Challenge Description
This is a challenge involving Vigenère cipher. After we run this command nc jupiter.challenges.picoctf.org 58295, we are greeted with a block of text:
Encrypted message:
Ne iy nytkwpsznyg nth it mtsztcy vjzprj zfzjy rkhpibj nrkitt ltc tnnygy ysee itd tte cxjltk
Ifrosr tnj noawde uk siyyzre, yse Bnretèwp Cousex mls hjpn xjtnbjytki xatd eisjd
Iz bls lfwskqj azycihzeej yz Brftsk ip Volpnèxj ls oy hay tcimnyarqj dkxnrogpd os 1553 my Mnzvgs Mazytszf Merqlsu ny hox moup Wa inqrg ipl. Ynr. Gotgat Gltzndtg Gplrfdo
Ltc tnj tmvqpmkseaznzn uk ehox nivmpr g ylbrj ts ltcmki my yqtdosr tnj wocjc hgqq ol fy oxitngwj arusahje fuw ln guaaxjytrd catizm tzxbkw zf vqlckx hizm ceyupcz yz tnj fpvjc hgqqpohzCZK{m311a50_0x_a1rn3x3_h1ah3xf966878l}
Tnj qixxe wkqw-duhfmkseej ipsiwtpznzn uk l puqjarusahjeii htpnjc hubpvkw, hay rldk fcoaso 1467 be Qpot Gltzndtg Fwbkwei.
Zmp Volpnèxj Nivmpr ox ehkwpfuwp surptorps ifwlki ehk Fwbkwei Jndc uw Llhjcto Htpnjc.
It 1508, Ozhgsyey Ycizmpmozd itapnzjo tnj do-ifwlki eahzwa xjntg (f xazwtx uk dhokeej fwpnfmezx) ehgy hoaqo lgypr hj l cxneiifw curaotjyt uk ehk Atgksèce Inahkw.
Merqlsu’x deityd htzkrje avupaxjo it 1555 fd a itytosfaznzn uk ehk ktryy. Ehk qzwkw saraps uk ehk fwpnfmezx lrk szw ymtfzjo rklflgwwy, hze tnj llvmlbkyd ati ehk nydkc wezypry fce sniej gj mkfys uk l mtjxotnn kkd ahxfde, cmtcn hln hj oilkprkse woys eghs cuwceyuznjjyt.
Determining the cipher
PicoCTF Hint: Perhaps looking at history will help
With this clue, I tried searching the years mentioned in the above text (1508, 1467, 1553, 1555). This led to the Wikipedia page for the Vigènere cipher. Indeed, some of these dates were mentioned again the “History” section of this page.
Getting the flag
Since the key is not provided, we can use a tool like thisto break the Vigenère cipher without knowing the key.
Using the above tool will allow us to obtain this decrypted block of text:
It is interesting how in history people often receive credit for things they did not create
During the course of history, the Vigenère Cipher has been reinvented many times
It was falsely attributed to Blaise de Vigenère as it was originally described in 1553 by Giovan Battista Bellaso in his book La cifra del. Sig. Giovan Battista Bellaso
For the implementation of this cipher a table is formed by sliding the lower half of an ordinary alphabet for an apparently random number of places with respect to the upper halfpicoCTF{b311a50_0r_v1gn3r3_c1ph3ra966878a}
The first well-documented description of a polyalphabetic cipher however, was made around 1467 by Leon Battista Alberti.
The Vigenère Cipher is therefore sometimes called the Alberti Disc or Alberti Cipher.
In 1508, Johannes Trithemius invented the so-called tabula recta (a matrix of shifted alphabets) that would later be a critical component of the Vigenère Cipher.
Bellaso’s second booklet appeared in 1555 as a continuation of the first. The lower halves of the alphabets are now shifted regularly, but the alphabets and the index letters are mixed by means of a mnemonic key phrase, which can be different with each correspondent.
We can retrieve the flag from the above text.
Flag
picoCTF{b311a50_0r_v1gn3r3_c1ph3ra966878a}