Symmetric Encryption Concepts

Posted by in Security+ | 0 comments

Symmetric encryption is a type of encryption that uses a single key to encrypt and decrypt data. If you’re planning to take the SY0-501 exam, you should have a basic understanding of cryptography concepts such as cipher types and cipher modes.

For example, can you answer this practice test question?

Q. Which of the following is a symmetric encryption algorithm that encrypts data 1 bit at a time?

A. Block cipher

B. Stream cipher

C. AES

D. DES

E. MD5

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation are available at the end of this post.

Block Versus Stream Ciphers

Most symmetric algorithms use either a block cipher or a stream cipher. They are both symmetric, so they both use the same key to encrypt or decrypt data. However, they divide data in different ways.

A block cipher encrypts data in specific-sized blocks, such as 64-bit blocks or 128-bit blocks. The block cipher divides large files or messages into these blocks and then encrypts each individual block separately. A stream cipher encrypts data as a stream of bits or bytes rather than dividing it into blocks.

In general, stream ciphers are more efficient than block ciphers when the size of the data is unknown or sent in a continuous stream, such as when streaming audio and video over a network. Block ciphers are more efficient when the size of the data is known, such as when encrypting a file or a specific-sized database field.

An important principle when using a stream cipher is that encryption keys should never be reused. If a key is reused, it is easier to crack the encryption.

Remember this

Stream ciphers encrypt data a single bit, or a single byte, at a time in a stream. Block ciphers encrypt data in a specific-sized block such as 64-bit or 128-bit blocks. Stream ciphers are more efficient than block ciphers when encrypting data in a continuous stream.

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.

Each 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 SY0-501 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.

Kindle edition also available.

Cipher Modes

Block ciphers can use a variety of different modes of operation. It’s important to have a basic understanding of these modes when choosing cipher suites.

The Electronic Codebook (ECB) mode of operation is the simplest cipher mode mentioned in this blog. Algorithms that use ECB divide the plaintext into blocks and then encrypt each block using the same key. This represents a significant weakness. If any of the plaintext blocks are the same, the resulting ciphertext is the same, making it much easier to crack. ECB is not recommended for use in any cryptographic protocols today.

Cipher Block Chaining (CBC) mode is used by some symmetric block ciphers. It uses an IV for randomization when encrypting the first block. It then combines each subsequent block with the previous block using an XOR operation. Because encryption of each block is dependent on the encryption of all previous blocks, CBC sometimes suffers from pipeline delays, making it less efficient than some other modes.

Symmetric Encryption Algorithm

Counter (CTM) mode effectively converts a block cipher into a stream cipher. It combines an IV with a counter and uses the result to encrypt each plaintext block. Each block uses the same IV, but CTM combines it with the counter value, resulting in a different encryption key for each block. Multiprocessor systems can encrypt or decrypt multiple blocks at the same time, allowing the algorithm to be quicker on multiprocessor or multicore systems. CTM is widely used and respected as a secure mode of operation.

It’s worthwhile noting that the CompTIA objectives list CTM and include CTM in the acronym list as Counter-Mode. However, it’s much more common to see it listed as CTR or CM.

Galois/Counter Mode (GCM) is a mode of operation used by many block ciphers. It combines the Counter mode of operation with the Galois mode of authentication. Note that it doesn’t authenticate users or systems, but instead provides data authenticity (integrity) and confidentiality. In addition to encrypting the data for confidentiality, it includes hashing techniques for integrity. It is widely used due to its efficiency and performance, allowing systems to quickly encrypt and decrypt data.

Remember this

The Electronic Codebook (ECB) mode of operation is deprecated and should not be used. Cipher Block Chaining (CBC) mode combines each block with the previous block when encrypting data and sometimes suffers from pipeline delays. Counter (CTM) mode combines an IV with a counter to encrypt each block. Galois/Counter Mode (GCM) combines Counter mode with hashing techniques for integrity.


