18.1.1. Introduction to Secure Communication

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.

In secure communication, it is crucial that two parties, Sita and Ram in this case, have already established a common key through a method known as key agreement. This key agreement ensures that both parties can encrypt and decrypt messages securely, maintaining their confidentiality even when using public channels for communication.

Think about exchanging secrets as friends. Before sharing sensitive information, like a surprise party plan, both friends need to agree on a unique code word that only they know, ensuring that if someone overhears their conversation, they won’t understand it.

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.

Once Sita and Ram have their common key, they need encryption algorithms that both parties know. Sita will use these algorithms to transform her original message (plain text) into an encrypted format known as ciphertext, which is unrecognizable ('garbled text'). This protects the message from anyone intercepting it.

Imagine writing a message in a secret code. Only you and your friend know how to decode it. If someone else finds your coded message, they won’t be able to understand it unless they know the code.

And by secure communication here I mean that, if there is a third party or Ravana, who knows the public description of your algorithm... the Ravana should not be able to come up with the values of m , m , m and so on.

In the context of secure communication, even if an unauthorized third party (like Ravana) knows the details of the encryption algorithms, they should not be able to decipher the original messages unless they have access to the common key. This ensures that the information remains confidential and only the intended recipients can understand it.

Consider it like sending a letter in a special envelope that only you and your friend can open. Anyone else seeing that envelope can't read the letter inside because they don't have the right tool to unlock it.

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.

Cryptographic algorithms are categorized mainly into two types: symmetric key encryption (where the same key is used for both encryption and decryption) and asymmetric key encryption (where public and private keys are used). In symmetric key systems, Sita and Ram share a common key and must keep it secret from others for secure communication.

Imagine having a diary with a lock. You and your best friend have the same key to that lock. You can both write messages in your diary securely, knowing that no one else can open it without the key.

Now, if that is the case assuming this setup has been done, the way symmetric encryption works is as follows. So, imagine Sita has some message, it could be an email, it could be just a hi message, it could be anything, it could be her banking password.

In symmetric key encryption, Sita prepares a message (like an email or a sensitive password) and uses an encryption algorithm along with the shared key to convert her original message into ciphertext. This ciphertext is then sent to Ram, who uses the same key to decrypt and read the original message.

Consider sealing a birthday gift in a box and locking it. You lock it using a key that both you and the birthday person have. The locked box (ciphertext) can be sent, and only those with the key can unlock it to access the gift inside (the original message).

Because everything will be now happening over a public channel... How at the first place they can do that?

Sita and Ram need to establish their common key securely over a public channel where anyone could potentially intercept their communication. This leads to the need for a secure key agreement method that enables them to share the key without it being compromised.

It’s like talking about a secret plan while standing in a busy market. You need to find a way to convey your secret code to your friend without letting anyone else hear it. Perhaps you write it down and pass it discreetly, ensuring that no one catches on.

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