Data Encryption and Symmetric Block Ciphers

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If you’re planning on taking the Security+ exam, you should have a good understanding of symmetric algorithms that use block ciphers to encrypt data.

For example, can you answer this question?

Q. Which of the following algorithms encrypts data in 64-bit blocks?

A. AES

B. DES

C. Twofish

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.

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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.

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. NIST is a U.S. agency that develops and promotes standards. They spent about five years conducting a review of 15 different symmetric algorithms and identified AES as the best of the 15.

AES can use key sizes of 128 bits, 192 bits, or 256 bits, and it’s sometimes referred to as AES-128, AES-192, or AES-256 to identify how many bits are used in the key. When more bits are used, it makes it more difficult to discover the key. AES-128 provides strong protection, but AES-256 provides stronger protection.

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Because of its strengths, AES has been adopted in a wide assortment of applications. For example, many applications that encrypt data on USB drives use AES. Some of the strengths of AES are:

  • Fast. AES uses elegant mathematical formulas and only requires one pass to encrypt and decrypt data. In contrast, 3DES requires multiple passes to encrypt and decrypt data.
  • Efficient. AES is less resource intensive than other encryption algorithms such as 3DES. AES encrypts and decrypts quickly even when ciphering data on small devices, such as USB flash drives.
  • Strong. AES provides strong encryption of data providing a high level of confidentiality.

Remember this

AES is a strong symmetric block cipher that encrypts data in 128-bit blocks. AES uses 128-bit, 192-bit, or 256-bit keys.

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 is now considered trivial. DES is not recommended for use today.

Block Ciphers

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.

Remember this

DES and 3DES are block ciphers that encrypt data in 64-bit blocks. 3DES was originally designed as a replacement for DES, but NIST selected AES as the current standard. However, 3DES is still used in some applications, such as when legacy hardware doesn’t support AES.

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Q. Which of the following algorithms encrypts data in 64-bit blocks?

A. AES

B. DES

C. Twofish

D. RC4

Answer is B. Data Encryption Standard (DES) encrypts data in 64-bit blocks similar to how 3DES and Blowfish encrypt data in 64-bit blocks.

Advanced Encryption Standard (AES) and Twofish encrypt data in 128-bit blocks.

Rivest Cipher 4 (RC4) is a stream cipher and it encrypts data one bit at a time.

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You'll see a list of authentication methods and authentication factors along with instructions on what you might be required to do on the actual exam to match the authentication methods with the authentication factors. You'll then have six questions that test your knowledge and ability to correctly answer the questions. This set also includes a link to a graphic showing the end solution for the overall performance based question simulation.

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Get the full bank of Security+ Practice Test Questions

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

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