Full Security+ Course

SY0-501 Full Security+ Course

Helping you Pass the First Time

Online access includes all of the content from the

CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide

  • Introduction
  • About the exam (including information on the number of questions, test duration, passing score, types of questions and more. Also includes a listing of the exam objectives)
  • 75 question pre-assessment exam
  • Mastering Security Basics (full content from Chapter 1 of the study guide including the exam topic review and 15 practice test questions)
  • Understanding Identity and Access Management (full content from Chapter 2 of the study guide including the exam topic review and 15 practice test questions)
  • Exploring Network Technologies and Tools (full content from Chapter 3 of the study guide including the exam topic review and 15 practice test questions)
  • Securing Your Network (full content from Chapter 4 of the study guide including the exam topic review and 15 practice test questions)
  • Securing Hosts and Data (full content from Chapter 5 of the study guide including the exam topic review and 15 practice test questions)
  • Comparing Threats, Vulnerabilities, and Common Attacks (full content from Chapter 6 of the study guide including the exam topic review and 15 practice test questions)
  • Protecting Against Advanced Attacks (full content from Chapter 7 of the study guide including the exam topic review and 15 practice test questions)
  • Using Risk Management Tools (full content from Chapter 8 of the study guide including the exam topic review and 15 practice test questions)
  • Implementing Controls to Protect Assets (full content from Chapter 9 of the study guide including the exam topic review and 15 practice test questions)
  • Understanding Cryptography and PKI (full content from Chapter 10 of the study guide including the exam topic review and 15 practice test questions)
  • Implementing Policies to Mitigate Risks (full content from Chapter 11 of the study guide including the exam topic review and 15 practice test questions)
  • 75 question post-assessment exam
  • Glossary

Get the SY0-501 Full Security+ Course Here


Test your readiness with these quality materials

Random 75-question tests

Random practice tests from the all of the practice test questions in the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide. All questions include explanations so you'll know why the correct answers are correct, and why the incorrect answers are incorrect.

3 sets Performance-based Questions

Three new sets of performance-based questions with a total of 30 questions. These new questions use a new testing engine that includes realistic drag and drop, matching, sorting, and fill in the blank questions.

Flashcard Set

  • 494 Online Security+ Glossary Flashcards
  • 222 Online Security+ Acronyms Flashcards
  • 223 Online Security+ Remember This Slide from the popular CompTIA Security+ Get Certified Get Ahead: SY0-501 Study Guide

Audio - SY0-501 Security+ Remember This Audio Files

Learn by Listening. Over one hour and 20 minutes of audio (MP3 downloads.)

Audio - SY0-501 Security+ Question and Answer Audio Files

Learn by Listening. Over two hours hour and 53 minutes of audio (MP3 downloads.)

Bonus #1

Audio from the end of chapter reviews from each of the chapters in the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide. Over one hour and 40 minutes of additional audio.

Bonus #2

Access to all of the online content that is available for free to anyone that purchases the CompTIA Security+ Get Certified Get Ahead: SY0-501 Study Guide. This includes labs, extra practice test questions, and supplementary materials

Bonus #3 

Access the study materials for a total of 60 days because sometimes life happens.

Get the SY0-501 Full Security+ Course Here

Q. Which of the following is a symmetric encryption algorithm that encrypts data 1 bit at a time?

A. Block cipher

B. Stream cipher

C. AES

D. DES

E. MD5

Answer is B. A stream cipher encrypts data a single bit or a single byte at a time and is more efficient when the size of the data is unknown, such as streaming audio or video.

A block cipher encrypts data in specific-sized blocks, such as 64-bit blocks or 128-bit Advanced Encryption Standard (AES) and Data Encryption Standard (DES) are block ciphers.

Message Digest 5 (MD5) is a hashing algorithm.

See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide for more information on cryptography concepts.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

CompTIA Security+ Get Certified Get Ahead: SY0-501 Study Guide

Subscribe To Our Newsletter

Join our mailing list and get a free excerpt of the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide.  This excerpt includes the introduction and Chapter 1. 

You have Successfully Subscribed!

Get Certified Get Ahead is a participant in the Amazon Services LLC Associates Program,
an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.

Copyright © 2015 Get Certified Get Ahead. All Rights Reserved.