Govur University Logo
--> --> --> -->
Govur University Logo
Sign In
...
Home → All Courses → Engineering and Technology Courses → How to Set Up End-to-End Encryption for Your Personal Communications → Flashcard

Describe the precise steps involved in a typical message encryption and decryption process within an end-to-end encrypted system.



The message encryption and decryption process in an end-to-end encrypted (E2EE) system involves a sequence of carefully orchestrated steps designed to ensure that only the intended sender and receiver can access the content of a message. The core principle of E2EE is that the message is encrypted on the sender's device and decrypted on the recipient's device, with no intermediate entities, like the server, having access to the unencrypted message content. The encryption and decryption typically involve a combination of symmetric and asymmetric cryptography, with the key exchange process preceding the actual message transmission.

Let's consider a scenario where Alice wants to send a secure message to Bob. The general process is as follows, assuming they have already established a secure shared secret key through a key exchange process using methods such as Diffie-Hellman, which is usually performed prior to transmitting messages:

1. Message Preparation: Alice composes her message, which may be text, images, or other digital content. This is the plain text data that will be encrypted.
2. Key Derivation: A session key (also called a message encryption key) is derived from the shared secret. Typically a new key is used for every message. The key is derived using a cryptographically secure key derivation function or KDF, along with a nonce. The nonce is a random number that's used only once for each message. This adds a crucial layer of security, meaning that even if two messages use the same key, the resulting ciphertext will be different. Using a new session key ensures that compromising one message doesn’t compromise other messages using the same main key, and adds the concept of ‘forward secrecy’ to the system.
3. Message Encryption: Alice now uses this derived symmetric session key and a chosen symmetric encryption algorithm (like AES or ChaCha20) to encrypt the message. The message and the nonce are used as input to the encryption algorithm to produce the ciphertext. Symmetric encryption is used here because it is much faster and more efficient for encrypting large amounts of data than asymmetric encryption.
4. Ciphertext Transmission: Alice sends the ciphertext (the encrypted message) and the nonce to Bob through the messaging server. Importantly, the server doesn't have the key and therefore cannot decrypt the message content.
5. Key Derivation (Receiver Side): Upon receiving the ciphertext and the nonce, Bob also needs the same key used to encrypt the message. Bob uses the same key derivation function with the same shared secret that was generated initially and also the nonce that was sent in order to generate the same symmetric key that Alice used. If he does not have the shared secret key, he will not be able to decrypt any messages. If it is the very first message, Bob can use his private key to perform the key exchange process with Alice before decryption.
6. Message Decryption: Now Bob uses the derived session key and the symmetric decryption algorithm to decrypt the ciphertext, recovering the original message from Alice. The decryption process reverses the encryption process with the key and the ciphertext being provided as an input to the decryption function, outputting the original plain text message.

To further clarify with an example, consider Alice sends the following message "Hello Bob!" using AES-256 and the nonce "123456". Here's how the steps will occur:

1. Alice composes her message: "Hello Bob!".
2. Alice derives a session key from the shared secret and nonce "123456".
3. Alice encrypts "Hello Bob!" using AES-256 and the derived session key and nonce to produce a ciphertext such as "A4B9C2D8E3F1". The nonce itself is also sent to Bob to decrypt the message.
4. The ciphertext "A4B9C2D8E3F1" and nonce "123456" are sent to Bob, possibly through a server. The server cannot decrypt it.
5. Bob also performs the same key derivation process as Alice from the shared secret key using the nonce "123456" to derive the same session key as Alice.
6. Bob decrypts "A4B9C2D8E3F1" using AES-256 and the derived session key to reveal the original message "Hello Bob!".

It is important to emphasize that in E2EE systems, this entire process is designed to ensure that the servers and any intermediary systems that are handling the message transmission do not have access to the decryption keys. This implies that the message contents are only visible to Alice, the sender, and Bob, the intended recipient. The message remains encrypted in transit and at rest on the server. Furthermore, the use of different session keys for each message, generated through the KDF and unique nonces, ensures the forward secrecy properties of the protocol. This means that even if one session key is somehow compromised, the security of previous and future messages is not affected because each message uses a unique session key that was derived from the main shared key. The combined use of symmetric and asymmetric cryptography, session keys, and key derivation functions is critical to the overall strength of the E2EE system ensuring both confidentiality and security.