'Copyright 2001 The American Cryptogram Association (ACA)  
'3613 Piedmont Drive, Plano TX 75075-6234  
'All rights reserved.  
  
'Contains digraph evaluation routines  
'which will be used in other Computer  
'Column programs.  See May/June 2001  
'Computer Column for further explanation  
'of these routines  
  
'PtaxStat.Bas (Ct$ from Novice Notes)  
Ct$="UXCUDZMVBAFWWPVDIKDOJISMAWRBBAY"  
Ct$=Ct$+"LOYXAKUXRJGDCJMYAPVRJWJADMU"  
Ct$=Ct$+"KLKLUAMKAOENYBFCCIQGFKQZAA"  
If Len(Ct$) mod 2 <> 0 then  
  Print "Cipher length must be even."  
  System 'Exit QBasic environment  
End If  
  
'Find period with best digraph ltrs  
For Per=3 to 16  
Depth = (len(ct$)/2)\Per  
Pairs$=""           'Each row is  
For Row=0 to Depth-1'2 letters deep.  
  RowS= Row*per*2   'Start of row txt  
  For Col=1 to Per  'Decode col pairs  
   Top$=Mid$(Ct$,RowS+Col,1)  
   Bot$=Mid$(Ct$,RowS+Col+Per,1)  
   Pairs$=Pairs$+PortaxDg$(Top$+Bot$)  
  Next Col  
Next Row  
Avg=DiAvg(Pairs$,1) 'Score the text   
If Avg > HiA Then HiA=Avg :HiP=Per  
Next Per  
  
Print "High-scoring period =";HiP;  
Print "with digraph average =";HiA  
Print "Probably cipher is ";  
If HiA < 580 Then Print "not ";  
Print "Portax." : System  
  
'Returns decryption of Portax  
'digraph Dg$ if Dg$ begins N-Z.  
Function PortaxDg$(Dg$)                         
Dec$="" 'Return null if decrypt fails  
T$=Mid$(Dg$,1,1) : B$=Mid$(Dg$,2,1)  
T=ASC(T$)-77 : T=T+T-1 : B=ASC(B$)-64  
If T > 0 Then  'Dg$ begins N-Z  
  If (B mod 2) = 0 Then T=T+1           
  If T<>B Then  
    Dec$= Chr$(77+(B+1)\2)+Chr$(64+T)  
  End If  
End If  
PortaxDg$ = Dec$  
End Function 'PtxDg$()  
  
'Corrected Nov/Dec 2001  
Function DiAvg(S$,A)  
 DLen%= Len(S$) - 1  
 If A<>1 then Dlen%=((Dlen%-1)\2)+1  
 DiAvg = Int(100*(DiSum(S$,A)/Dlen%))  
End Function 'DiAvg  
  
Function DiSum(S$,A) Static  
IF Init%=0 Then  
DIM Di%(26,26)  
d$=d$+"47874675736879373989676574742"  
d$=d$+"08111630721710653712060825273"  
d$=d$+"28727621822647613040765686558"  
d$=d$+"43665753677656062978887667458"  
d$=d$+"79775998577673745376447226538"  
d$=d$+"40757624050755475577326557527"  
d$=d$+"66635051854494348315548426576"  
d$=d$+"25050758777744258797647884735"  
d$=d$+"05500040003000005000006000005"  
d$=d$+"43274246224365313653040508557"  
d$=d$+"85448258548524666550718643842"  
d$=d$+"47104647613656140608678869684"  
d$=d$+"66568535896563626677686663678"  
d$=d$+"97729789684537333732672173276"  
d$=d$+"07666030400000000000000000000"  
d$=d$+"06000008667966583666686368865"  
d$=d$+"60718676865784666687458974706"  
d$=d$+"28665865983366596278874707266"  
d$=d$+"76646462377856088833434361008"  
d$=d$+"00070000050001021003073347328"  
d$=d$+"72244673055521403141424203510"  
d$=d$+"11035012520223066666655633565"  
d$=d$+"86357643624240005000300200300"  
d$=d$+"010200044"   
For I%=0 to 675 'Build Di% Table  
  C$=Mid$(D$,I%+1,1): V% = Val(C$)  
  Di%((I%\26)+1,(I% mod 26)+1)=V%  
Next I%  
d$="" : Init%=1 'Initialization done  
End If  
TLen%=Len(S$)-1 : Sum%=0 :Stp%=2  
If A=1 Then Stp%=1  
For I%=1 to TLen% Step Stp%  
  R%=Asc(Mid$(S$,I%,1))-64  
  C%=Asc(Mid$(S$,I%+1,1))-64  
  Sum%=Sum%+Di%(R%,C%)  
Next I%  
DiSum = Sum%  
End Function 'DiSum