18.2. The Symmetric Key Encryption Process

Assuming that the key agreement has been achieved, the second problem that is addressed by cryptography. The setting here is the following, we will assume that Sita and Ram have already executed the key agreement protocol over the internet, and they have agreed upon a common key. Using this common key, we require algorithms which are publicly known, according to which Sita can encrypt her message into some scrambled text and communicate to Ram. Ram should be able to convert back those scrambled text to the original contents using the same key.

The first step in symmetric key encryption is establishing a common key between the two parties, in this case, Sita and Ram. This key is agreed upon through a key agreement protocol securely executed over the internet. After the agreement, Sita will use this shared key to encrypt her plaintext message, transforming it into ciphertext before sending it to Ram. The encryption must ensure that a third party (e.g., Ravana) who knows the algorithm but not the key cannot decrypt the message. This highlights the importance of confidentiality in communication.

Consider Sita and Ram as two friends who want to share secrets. They first agree on a secret code (the key). After they agree, Sita writes her secret message in that code and sends it to Ram. If someone else tries to read the message, they will see nothing meaningful, as they do not know the code, ensuring privacy.

There are two classes of cryptographic algorithms we use. The first is private key or symmetric key encryption. In this, it is ensured that a common key is shared between Sita and Ram, and no one else knows its value. Sita encrypts her message, transforming it into something that appears nonsensical or meaningless, called ciphertext. Ram then uses a decryption algorithm with the same key to convert the ciphertext back into the original plain message.

Symmetric key encryption uses the same key for both encryption and decryption, which is why it's called 'symmetric.' Sita has her plaintext (the original message), and using an encryption algorithm, she transforms this into ciphertext that can be safely sent over the internet. Upon receiving the ciphertext, Ram will use the shared key and a decryption algorithm to revert it back to the original plaintext.

Imagine Sita has a locked box (the encryption algorithm) and a key (the common key). She places her letter (the plaintext) inside and locks it. She then sends the locked box to Ram. Once Ram receives it, he uses the same key to unlock the box (the decryption algorithm) and read the letter, perfectly restoring it to its original form.

The reason it is called symmetric key encryption is due to the symmetry of using the same key for both encryption and decryption. Both parties must agree upon the key beforehand for secure communication.

The key in symmetric key encryption is pivotal because it is the secret code that guards the information being exchanged. Both Sita and Ram must keep this key confidential from any potential eavesdroppers. If an unauthorized person learns the key, they could easily decrypt any messages sent between Sita and Ram. Therefore, careful handling of the key is critical to maintaining the security of their communication.

Think of the key as a special passcode that allows Sita and Ram to access a shared vault where they store their secret messages. If anyone else learns this passcode, they can access the vault and read all the secrets, so they must keep it safe.

Sita and Ram must establish a common key securely over a public channel. The question of how they can do this arises, as initially, they do not know each other (like a new user wanting to transact online). The Diffie-Hellman protocol was developed to solve this problem.

The Diffie-Hellman protocol enables Sita and Ram to agree on a secret key without having to send the key over the internet. They each create a random component and mix it with publicly known information to generate a key that only they know. Even if someone observes their communication, they cannot derive the key without knowing the individual components each party created.

Imagine Sita and Ram are mixing colors to create a unique hue. They each have a secret color and combine it with a publicly known color. They send each other the mixed color. Upon receiving the mix, they add in their secret color, creating a unique final color that only they can reproduce, ensuring it's kept secret from onlookers.

By using well-constructed asymmetric tasks, Diffie and Hellman showed how Sita and Ram could agree on a common secret key using groups where deriving the key back from public information is computationally hard, like solving a discrete logarithm.

The conclusion emphasizes that through careful design of the cryptographic protocols, secure communication can be established over potentially insecure channels. The underlying math ensures that while anyone can see the public data exchanged, deriving the secret key from it without the proper mathematical tools is impractically difficult.

Think of it like a secret handshake that only Sita and Ram know. Even if others see them perform the handshake, without the specific moves and steps, there's no way for anyone else to figure out the 'secret' part of their communication.

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