Symmetric Encryption Protocols

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Symmetric encryption uses the same key to encrypt and decrypt data. If you’re planning to take the Security+ exam, you should have a basic understanding of appropriate cryptographic methods. This includes using symmetric encryption protocols.

For example, can you answer this question?

Q. Your organization is planning to implement videoconferencing, but it wants to protect the confidentiality of the streaming video. Which of the following would BEST meet this need?

A. PBKDF2

B. DES

C. MD5

D. RC4

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

AES

The Advanced Encryption Standard (AES) is a strong symmetric block cipher that encrypts data in 128-bit blocks. The National Institute of Standards and Technology (NIST) adopted AES from the Rijndael encryption algorithm after a lengthy evaluation of several different algorithms. It is one of the primarily used symmetric encryption protocols in use today.

 

 

DES

Data Encryption Standard (DES) is a symmetric block cipher that was widely used for many years, dating back to the 1970s. It encrypts data in 64-bit blocks. However, it uses a relatively small key of only 56 bits and can be broken with brute force attacks. In the ’70s, the technology required to break 56-bit encryption wasn’t easily available, but with the advances in computer technology, a 56-bit key has been considered trivial for many years. DES is not recommended for use today.

3DES

3DES (pronounced as “Triple DES”) is a symmetric block cipher designed as an improvement over the known weaknesses of DES. In basic terms, it encrypts data using the DES algorithm in three separate passes and uses multiple keys. Just as DES encrypts data in 64-bit blocks, 3DES also encrypts data in 64-bit blocks.

Although 3DES is a strong algorithm, it isn’t used as often as AES today. AES is much less resource intensive. However, if hardware doesn’t support AES, 3DES is a suitable alternative. 3DES uses key sizes of 56 bits, 112 bits, or 168 bits.

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The table below summarizes the important symmetric protocols covered in the CompTIA Security+ exam.

Symmetric Encryption Protocols

Symmetric encryption protocols

Remember this

If you can recognize the symmetric algorithms such as AES, DES, 3DES, Blowfish, and Twofish, it will help you answer many exam questions. For example, if a question asks what you would use to hash and it lists encryption algorithms, you can quickly eliminate them because they don’t encrypt data. You should also know the size of the blocks and the size of the keys listed in table.

 

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RC4

Ron Rivest invented several versions of RC, which are sometimes referred to as Ron’s Code or Rivest Cipher. The most commonly used version is RC4 (also called ARC4), which is a symmetric stream cipher and it can use between 40 and 2,048 bits.

It’s worthwhile pointing out that this is the same RC4 used in WEP. WEP’s vulnerabilities weren’t because it used RC4, but instead because it did not follow a basic rule of a stream cipher: Don’t reuse keys.

When implemented correctly, RC4 has enjoyed a long life as a strong cipher. For many years, it has been the recommended encryption mechanism in Secure Sockets Layer (SSL) and Transport Layer Security (TLS). SSL and TLS encrypt Hypertext Transfer Protocol Secure (HTTPS) connections on the Internet.

However, experts have speculated since 2013 that agencies such as the U.S. National Security Agency (NSA) can break RC4, even when implemented correctly such as in TLS. Because of this, companies such as Microsoft recommend disabling RC4 and using AES instead. Even though AES is a block cipher and RC4 is a stream cipher, TLS can implement either one.

Note that while RC4 is at the end of its lifetime, the SY0-401 exam objectives (and many questions) were created in 2014 and RC4 was still used then. In other words, while many security experts recommend NOT using RC4 today, the exam doesn’t necessarily keep up with the security experts.

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Blowfish and Twofish

Blowfish is a strong symmetric block cipher that is still widely used today. It encrypts data in 64-bit blocks and supports key sizes between 32 and 448 bits. Bruce Schneier (a widely respected voice in IT security) designed Blowfish as a general-purpose algorithm to replace DES.

Interestingly, Blowfish is actually faster than AES in some instances. This is especially true when comparing Blowfish with AES-256. Part of the reason is that Blowfish encrypts data in smaller 64-bit blocks, whereas AES encrypts data in 128-bit blocks.

Twofish is related to Blowfish, but it encrypts data in 128-bit blocks and it supports 128-, 192-, or 256-bit keys. It was one of the finalist algorithms evaluated by NIST for AES. However, NIST selected another algorithm (Rijndael) as AES.

Remember this

RC4 is a strong symmetric stream cipher. Blowfish is a 64-bit block cipher and Twofish is a 128-bit block cipher. Although NIST recommends AES as the standard, Blowfish is faster than AES-256.


 

Q. Your organization is planning to implement videoconferencing, but it wants to protect the confidentiality of the streaming video. Which of the following would BEST meet this need?

A. PBKDF2

B. DES

C. MD5

D. RC4

Answer is D. Rivest Cipher 4 (RC4) is a symmetric encryption stream cipher, and a stream cipher is often the best choice for encrypting data of an unknown size, such as streaming video. Encryption is the best way to ensure the confidentiality of data.

Even though RC4 is at the end of its lifetime, the SY0-401 exam objectives (and many questions) were created in 2014 and RC4 was still used then. This is also a great example of picking the BEST possible answer of the available choices. If you take the exam, CompTIA won’t allow you to insert your own BEST answer.

Password-Based Key Derivation Function 2 (PBKDF2) is a key stretching technique designed to protect passwords against brute force attempts and is not used for streaming data.

Data Encryption Standard (DES) is an older block cipher that is not secure and wasn’t secure even when the SY0-401 objectives were released in 2014.

Message Digest 5 (MD5) is a hashing algorithm used for integrity.

See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information cryptography.

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