texttt J & \texttt C \end {array}$

Hence for the fourth change, we have:

S psll feb I will not

ho ahlo be dro bqsr geno axasf. be able to use this code again.

Bqo ofgensfx et bqo morraxo S loaio fozb bsmo psll vebabo The encoding of the message I leave next time will rotate

efgo oagq bsmo bqo morraxo gefbasfr bqo lobbov ... poll, jed psll once each time the message contains the letter ... well, you will

tsxdvo sb edb. Bqo bsmo atbov bqab, bqo geno psll ho bqo ramo figure it out. The time after that, the code will be the same

efo bqab bqsr lobbov hoxaf psbq, ozgoub bqab bqo morraxo psll ho one that this letter began with, except that the message will be

rulsb sfbe xvedur et osxqb gqavagbovr. Bqoro xvedur psll ho split into groups of eight characters. These groups will be

pvsbbof sf vafnem evnov, hdb remo et bqom eiovlau. written in random order, but some of them overlap.

Haryovqedfn psll ho wdsgylj apavo et afj makev uelsgo Baskerhound will be quickly aware of any major police

euovabsef, re uloaro ho rdhblo. Lob remoefo yfep pqovo jed avo operation, so please be subtle. Let someone know where you are

xesfx sf garo jed nsrauuoav. going in case you disappear.

It remains to establish where the cipher breaks are.

Just before the S psll</tt> of paragraph $6$, we have zwzre</tt>, which is the same as axasf</tt> but one letter displaced.

Hence zwzre</tt> is code for again</tt>, and we see that the $4$th cipher encoding ends here.

This takes us back at least to <tt>R orkk</tt> which is <tt>I will</tt>.