3.5. The KRB_PRIV Exchange

Overview

The KRB_PRIV message provides both confidentiality and integrity protection for application data exchanged between principals. The user data is encrypted and integrity-protected.

Purpose

KRB_PRIV is used when:

• Both confidentiality and integrity protection are required
• Sensitive data must be protected from eavesdropping
• Complete security for the data exchange is needed

Message Structure

KRB_PRIV contains:

• Encrypted data including:
  • User data
  • Timestamp (for replay detection)
  • Sequence number (optional)
  • Sender and recipient addresses (optional)

Generation of KRB_PRIV Message

Sender:

1. Constructs plaintext message with user data
2. Adds timestamp and optional sequence number
3. Encrypts entire message using shared key
4. Transmits encrypted message

Receipt of KRB_PRIV Message

Receiver:

1. Decrypts message using shared key
2. Verifies timestamp (replay protection)
3. Checks sequence number if used
4. Extracts user data

Security Properties

• Confidentiality: Encryption protects data from eavesdropping
• Integrity: Encryption scheme provides integrity protection
• Replay Protection: Timestamp and sequence number
• Complete Protection: Both confidentiality and integrity

Comparison with KRB_SAFE

• KRB_PRIV provides confidentiality; KRB_SAFE does not
• KRB_PRIV has higher computational overhead
• KRB_SAFE may be preferred when confidentiality is not required

Reference

For complete technical details, refer to RFC 4120 Section 3.5.