Adding Ceasar and Vigenere easy challenge + starting to work on tutorials

challenges/chapter_1/stega/ceasar.md

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+# Ceasar
+
+Description: Julius Ceasar was a smart man.
+
+Lq fubswrjudskb, d Fdhvdu flskhu, lv rqh ri wkh vlpsohvw dqg prvw zlghob nqrzq hqfubswlrq whfkqltxhv. Lw lv d wbsh ri vxevwlwxwlrq flskhu lq zklfk hdfk ohwwhu lq wkh sodlqwhaw lv uhsodfhg eb d ohwwhu vrph ilahg qxpehu ri srvlwlrqv grzq wkh doskdehw. Iru hadpsoh, zlwk d ohiw vkliw ri 3, G zrxog eh uhsodfhg eb D, H zrxog ehfrph E, dqg vr rq. Wkh phwkrg lv qdphg diwhu Mxolxv Fdhvdu, zkr xvhg lw lq klv sulydwh fruuhvsrqghqfh. Eb wkh zdb, khuh lv brxu iodj: {iodj_DyhFhdvdu}

challenges/chapter_1/stega/ceasar_solution.md

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+{flag_AveCeasar}

challenges/chapter_1/stega/vigenere.md

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+# Vigenere
+
+Description: Ceasar is too easy, things should be more complicated.
+
+Gwxiz mgwpu jsysi wdb gqrx, tsiakggikym ijmcmjgdvm iymk, wzl js rmlwhwj xrqgsm qtgvhzhpvz yg prfjzk ig hfpjzk qnkee vtouhe. {wpvo_KrpvptoqurVwTsjt}

challenges/chapter_1/stega/vigenere_solution.md

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+{flag_EncryptionIsCool}

tutorials/command_line.md

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+# Introduction
+
+The command line interface is run in a terminal (or prompt). It is the direct interface with your computer. There you are able to do the same things are you are able in a graphic interface such as visiting a folder, reading and writing files.
+
+But the command line interface allows you access to a number of other programs which are really useful in a developer's every day life or while participating in CTFs. You should not be an expert, but with little knowledge of the command line you will already be able to do a number of things faster and quicker.
+
+Terminals in Linux and OSX are quite similar, if you run Windows, things are a bit different.
+
+## Understanding paths
+
+TODO
+
+## Basic command line 
+
+* List the files in the current folder: `ls`
+* See where you are: `pwd`
+* See who you are: `whoami`
+* Go in a folder: `cd INSERT_HERE_THE_NAME_OF_YOUR_FOLDER`
+* Go in the parent folder: `cd ..`
+* Go to your home directory: `cd ~` or `cd`
+* Print a file in the terminal: `cat THE_NAME_OF_YOUR_FILE`
+* Copy a file: `cp YOUR_FILE YOUR_DESTINATION`
+
+## Run programs in command line
+
+TODO