Diffie-Hellman might sound a little foreign to you, but if you plan to take the Security+ exam, it’s worth knowing what they are.
Diffie-Hellman Sample Question
As an example, can you answer this sample question from the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide
Q. Your organization is investigating possible methods of sharing encryption keys over a public network. Which of the following is the BEST choice?
A. CRL
B. PBKDF2
C. Hashing
D. ECDHE
Answer below
The 601 Version of the Study Guide
The CompTIA Security+: Get Certified Get Ahead: SY0-601 Study GuideEach of the eleven chapters presents topics in an easy to understand manner and includes real-world examples of security principles in action.
You’ll understand the important and relevant security topics for the Security+ exam, without being overloaded with unnecessary details. Additionally, each chapter includes a comprehensive review section to help you focus on what’s important.
Over 300 realistic practice test questions with in-depth explanations will help you test your comprehension and readiness for the exam. The book includes:
- A 75 question pre-test
- A 75 question post-test
- Practice test questions at the end of every chapter.
Each practice test question includes a detailed explanation to help you understand the content and the reasoning behind the question. You’ll be ready to take and pass the exam the first time you take it.
If you plan to pursue any of the advanced security certifications, this guide will also help you lay a solid foundation of security knowledge. Learn this material, and you’ll be a step ahead for other exams. This SY0-601 study guide is for any IT or security professional interested in advancing in their field, and a must-read for anyone striving to master the basics of IT security.
Diffie-Hellman
Diffie-Hellman is a key exchange algorithm used to privately share a symmetric key between two parties. Once the two parties know the symmetric key, they use symmetric encryption to encrypt the data.
Whitfield Diffie and Martin Hellman first published the Diffie-Hellman scheme in 1976. Interestingly, Malcolm J. Williamson secretly created a similar algorithm while working in a British intelligence agency. It is widely believed that the work of these three provided the basis for public-key cryptography.
Diffie-Hellman methods support both static keys and ephemeral keys.
Static Versus Ephemeral Keys
The two primary categories of asymmetric keys are static and ephemeral. In general, static keys are semipermanent and stay the same over a long period of time. In contrast, ephemeral keys have very short lifetimes and are re-created for each session.
Static Keys
RSA uses static keys. A certificate includes an embedded public key matched to a private key and this key pair is valid for the lifetime of a certificate, such as a year.
Certificates have expiration dates and systems continue to use these keys until the certificate expires. A benefit of static keys is that a CA can validate them with a CRL or using the Online Certificate Status Protocol (OCSP).
Ephemeral Keys
An ephemeral key pair includes a private ephemeral key and a public ephemeral key. However, systems use these key pairs for a single session and then discard them. Some versions of Diffie-Hellman use static keys and some versions use ephemeral keys.
Perfect Forward Secrecy
Perfect forward secrecy is an important characteristic that ephemeral keys comply with in asymmetric encryption. Perfect forward secrecy indicates that a cryptographic system generates random public keys for each session and it doesn’t use a deterministic algorithm to do so.
In other words, given the same input, the algorithm will create a different public key. This helps ensure that systems do not reuse keys.
Diffie-Hellman Versions
RSA is based on the Diffie-Hellman key exchange concepts using static keys.
Two Diffie-Hellman methods that use ephemeral keys are:
- DHE. Diffie-Hellman Ephemeral (DHE) uses ephemeral keys, generating different keys for each session. Some documents list this as Ephemeral Diffie-Hellman (EDH).
- ECDHE. Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) uses ephemeral keys generated using ECC.
Another version of Diffie-Hellman, Elliptic Curve Diffie-Hellman (ECDH), uses static keys.
Remember this
Ephemeral keys have very short lifetimes and are re-created for each session. In contrast, static keys are semipermanent and stay the same over a long period of time. Diffie-Hellman Ephemeral (DHE) and Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) both use ephemeral keys
Diffie-Hellman is a secure method of sharing symmetric encryption keys over a public network. Elliptic curve cryptography is commonly used with small wireless devices. ECDHE is a version of Diffie-Hellman that uses elliptic curve cryptography to generate encryption keys.
Diffie-Hellman Answer
Q. Your organization is investigating possible methods of sharing encryption keys over a public network. Which of the following is the BEST choice?
A. CRL
B. PBKDF2
C. Hashing
D. ECDHE
Answer below
D. Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) allows entities to negotiate encryption keys securely over a public network.
Password-Based Key Derivation Function 2 (PBKDF2) is a key stretching technique designed to make password cracking more difficult.
A certificate revocation list (CRL) identifies revoked certificates and is unrelated to sharing encryption keys.
Hashing methods do not support sharing encryption keys over a public network.
See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide
Pass the Security+ exam the first time!
CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide
