rtevsbc cit ktggqut

Computer Science Level 3

Cryptography began thousands years ago, before the invention of computers. Modern or not, they have the same purpose - to secure communication in the presence of third parties.

Chris received this piece of encrypted text. Who is most likely to be the author?

cit_yvthftnes_jy_mtcctvg_on_ctzc_iqg_wttn_gcfrotr_yjv_fgt_on_evsbcqnqmsgog_
qnr_yvthftnes_qnqmsgog_on_bqvcoefmqv_rqconu_wqel_cj_cit_ovqho_
kqcitkqcoeoqn_qm_lonro_dij_yjvkqmms_rtxtmjbtr_cit_ktcijr_cit_eobitvg_
wvtqlqwmt_ws_ciog_cteinohft_uj_wqel_qc_mtqgc_cj_cit_eqtgqv_eobitv_
onxtnctr_ws_pfmofg_eqtgqv_gj_ciog_ktcijr_ejfmr_iqxt_wttn_tzbmjvtr_
on_emqggoeqm_coktg

Details and Assumption

_ denotes a space

You can learn about encryption techniques here .

Non of the above Victor Hugo William Shakespeare J.K. Rowling

1 solution

Agnishom Chattopadhyay
Jul 5, 2016

This is a substitution cipher. That means that the ciphertext has been formed by replacing each symbol (or letter) of the plaintext with a definite symbol (or letter) to form the ciphertext.

How many such rules for substitution could be there on a 26 letter alphabet? That's easy: it's 26!

Thankfully, we do not have to bruteforce through all the possible 26! keys to see if we are getting a meaningful plaintext. We have Letter Frequency Analysis to the rescue. It turns out that the occurences of the letters follow a frequency distribution depending on the used language.

Letter frequencies

Given this, it is easy to guess that most frequent symbol is a substitute for e , the next most frequent symbol is a substitution for t and so on.

This is what the decrypted text looks like

the_ frequency_ of_ letters_ in_text_ has_ been_ studied_ for_ use_ in_ crypt analysis_ and_ frequency_ analysis_ in_ particular_ dating_ back_ to_ the_iraq i_mathe matician_al_ kind i_ who_ formally_ developed_ the_ me thod_ the_ ciphers_ breakable_ by_ this_ technique_go_ back_ at_ least_ to_ the_ caesar_ cipher_ invented_ by_ julius_ caesar_s o_this_ me thod_ could_ have_ been_ explored_ in_ classical_ times

ETAOIN SHRDLU is the author.

Hasmik Garyaka - 3 years, 7 months ago

rtevsbc cit ktggqut

1 solution

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