Cryptography Lesson #1 - Block Ciphers (Part 1)

Cryptography Lesson #1 – Block Ciphers (Part 1)



welcome to cryptography lesson number one in this lesson we will focus on one of the fundamental cryptographic tools that has a wide variety of uses in the field today block ciphers before we talk about how block ciphers work though let's take a step back for a moment and attempt to understand the importance of block ciphers in the field of cryptography imagine you wake up one day and are ready to start your day with a nice bowl of cereal people would typically view this as you getting some cereal and some milk but they're actually forgetting some things that are just as important as a cereal milk the spoon and the bowl have you ever tried eating cereal without a bowl and a spoon simply put it just doesn't work and that's exactly how important block ciphers are to cryptography using cryptographic schemes without block ciphers is like trying to eat cereal without a spoon or a bowl and of course like spoons and bowls block ciphers come in all different shapes and sizes and can be used in all sorts of different ways to illustrate how block ciphers work let's consider a game commonly found on the back of cereal boxes it reads decrypt the following message using the table above in this example we're going to call this message the ciphertext the game is asking us to translate it to plaintext to solve this puzzle we would simply look at the letters in the ciphertext row of this table and see what plaintext they correspond to in this case they would spell the word face now can you spot the difference between the plaintext and the ciphertext in this example as you can see the ciphertext characters are simply the plain text characters shifted with wraparound to the right by one in other words when making this puzzle a plaintext word was chosen and this table was used to come up with the encrypted version of that work this is very similar to the function of a block cipher specifically a block cipher will take an input and translate it to something else using a table much like the one in this example however as we saw with this example a block cipher such as this does not provide much in the way of security in fact we were able to quickly solve the puzzle to improve the security of block ciphers the table itself actually consists of several rows which are selected from using a secret key if someone wanted to decode a without knowing the key they would have to guess which row to use with a large enough block cipher the probability of them choosing correctly is extremely small

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