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Module-2 Part-1 (Classical Cryptography)

Welcome to the second module of Year of Security! This is Week 1 of Module 2 : Cryptography.

The roots of cryptography date back to history when they were first used for being able to send messages to another party such that even if an enemy intercepted the message, they couldn’t make sense of it. This is the principle of confidentiality: no one except the authorised parties should be able to make sense of the data. Classical ciphers were used to encrypt a plaintext message (in a particular language, we usually use English in our examples) into unintelligible ciphertext which cannot be decrypted without knowledge of a certain secret key known only to the autrhoised participants. Of course, “cannot be decrypted” is an idealisation and classical ciphers, despite trying their best to be hard to decrypt without a key, are vulnerable to all sorts of attacks that would enable an attacker to decrypt a ciphertext without knowledge of the secret key. Analysing cryptographic protocols rigorously is the field of cryptanalysis. In this week, we explore a few classical ciphers. We then move on to study the two most common forms of encoding which, though aren’t technically part of cryptography, are invaluable to solving practical challenges in modern cryptography which would be covered in the subsequent weeks.

Classical Ciphers

Caesar Cipher

The Caesar Cipher, famously used by Julius Caesar in wartime (though he technically used a fixed value of the key), is one of the simplest and most popular classical ciphers out there.

EXERCISE: Decrypt HAAHJR HA KHDU [Caesar; Key=7] ANSWER: ATTACK AT DAWN

ROT13 Cipher

The ROT13 is nothing but the Caesar cipher with key=13. It’s special because the encryption and decryption functions are the same - making it a ‘mirror cipher’ or a ‘reciprocal cipher’. It’s commonly used to obfuscate answers to newspaper puzzles.

EXERCISE: Decrypt EVAT NEBHAQ GUR EBFVR [ROT13] ANSWER: RING AROUND THE ROSIE

Vigenère Cipher

The Vigenère Cipher, also known as the Polyalphabetic Cipher, was once considered one of the hardest ciphers to crack; so much so that it was also called le chiffrage indéchiffrable (the indecipherable cipher). Unfortunately for the cipher though, it’s very easy to crack using frequency analysis.

EXERCISE: Decrypt Eiec fwijd bf Qpamvrvcs Isaasxi'd ugzc oa tsc uzqpgayydgkif oq Taownrrp aaxzee? [Vigenère; KEY=ANALYSIS] ANSWER: Ever heard of Friedrich Kasiski's work on the cryptanalysis of Vigenere cipher?

Atbash Cipher

If you have ever had to study Mental Ability for any competitive examinations (trauma moments, I know), then you must have met the Atbash Cipher innumerable times!

EXERCISE: Decrypt Ivzw Gsv Wz Ermxr Xlwv? [Atbash] ANSWER: Read The Da Vinci Code?

Monoalphabetic Substitution Cipher

The monoalphabetic substitution cipher is a great example of a cipher that has a forbidding large key space, something that would be inhibitive to even the beefiest of computers, yet can be cracked very easily using statistical methods.

Cracking Monoalphabetic Substitution Cipher - Frequency Analysis

Using frequency analysis to crack the monoalphabetic substitution cipher is Crypto101 and one of the most basic examples of cryptanalysis.

Transposition Ciphers

Another interesting class of classical ciphers is the transposition ciphers. As the name suggests, they transpose (move around) parts of the plaintext to create the ciphertext.

Morse Code

Morse Code is NOT a cipher! It’s an encoding format, albeit one used quite often in Classical Cryptography.

EXERCISE: Decode -- --- .-. ... . / .... .- ... / .- / ...- . .-. -.-- / .. -. - . .-. . ... - .. -. --. / ... . - / --- ..-. / ..- ... .- --. . ... / .. -. / .... .. ... - --- .-. -.-- [Morse] ANSWER: MORSE HAS A VERY INTERESTING SET OF USAGES IN HISTORY

The Enigma Machine

The Enigma Machine is a specific instance of a set of mechanical cryptographic machines called rotor machines. The Enigma Machine is undoubtedly the most popular of the rotor machines. The Germans used Enigma machines during World War II to communicate messages secretly and many countries invested huge capital funding to try and break the Enigma. It seemed almost impossible to crack the Enigma. Enter Alan Turing: he created an electro-mechanical device called the Bombe to crack the Enigma (along with his group of cryptographers at Bletchley Park). The Allied victory is doubtful to have occurred if not for the cracking of the Enigma.

Encodings

Note: Encodings are not ciphers, and not technically a part of cryptography per se. They are just ways to describe data in different formats.

Hexadecimal (Hex)

Hexadecimal encoding is achieved by taking the ASCII value of each character and converting that to its hex equivalent and concatenating all the formed “hex characters” together.

EXERCISE: Decode 48657861646563696D616C206973206A757374206F6E65206F66207365766572616C2028696E66696E6974656C792920706F737369626C65207261646963657321 [Hex] ANSWER: Hexadecimal is just one of several (infinitely) possible radices!

Base64

Base64 is a kind of encoding where each character represents one of 64 integers. In Base64 encoding, another special character “=” is used to represent padding. This is needed because ASCII strings are always multiple of 8 bits whereas Base64 strings are multiple of 6 bits, hence whenever we wish to encode an ASCII string that’s not a multiple of 6 bits, we need to pad the Base64 encoding to make the encoding-length a “multiple of 8 bits”.

EXERCISE: Decode QmFzZTY0IGhhcyBhIGxvdCBvZiBjb3VzaW5zIHN1Y2ggYXMgQmFzZTMyLCBCYXNlNTgsIGV0Yy4= [Base64] ANSWER: Base64 has a lot of cousins such as Base32, Base58, etc.

Challenges

We have 2 challenges lined up for you this time:

  1. Ones And Zeroes
  2. Tripartite

Don't be disheartened if solving them isn't obvious to you. Challenges in classical cryptography often require a lot of creativity and ingenuity, and it's not necessary nor important in your cybersec career to be really strong in classical cryptography. Consider these challenges as puzzles and give them a whirl! And remember, neither have we covered all the classical ciphers in this article (there are too many out there, all for you to explore) nor can there ever be an exhaustive list of ways one could encrypt messages.
From next week, the content will be a lot more structured and the context of the challenges will be a lot more obvious as we transition into modern cryptography, which is the actual cryptography used in the digital world. So enjoy the puzzles and creative freedom you possess while they last!

All in all, you would need to solve atleast 2 challenges throughtout the module but we encourage that you solve all of these, for things are more fun that way 😊. Make sure you go through the respective README.md files before jumping into the challenge. Have fun! 😁

Discussions among mentees are encouraged and we request you to use our Discord Server for the same.

Created with ❤️ by CSec