Diffie-Hellman and Security+

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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

CompTIA Security+ Study Guide

The 501 Version of the Study Guide

The CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide is an update to the top-selling SY0-201, SY0-301, and SY0-401 study guides, which have helped thousands of readers pass the exam the first time they took it.  It includes the same elements readers raved about in the previous three versions.

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  • 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.

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Kindle edition also available.


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 for more info on ECDHE.

Security+ Practice Test Questions

SY0-501 Practice Test Questions 

Over 300 realistic Security+ practice test questions

All questions include explanations so you'll know why the correct answers are correct,

and why the incorrect answers are incorrect.

Pass the Security+ Exam

the First Time You Take It

Multiple quiz formats to let you use these questions based on the way you learn.
  • Learn mode - randomized. View each of the questions in random order. Learn mode allows you to keep selecting answers until you select the correct answer. Once you select the correct answer, you'll see the explanation. Click here to see how learn mode works.
  • Learn mode - not randomized. View each of the questions in the same order. Use this if you want to make sure that you see all of the questions. Learn mode allows you to keep selecting answers until you select the correct answer. Once you select the correct answer, you'll see the explanation. Click here to see how learn mode works.
  • Test mode - randomized. View each of the questions in random order. In test mode, you can only see the correct answers and explanations after you complete the test. Click here to see how test mode works.
  • Test mode - not randomized. View each of the questions in the same order. In test mode, you can only see the correct answers and explanations after you complete the test. Click here to see how test mode works.
  • Test mode - 75 random questions. View 75 random questions from the full test bank similar to how the Security+ exam has a potential maximum of 75 multiple choice questions. In test mode, you can only see the correct answers and explanations after you complete the test. Click here to see how test mode works.

Get the full bank of SY0-501 Practice Test Questions Here

 SY0-501 Practice Test Questions


INCLUDES QUESTIONS TO HELP YOU PREPARE

FOR THE NEW PERFORMANCE BASED QUESTIONS 

Bonus - Performance Based Questions

Three sets of performance-based questions including over 30 questions. These questions show you what you can expect in the live exam. They include drag and drop, matching, sorting, and fill in the blank questions. See a demo here.

Bonus - Extra Practice Test Questions

New multiple-choice questions in the extra test bank. Questions are added occasionally. You can see what has been added recently here.

Get the full bank of Security+ (SYO-501) Practice Test Questions Here

Get the full bank of Security+ Practice Test Questions

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CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide

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