Week 5 - Cryptography Part-II

Learning Objectives

• List the various ways in which cryptography is used
• Define digital certificates
• List the various types of digital certificates and how they are used

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Asymmetric Cryptography

Unlike symmetric encryption, which uses the same key for both encryption and decryption, asymmetric encryption uses a pair of keys: one for encryption and another for decryption. This type of encryption is also known as public key cryptography.

Encryption Process

• Data is encrypted using the public key. (mostly)
• The encrypted data can only be decrypted using the private key.
• The private key must be kept secure, while the public key is shared.

Detailed diagram

sequenceDiagram
    participant Alice
    participant Bob

     Public Key Distribution
    Alice->>Bob: Requests Bob's public key (via website or secure channel)
    Bob->>Alice: Sends Bob's public key

     Transmission
    Note over Alice, Bob: The message is encrypted and sent over an insecure channel.
    Note over Alice, Bob: Even if intercepted, it cannot be decrypted without the private key.

     Final Security Assurance
    Note over Alice, Bob: Only Bob can decrypt the message, as the private key is secret and computationally infeasible to reverse-engineer.
    Note over Alice: Anyone can encrypt using Bob's public key, but only Bob can decrypt using his private key.

     Step 1: User requests a digital certificate
    User->>TTP: (1) Request digital certificate with public key
    TTP->>TTP: (2) Verify User's identity
    TTP->>User: (3) Issue digital certificate (signed with TTP's private key)
    
     Attack Scenario
    Attacker->>Receiver: (A1) Sends fake public key (claiming to be User)
    Receiver->>TTP: (A2) Verify certificate
    TTP--XReceiver: (A3) Certificate is invalid!
    Receiver->>Attacker: (A4) Reject communication