SSCP Systems Security Certified PractitionerCryptographyMedium
A security auditor is reviewing a company's SSL/TLS configuration for its web servers. The auditor discovers that the servers are configured to use ephemeral Diffie-Hellman (DHE) key exchange with certificates that are signed using RSA. The auditor notes that even if the server's long-term RSA private key were compromised in the future, past communications would remain confidential. This property is primarily due to which cryptographic concept?
- AKey escrow
- BCertificate pinning
- CKey stretching
- DPerfect Forward Secrecy (PFS)
Show answer & explanationAnswer & explanation
Correct answer: D. Perfect Forward Secrecy (PFS)
Perfect Forward Secrecy (PFS) ensures that a compromise of long-term secret keys (like the server's RSA private key) does not compromise the confidentiality of past session keys. This is achieved by using ephemeral keys for each session, such as those generated by DHE, which are discarded after the session ends.
Why the other options are wrong
- A. Key escrow involves storing a copy of a private key, which would compromise past sessions if accessed.
- B. Certificate pinning helps prevent MITM attacks by pre-associating a host with its expected certificate, but doesn't guarantee confidentiality of past sessions if the server's private key is compromised.
- C. Key stretching is used to strengthen weak passwords against brute-force attacks, not to protect past session confidentiality from long-term key compromise.
Perfect Forward Secrecy (PFS)
A property of a key-agreement protocol that guarantees that if the long-term secret keys are compromised, past session keys remain secure and cannot be derived from the compromised long-term key. This is typically achieved using ephemeral keys.
- Protects past communication confidentiality.
- Achieved by using ephemeral session keys (e.g., DHE, ECDHE).
- Compromise of long-term key does not affect past session keys.
Memory trick: PFS makes sure past secrets stay secret, no matter what happens to the main key.