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How to decode Playfair when only a small part of key is available? What is the plain text for this message? Can someone explain how to recover the plain text for this message? Is there a program that I can download and execute on my PC to test possible keys in bulk to solve the following problem?

The cypher used is Playfair. The partial key as a word is: ?A?D????BCG??KLNQ???V???Z. The partial key as a 5x5 table is:

? A ? D ?
? ? ? B C
G ? ? K L 
N Q ? ? ? 
V ? ? ? Z

Note that the symbol "?" indicates an unknown letter. The plain text is supposed to be in English or Portuguese. Only one of these following 3 texts is encoded by the Playfair cypher, but I don't know which.

TXT1

WL_SWVJDTA_GSFSSL_SKKGJAZVWVJ_GEMWHUG_FSFS_WSWTW_SSG_SHWGWID_F_WELAWKS-_SWFKWGFELN__OZSE.ESEV_SA,AFUEAW_DWID_WEYFGSEGSMK_VAWW_XUDSKMJV_J,AL,FI_LVF_MWSLN__GF__OZHFFL_FKK_GAWLSGFWX__WGJ_S_L_GWKJSE_MYHKGWSD_AKTF.WGLDFVASLL_FKWUA_FW_LMUGSKGAF_GAWH_LKLSGGGIGSL_GU_F_WLJMFXSFJ_WNNGUBF__YFWVWMS__F,SWJ_WA_K_GGLSFKWAVAA_SVFYJSGWJE_G_NLU_F_GLSWWLJJ,M_JAUEWWLMJLSSLXDGUVFSW_GSKEWAMFG_FAUSL_F_SSKVMLX__JKS_JNSNVVEYSDJHM_AGUEWSWWWLMFJ_AJLS__SGYFGWF_WES-_DWX

TXT2

SNPQUNBFMIENSKVNSXPQAZMEFONUGSNFRHOEKHBSQRQKRHUPXOUOTULAADZAMARNQEQUSCCORLPQLMTUDECUGXACQKCZKUSIDQOLVFRHEHUCULMIENSKVNKSQRTULAADUDQUHOKUDSERLGPAMACKOCTUQRUBPSZPQUSMQPULMIENSKVNPZPUQKAZAMKEXIMXNDAXACCNIUNEACMEFONUACUMPQUSSLUDQRTPUCAQKGDUIKTUCLGQEIPIZXKGKCDUHRQTMPMBDQSMQPGCAMRHPOXOTNRLOQABNLEPZPIEOFPUUKPISXSKMUDASKISWESKPSPQPTDQQCEHOKQOTUACAFPELMSKPSZAUPPESDPXACMECOROTVRLEHUCVNOUPQEAOQXIPQXBKGOCRHLDMQPSQKSXGSFMPEKMUCSMQCUPXDBPSXMEZPBWCDMPCAKSRETFACPOBDMPMBOHQTQBPOBCNTGSOUQOEDDNSMQPTPEPGKZPMCMAOEXMMLDUEHSIAOBUGSUDGKQOPYPQMLVNKSNXSXSMSPRNQEQUSCAFERSKREGKYDGSMEIZHEMDHRSKPSXHQKUCOEGSHKTPNTQBWDTPEROEOCLMAOBQUPMQSDCMGSHNQEQUSCRMMIQKCUDRSDMEIOCOMFHOHCPOGKYDGSUDIUSNPQUNBFMIENSKVNCAUORHRLNOPQTULAMEFOUCUDTUERGBMARSTCKSQOTPEHIPMHQKIXCAUORHRLSDMEQUMAUCUCBUMASGMUPQOKQUACMESVUPBUQMAUSLCABYMIDHPAHRBULMEHSNPQUNBFMIENUDMQMPMWCAEHXACHEOOVEGLMDQSMQTRHPOGKYD

TXT3

VBRCTB%RS1W2G_0CBKDAPQBG___GXGW,O_QCWEF2GQDGA9QG-NDRY__1U-BWCT__YRKKTYXCKBZAB_HJVG_BXY_YDUR_B39_0JS2QG-_AD__TGVKHBD__GCBDT__0KBJD_.BDYG_CD2R0Y1H_DYJGUKWJKKB,C_BHWOB_NCODXZKDG__ZB__S2_20BDECTYDDKCYY_HY5UDY_GKOT1W_CC2
_CKDPQ_0G%X9GDTKNJFJ_CC_V_TC_B_BâUKTGKXTBFAP5BK2G30T_,_-BDURGBB_DDH,N0YB_BYGBK8_A__0
SB-CD_S2J.T_CBUJU_W_UDJVBUKYDUTY€YTYCJXHC_AD0C_0K_
K2_EYGG_CCVWGBVB_K%VWCMDDG__CTOY1CNDSBYA30__WDBJC,QHYGYG_YKYN_BCKO™VUKC.X
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Playfair key grids are often generated by filling in the letters of a key phrase, then completing the grid with the remaining unused letters of the alphabet.

It looks to me like the partial grid you provided has been created in this way. If so, you can fill in several more letters straight away (assuming there is no J):

? A ? D ?             ? A ? D ?
? ? ? B C             ? ? ? B C
G ? ? K L     -->     G H I K L
N Q ? ? ?             N Q ? ? ?
V ? ? ? Z             V W X Y Z

Furthermore, the three cells between Q and V are most likely to be RST, RSU, RTU, or STU. For each of those four possibilities, there are now only six cells remaining to be filled. With a computer, it wouldn't take long to test every possible combination (4×6! = 2880). You could probably even solve this with pen and paper by attempting a partial decryption with the parts of the key you already have, then working back from the plaintext to figure out the others.

I think it's safe to assume that TXT2 is the Playfair encrypted message. It has an even number of characters, no digits or punctuation, and no letter J. Also, when split into pairs, there is no pair in which the same character appears twice (something that cannot happen in Playfair encryption).

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  • $\begingroup$ thanks you were really helpful $\endgroup$ Feb 19 at 14:47
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The text that can be decoded by playfair is the second one.

To find the ansewer i used this tool: https://github.com/N8Stewart/PlayfairCrack

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