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Lecture Five: Confidentiality -2 1. What is Key Exchange (KEX)? • Definition: Key exchange is a method for securely sharing encryption keys between two parties so they can communicate privately. For example, when two people want to send encrypted messages, they need to exchange keys in a way that no one else can intercept them. • Importance of key exchange: Key exchange ensures that two people can communicate securely by sharing an encryption key without it being intercepted. For instance, when you connect to a website via HTTPS, your browser and the website exchange keys so that all the data you send (like passwords) is encrypted and safe from hackers. 2. Types of Key Exchange Protocols: • Diffie-Hellman (DH) Key Exchange: This is a common key exchange method where two parties can create a shared secret key even if someone is listening to their communication. Even if someone is listening to their conversation, they won’t be able to figure out the key. For example, when two people use Diffie-Hellman to exchange keys, hackers can’t figure out the shared key even though they may intercept the communication. • Elliptic Curve Diffie-Hellman (ECDH): This is a faster and more secure version of Diffie-Hellman that uses elliptic curves to generate keys. It is often used in modern applications like secure messaging and VPNs because it’s more efficient. 3. Problems with Key Exchange and preventing attacks: • Man-in-the-Middle Attack: An attacker could intercept the key exchange and pretend to be one of the participants. For example, if someone positions themselves between you and a website during the key exchange process, they could steal your keys and decrypt your data. • Preventing Attacks: To avoid attacks like Man-in-the-Middle, systems use digital certificates to verify the identities of the participants. For instance, when you visit a secure website, the website provides an SSL/TLS certificate that proves it’s legitimate, preventing attackers from stealing the exchanged keys. 4. Electronic Codebook Mode (ECB): • Electronic Codebook Mode: ECB is a block cipher mode that encrypts each block of data independently using the same key. While simple, it has significant weaknesses because identical plaintext blocks result in identical ciphertext blocks. • Problems: ECB doesn't provide proper security for a secure channel because patterns in the plaintext are preserved in the ciphertext, making it easy for attackers to detect and exploit the structure of the data. • Example: If an image is encrypted using ECB, an attacker can still recognize patterns in the encrypted image, making it easy to detect parts of the original message. 5. Cipher Block Chaining (CBC): • Cipher Block Chaining: In CBC mode, each plaintext block is XORed with the previous ciphertext block before encryption, ensuring that each block depends on the previous one. This makes it more secure than ECB. • CBC Padding Oracle Attack: A vulnerability in CBC arises if the padding used in the last block is incorrect and the system leaks information about the padding. Attackers can exploit this information to decrypt the message without knowing the key. • Example: If an attacker can guess whether the padding is correct, they can gradually decrypt the entire message by analyzing the feedback from the system. 6. Counter Mode (CTR) and Counter Mode with Galois/Counter Mode (GCM): • Counter Mode (CTR): CTR mode turns a block cipher into a stream cipher by using a counter that is incremented for each block. This makes it fast and highly parallelizable. • Galois/Counter Mode (GCM): GCM is an extension of CTR mode that provides both encryption (confidentiality) and message integrity (authentication). It is widely used in secure communication protocols like TLS because it’s efficient and secure. • Example: CTR and GCM are used in systems that need both speed and security, such as securing network traffic in real-time applications. 7. AEAD Cipher Modes (Authenticated Encryption with Associated Data): • AEAD: AEAD cipher modes provide both confidentiality (encryption) and message integrity (authentication), ensuring that a message is not only encrypted but also protected from tampering. • Confidentiality, Message Integrity, and Sender Authenticity: AEAD ensures that data cannot be read by unauthorized parties (confidentiality), that it hasn’t been altered (integrity), and that it was sent by the correct sender (authenticity). • Example: GCM is an AEAD mode used in many secure communications systems, such as VPNs and HTTPS, because it encrypts data while also ensuring its integrity and authenticity.