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Making Sense Of Cryptography

A Beginner’s Guide by Spandana Bansod


There’s no denying you’ve heard it before, it’s everywhere. It’s the new buzzword. For now, it may seem like rocket science, you may just not be able to decrypt it at all (get the pun?). Sure, you won’t instantly become a cryptographer after reading this post, but you surely will get an idea what the basic terms mean, and how it isn’t really as difficult as you probably think now. So…


What does Cryptography even mean?


Just one security system is not completely secure, so it needs multiple layers of security. This is called defense in depth. This is also where cryptography is used. Cryptography, at its most basic level, means encryption and decryption.


Encryption is converting plain text, like the blog post you are reading right now, to a bunch of random characters you can’t even read, called ciphertext, that is, until you decrypt it, which is, convert the ciphertext back into plain text.


People have always needed to keep certain information secret, so they used cryptography even before computers. But since then, the field has constantly evolved and got more and more complex, as a result, there are innumerable ciphers (you’ll get what a cipher is in a moment). To organize them, we sorted them into 2 main types, historical and modern ciphers. We’ll do historical ciphers first, because well, they are historical...






What are Historical Ciphers?

Historical ciphers are pen and paper ciphers, because well, they didn’t have computers back then, and usually, are pretty easy to crack. Caesar Cipher, by Julius Caesar, is one of the earliest and most basic forms of a historical cipher. it moved every letter some number of places forward, if a Caesar Cipher moved every letter 3 places forward, ‘Blog Post’ would become ‘EORJ SRVW’.


The Caesar Cipher is an example of a substitution cipher. Substitution ciphers replace every letter with another letter, number, or some other character. The disadvantage to this, however, is that letter frequencies are still the same. Letter frequencies are how frequently a letter appears. For example, if you know that the letter ‘e’ is the most commonly used letter of the English alphabet, you can probably figure out, with long enough ciphertext and some guesswork, the character which has replaced the letter ‘e’, and so on.


The Vigenère Cipher


To somewhat combat this, polyalphabetic ciphers were introduced. The name may sound complicated, but the ciphers really aren't. A very basic and pretty well known example of polyalphabetic ciphers is the Vigenère Cipher. To understand it, let’s try encrypting and decrypting the word ‘thumbnails’ using the cipher. We’ll also need a keyword, so let’s use the word ‘kid’. Repeat the letters of the keyword as many times as needed to substitute the letters of the plain text. The resulting string of letters will be the key. In our case, the key is ‘kidkidkidk’.


Encryption


Now, to encrypt it, go to the row of the first letter of your key(refer to the chart below), and the column of the first letter of your plain text, and their intersection will be the first letter of your encrypted message. For this example, the intersection of ‘k’ (green), the first letter of my key, and ‘t’ (red), the first letter of my plaintext will be ‘D'(circled). Repeat this with the other letters of your plain text until it is all encrypted. The encrypted message for our example would hence be ‘DPXWJQKQO’.


Decryption


To decrypt It, go to the row of the first letter of your key, and find the first letter of your encrypted message in the row. The name of the column you are currently in, is the first letter of your plain text. In the example we used, the row of the first letter of the key is the letter ‘k’ (green), and in it’s row, the letter is in the 20th column, which is, the letter ‘t’(red), the first letter of our plain text.


Don’t get it? I didn’t get it the first time either. Try rereading the method, while using your own plain text and key. Doing is the best way of learning, after all.


Modern Ciphers

Modern ciphers are just what the name says, modern and recently discovered. They run on computers, and are probably protecting all sorts of your stuff from getting in the hands of hackers, right now, as you continue to read this post.


Modern Ciphers are further divided into 2 main categories: symmetric key ciphers, and asymmetric key ciphers.


Symmetric Key Ciphers

Symmetric key ciphers are ciphers where the same key is used for both encryption and decryption. Just like the Vigenère cipher, where we used the same key in our example, ‘kid’ both times, when we were encrypting and also when we were decrypting.


Asymmetric Key Ciphers

Asymmetric key ciphers are generally more advanced (plenty exceptions exist, though) than symmetric ciphers. They are exactly the opposite of symmetric ciphers, where both encryption and decryption require different keys. They’re kind of like locks, the lock is the public key, everyone can access it, but that’s of no use if you don’t have the key, which are called private keys, only available to those who are intented to decrypt it.

Summary

If you forget everything else, remember this:

People have needed privacy and security from the very beginning, and that need is what gave birth to cryptography. Encryption and decryption are the most basic but fundamental concepts in cryptography. Ciphers are divided into 2 major parts, historical and modern, based on the time periods they were mostly used in. Historical ciphers originally didn’t use computers. One of the most popular and basic historical ciphers was the Caesar Cipher, which moved every letter some number of places forward; this number was it’s key. The Vigenère cipher uses a chart called the Vigenère chart/table to both encrypt and decrypt a message, with the key being a keyword. Modern Ciphers are further divided into two parts; symmetrical and asymmetrical. Symmetric Ciphers use the same key for both encrypting and decrypting, while asymmetric key ciphers use a public key accessible to everyone, and a private key accessible only to the intended decryptor.



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