Follow the Playfair analysis tips and refer to the example analyses in the Playfair module if you get stuck. Take notes as you go, in chronological order, about clues you find and guesses you make. This makes it vastly easier to recover from a wrong guess.

You'll need "pair bigram" frequency stats from Boxentriq(remember to set both n-gram size and step size to 2).You'll probably want to use one or the other automated worksheet. You may use other programs similar to the above for calculating statistics, trying out decipherments that you propose, etc. You may not use programs that decide which decipherments to try or that find the key for you, unless you write them yourself.

As always, report the key, the plaintext, your cryptanalytic reasoning (also made vastly easier by taking notes as you go), and your critical thinking answer. You don't need to submit every word of your working notes; you can summarize clues, guesses, processes, and results at about the same level of detail as is found in the analysis tips.

QC BM PI OC WH BO SO IX WD BM MY BI LF KO ON SM KO TQ UL IF GM KF QC BM YS TQ UL IF GL CX DY FW FM YZ CT HW FG KQ TC AS QH BM YS BK NW NI EL DT IK OF RV LC WS YG GW SH TK OF TC BY HX DU LU SG LK OF NC HW DU DW YG WG FL BK QY CM TC BG UL CK IX WD GB VK HW DM BL MU KD PZ CM BS PA FE PO ED BY PA FE PO GH ZK LD OK HB BI SN FD CM NC HW DU DW YG WG FL SG ZK HE NG EZ CM CT FU FL YB PO EG MU NG EZ CM CT WG FH EF FH GK XL GN QH WD OK DB HB HW CL VH UD TC UL PT AT UD SB TK OF PO EY KO EO FE PI KZ CM BL MU KD PZ FC FL WI SH BY EL CH WD TQ UL IF LU KL GM BM UY YA QK OF TL KF NC HW DU DW YG WG FL WM TQ GT QW OI NG GI VU MC HW DU LE SY OI WM TQ IQ VN SG HB HW DU LE DU FL IO GL CX WD MC HW KO CA PO CY IB WD TQ UL IF DU LW SO LF CK IP KC CW WD BM MY BI LF SP PO WR FH EK FE KO TK YC FW FM MT FW CW YP LU NK OF WG EY IB KI ON CM MT SO CQ OF GN QY UL FM WD OM MH KC FW DM FE KO HW FE YZ LK KF IK OF HW QT WD YG EF GY YP MO NE QK DB WC LF CY KN FL DF BI HW LU YZ CF YD IN NI BL LF HW DM PS EF PO MO NI AK CP KZ CT YB YZ AL KP HW CW TK YK TQ UL IF LE SY OI BS LF SY SH TK OF BI OI CQ DF NC AK MU QT WC LF EG KZ PO BK VU AK MW OC WH WG CO IX NG FW LU QC KO HW CW TK DI TC IO LE MO IM NG HW FW LW SX SH SA ZK LA PO CK IX WD EN SB DZ DM WL BS NI CX WD MT KZ CH BY HW VL OG DW KO CH WD GB QK HX SY KO HW FE LS GS KF UL NK DB HB HW SB CN NW UL GY LV AY BS KQ YQ NB LA TL HW GY IF KO CA BY GK TM QC BM TQ IX WD PS FY SB WC VK NG SG UV PA UL EY LW IK QH BY WI SM GP NW UL OP KF VL GN ZD IB WC OP BI SX WD EF YI PO CK HX SY KO HW DW PO CY NG HW LW PO XW IA SB HW DU FM IK IX FE KM OC OI QK OF HW FM BO CM WD BY EH QK HB WC OP KF CK TI YZ EF EK IO OB OZ HW LW PO BY CH UV DU FE BY KQ UD GY PC OM WL TK MC MD MT DU FE YZ OY KF CK TC PO BS KF WD AL UF EH QC WH WD EL YZ SD MO SW BY CI LA CM DV LA LO LB KO EA PO SH TK OF TC WD BM MY BI LF BM YT QT LQ TC SY HW CW TK YK TQ UL IF LE SY TK TC TK MT SY SA YV NA QK SB GM TQ HX SY HW FC UD SB CM CT KN FL DF NG HW CW DY FW LE ZY XF SN OF HW GL HW DU TQ IP KC CL CT LU CN HX SY EF FM PO EH KC GM YS QW MW IP FL CK OF QC BM YT GM RP SY BO UD SB QK UF QY AL IB TY BI LF QC BM TC WD AS CQ FL CI QW EN LB DU AI VL GK GP PS LX BK QT BM QC KO CB KG LC

Notable patterns in this ciphertext:

The AB BA pattern is matched only by IN NI.

The AB XX AB pattern matches FH EF FH, DU LE DU, TK TC TK, and HW GL HW.

The AB XX BA pattern matches UL IF LU, CK IP KC, YG EF GY, and IB TY BI.

Matches occur in the text in the same order they are given here.

Top 20 digraphs in the cipher text:

('HW', 30), ('WD', 19), ('PO', 16), ('KO', 13), ('UL', 13), ('OF', 13), ('DU', 13), ('BM', 12), ('TQ', 11), ('TK', 11), ('SY', 11), ('TC', 10), ('BY', 10), ('FL', 10), ('CM', 10), ('FE', 10), ('LF', 9), ('QC', 8), ('BI', 8), ('KF', 8)

In your analysis, make sure to indicate:

• How you filled in the initial letters in your guessed grid and what frequency patterns you relied on (give three examples)
• How you used cribs, wordspotting, alphabetic structure or other techniques to place more letters in the grid (give three examples)

Critical Thinking, Episode V: Kerckhoff Strikes Back

Suppose we were an organization using this kind of cipher as our standard way of communicating secretly. While keeping Kerckhoff's principle in mind (only your key and message are secret, not the choice of cryptosystem itself), consider the following proposal to alter the cryptosystem used above:

We create a playfair square as normal, and also create a monoalphabetic cipher.

We alternate between enciphering a pair of letters using playfair, and then the following letter with our MA cipher, then the next to letters with playfair, the following letter with MA, etc.

In other words, we skip a letter between every pair of letters we encipher using playfair, and then we encipher the leftover letters using our MA cipher.

Would the change to the cryptosystem make it more secure (take significantly more time to cryptanalyze, mitigating an existing weakness), less secure (take significantly less time to cryptanalyze, create a new major weakness), or neither (only trivial change, no major impact on strengths or weaknesses).

Why? How? What strengths/weaknesses change?