Question Type: Computer Networks/Cryptography

This question tests your understanding of encryption keys used in network communication. Let's analyze the options:

• Single Key: While conceptually simple, a single key shared between communicating parties presents significant security challenges. If the key is compromised, all communication is compromised. This is not a common practice in secure communication.
• Session Key: A session key is a symmetric encryption key used for a single communication session. It is a temporary key, specifically generated for a particular communication exchange. This is a standard practice for securing communication between end systems, offering strong security without the vulnerabilities of long-term key management.
• Private Key: A private key is part of an asymmetric encryption system. It is a secret key used to decrypt messages encrypted with a corresponding public key. While private keys are crucial for secure communication, they are not usually directly used as the key for the communication session itself.
• Public Key: A public key is part of an asymmetric encryption system. It is a publicly available key used to encrypt messages that can only be decrypted by the corresponding private key. It is not directly used as the key for encrypting the bulk communication session data.

Step-by-step logic: The question asks about the commonly used key for end-system encryption. The most common and secure practice is to use a temporary symmetric session key for the duration of the communication session. Public and private keys are used for key exchange and authentication in many systems but not as the primary encryption key for the actual data transfer in a session.

Core Logic/Pattern: The core concept is understanding the different types of cryptographic keys and their roles in secure communication. Session keys are temporary and provide a balance between security and key management complexities.

Eliminating Incorrect Options: Options 1, 3, and 4 are incorrect because while they are used in secure communication, they are not the commonly used key for encrypting the communication *between* end systems during a session. A session key is explicitly created for the single session to reduce security risks associated with long-term key usage.

Correct Answer: Session Key