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'Probably a Code?' printed from https://nrich.maths.org/
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When cracking text-based codes, a first step is often to look for repeated strings of characters resulting from structure in the underlying cypher-text. This can give a way in to cracking the code.
Alison and Steve are considering the following coded message:
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vrucseviffsrzpvekpqmyfvfvarvfhfzecsxzvvorjbtlrkaqmcvbgyfsct
nuznglyrkqsqewtauvzpievmblcciabcfvofpymgrfnpkjuszcjrjuaur idgznelhpycaoljzbjkrcqioguwavhlvflejbcuqkpkzsvmnpaxivovjjt lrqmyovtpjhfkzccrctgksiwoauvzgnvfvuabnitkfnpkjuziohqmipjv eoipivaolnwwtlgravlbwbjvfvqefpcqolfzvuwviqvlqsgvovjekuqfnr ybdqulzplcfqimctfsmevzvuqyrwmtcrebcuqmqxpqzbtfvedcpak wdlcvzubnumfaurbvorgwnlvjbjlfvivvswzqzgrvfkrjwnhgzwppgv dgycimulakakafvthabdgktnxqphgzwphfkpgyrxqqubwjghhkgcu qumnptybvorimohexitlgkpgzhequmbimxlemqupocmkafszqhqu qurwlavzxzzvpaxbjlufzkgbeipkqznhbfzvihcvzrlglinzccmpkblzv orimhvenqvolfctsrrdgtljquariqypycxwaffugaelavpagzgjruqpna rdknnkwtzgymtlfewyhauntvfkitlorvkzuvlcuqjiksveoqcriiehyda ghjvucfovecmgvlvvncipkflzrhfjqpnveequqvzuhauqpirrcvfrmm tfevokvayqvoribqkvjkqcrimfvakpgonsqvhocmisbsmkafgzqkhtb kvajipksvivbevaohlsmypgywwaroiowyviuauvxjlafugunfnvorym ccretaibuqgzhelqbokmfslrzgpakpqzrlvfpftwxlevluvyzbwkrjejhg diaubkjglkgmearuqphpfcpaepwhlgvzphycqiogrqohlkpgyruqujb mmtauvequqiwwzcfegyjyqeonkbthpkavoremgkyvipkzrgtltltcar rbjvhjipkpvtgzgzinvojmtcnkqqufkpcaevywpevwpslkpkzifgcnrk wtlaumtauvqtzrvukutvkelakzkjvkqgzpfvupfkmpasfzgcriquonct uhgzivlzpitkrebebezwupgpekaukpgzvxpvvsrxcygfnvornwtsqem xlesmhvevdkzvkmfhauucfgimckncipkavdgyovnqyrzuryvebgkop bjlsfwvvsdipauvaghevualakqelzvvvznelvorpitlflnhppzmpagfkq udlmthycnghefnfhaxmtveumcaurvfabzvfbpvugabtwotrekgauz anhofzkvhjdqfnxmypgybjlwfgcjuztfmrvtuduvvjlrdjcyxjqphyzb vsrswcajzbjovjpqsvuiatnkmuvarvgecvlkavfvqmqzaevivzabcyqu unkqxlezdgyolbubcgwupaxinsgymulpfvllpkctlfkwdlsrtullfceha ewvjbebgzgkpgpavavpzrjnlovvgmvkejppyquonctevawmtvartn tneskuqkwvorciuatvvgynkqquoplkzpfdgyveocwnjacnremcygy mrvyvbqaufagjblvvyvvavvevieojyqeonkxtlfvvvzbdipfzfvvofrzg yrhckzvkmqyopiujribcpazviauvagjevbqmgymohtemvduzkjpsr bcsygwuzvstgjnewpslsmgmsvkvlqsgcuhelgygrskutjceonjukur |
Alison computes the frequency of each letter, finding these results:
| 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 |
| 85 | 55 | 72 | 22 | 73 | 77 | 85 | 48 | 61 | 58 | 90 | 77 | 61 | 55 | 55 | 87 | 80 | 78 | 41 | 58 | 94 | 166 | 44 | 21 | 53 | 80 |
After some fiddling around on a spreadsheet, Steve notices that there are seven repeated strings of 5 letters, highlighted as shown below:
Steve dismisses this as coincidence, saying "I would guess that these repeated 5-letter sequences are consistent with randomly generated text based on these frequencies of letters". Alison is not convinced and thinks that this structure of 5 letter sequences would not likely occur randomly for these frequencies of letters.
Investigate.
Challenge: Crack the code, regardless of who is right!