18.2.1. Encryption and Decryption Algorithms

And assuming that the key agreement has been achieved, the second problem that is addressed by the cryptography, the second core problem, I should stress here, it is not the case that secure communication is the only problem, the second core problem addressed by cryptography startup secure communication. So, the setting here is the following, we will assume that Sita and Ram has already executed the key agreement protocol over the internet, and they have agreed upon a common key. And now using this common key, we would require Sita and Ram to come, we would require some algorithms which are publicly known, according to which Sita can convert or encrypt her message into some garbled text into some garbage and communicate to Ram and Ram should be able to convert back those garbage or scrambled text back to the original contents using the same key, k which Sita has.

In this part, we set the stage for understanding encryption and decryption in cryptography. The first step is the 'key agreement' process, where two parties (in this case, Sita and Ram) establish a shared secret key that will allow them to communicate securely. This process is crucial because it ensures that only Sita and Ram know the key. Once they have this key, they can use it alongside publicly known algorithms to encrypt messages. When Sita sends a message, she encrypts it to make it unreadable to anyone who intercepts it, such as a third party. The intended recipient, Ram, can use the same key to decrypt this message and read it as intended.

Think about sending a letter in a sealed envelope. Sita writes her message, puts it in an envelope, and uses a special lock (the key) that only she and Ram have. Anyone who tries to open the envelope without the lock cannot read the message inside. This way, even if someone sees the envelope, they can't understand the content without the correct key.

So, it turns out that there are two kinds of, two classes of cryptographic algorithms which we use. The first category is that of private key or symmetric key encryption. In the symmetric key encryption, the setting is the following.It willbe ensured that a common key is already shared between Sita and Ram by some mechanism, say, by running a key agreement protocol and no one else apart from Sita and Ram knows the value of that key.

This chunk introduces symmetric key encryption. In symmetric key encryption, both parties (Sita and Ram) share the same key for both encryption and decryption processes. This means that the same key is used to encrypt a message and later decrypt that same message, which makes it essential that this key remains secret between just the two parties.

Imagine Sita and Ram both have identical keys for a treasure chest. Sita places her treasure (message) inside, locks the chest with her key, and then sends the chest to Ram. Since Raj also has the same key, he can unlock the chest and retrieve the treasure. If someone else tries to open it without the key, they won't be able to get to the treasure.

So, she has some message which is abstracted as a binary string, we call her message as plain text. We want to design an algorithm which we call as an encryption algorithm which takes a message m and the key k both of which are binary strings. And it should produce another binary string which we call a ciphertext.

This section discusses the core functions of encryption algorithms. Each message Sita wants to send is converted from plain text (readable message) into ciphertext (scrambled message). The encryption algorithm takes the original message and the shared key, turning them into an unreadable form that looks like gibberish to anyone intercepting the communication.

Think of an encryption algorithm like a blender that takes a fruit smoothie (plain text) and turns it into a thick puree (ciphertext). Anyone who sees the puree won’t be able to tell what fruits were used. Only Sita and Ram, with their key (recipe), can turn the puree back into a smoothie.

Once Ram obtains this scrambled message, he will have a decryption algorithm, he will have in a sense, he will know that Sita has used an encryption algorithm whose details are publicly known and the corresponding matching decryption algorithm also will be publicly known.

This part explains what happens on the recipient's side. After receiving the ciphertext, Ram will use a decryption algorithm (which is the opposite of the encryption algorithm) along with the same shared key to convert the scrambled message back into its original plain text form. The encryption and decryption algorithms are linked, allowing this reversed operation to occur.

Returning to the treasure chest analogy, Ram has a key identical to Sita’s. When he receives the locked chest, he uses his key to unlock it and reveal the treasure inside. The treasure (the original message) is now accessible to him again.

So, the reason it is called symmetric key encryption is because of the symmetry, namely, the same key is used both for encrypting the message as well as for decrypting the message.

This section clarifies the term 'symmetric' in symmetric key encryption. The same key is utilized for both encrypting and decrypting the message, creating a straightforward and efficient means of symmetric encryption. This symmetry is what makes this method efficient since only one key needs to be securely shared between the communicating parties.

It's like a hotel key system where each guest gets the same key to access their room. Anyone with the key can open the door, but it also means that the hotel must ensure that only the right guests receive those keys to keep the rooms secure.

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