A Key Distribution Center (KDC) securely distributes session keys in symmetric encryption using a combination of pre-shared keys and secure message structures. Here's how it works:
1. Pre-shared Keys Establishment
Each user or service (referred to as a "principal") in the system shares a long-term symmetric key with the KDC. These keys are securely pre-distributed during setup and are not transmitted over the network.
2. Session Key Generation
When a user (e.g., User A) wants to communicate securely with another entity (e.g., Service B), User A sends a request to the KDC. The KDC generates a unique session key specifically for this interaction.
3. Secure Distribution of the Session Key
The KDC securely delivers the session key using the following process:
- Encrypts the session key for User A:
The session key is encrypted using User A's pre-shared key with the KDC. This ensures only User A can decrypt it.
- Encrypts the session key for Service B:
A second copy of the session key is encrypted using Service B's pre-shared key with the KDC.
4. Ticket Creation
The encrypted session key for Service B is wrapped into a "ticket." This ticket contains:
- The encrypted session key for Service B.
- Additional metadata (e.g., validity period, user identity).
The ticket is designed to be delivered by User A to Service B without exposing the session key.
5. Delivery to User A
The KDC sends the following to User A:
- The session key encrypted with User A's key.
- The ticket (encrypted with Service B's key).
User A can decrypt the session key using their own pre-shared key but cannot access the ticket's contents.
6. Secure Session Establishment
User A forwards the ticket to Service B as part of their initial communication. Service B decrypts the ticket using its pre-shared key and retrieves the session key. Both parties now have a shared session key, allowing secure symmetric communication.
