AES

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Questions and Answers

Which of the following is a true statement about modern symmetric block cipher design principles?

  • Confusion refers to spreading the information and any minor changes in plaintext should lead to a large change in the ciphertext. (correct)
  • Confusion and diffusion are achieved by substitutions only.
  • Permutations act on smaller groups of bits.
  • Diffusion refers to hiding the relation between ciphertext and key.

What is the main reason DES is no longer considered secure?

  • Its block length is too short.
  • Its key length is too short and advancements in computing power made it vulnerable to exhaustive keysearch. (correct)
  • It is too slow for modern encryption needs.
  • It uses a weak substitution-permutation network.

What is the block length of AES?

  • 128 bits (correct)
  • 256 bits
  • 64 bits
  • 192 bits

What is the maximum key length for AES?

<p>256 bits (B)</p> Signup and view all the answers

What is the purpose of the Mix Columns step in AES encryption?

<p>To mix the columns of the state matrix (A)</p> Signup and view all the answers

What is the purpose of the Add Round Key step in AES encryption?

<p>To XOR the round key with the current block (D)</p> Signup and view all the answers

What is the purpose of the Shift Rows step in AES encryption?

<p>To shift rows cyclically (B)</p> Signup and view all the answers

What is the purpose of the SubBytes step in AES encryption?

<p>To substitute bytes using a fixed table (C)</p> Signup and view all the answers

What is the main difference between DES and Triple DES?

<p>Triple DES uses three different keys for encryption and decryption. (C)</p> Signup and view all the answers

What is the purpose of the key expansion algorithm in AES?

<p>To generate the round keys from the original key. (D)</p> Signup and view all the answers

What is the main advantage of using a variable key length in symmetric block ciphers?

<p>It makes the cipher more secure. (D)</p> Signup and view all the answers

What is the main reason DES is no longer considered secure?

<p>Its key length is too short and advancements in computing power made it vulnerable to exhaustive keysearch. (C)</p> Signup and view all the answers

What is the block length of AES?

<p>128 bits (C)</p> Signup and view all the answers

What is the purpose of the Mix Columns step in AES encryption?

<p>To mix the columns of the state matrix (B)</p> Signup and view all the answers

What is the purpose of the Shift Rows step in AES encryption?

<p>To shift rows cyclically (D)</p> Signup and view all the answers

What is the purpose of the SubBytes step in AES encryption?

<p>To substitute bytes using a fixed table (A)</p> Signup and view all the answers

What is the purpose of confusion in modern symmetric block cipher design?

<p>To hide the relation between plaintext and key (A)</p> Signup and view all the answers

What is the purpose of diffusion in modern symmetric block cipher design?

<p>To spread the information in the plaintext (D)</p> Signup and view all the answers

What is the combination of substitutions and permutations used for in modern symmetric block cipher design?

<p>To achieve confusion and diffusion (C)</p> Signup and view all the answers

What is the key length requirement for Data Encryption Standard (DES)?

<p>56 or 40 bits (D)</p> Signup and view all the answers

What made Data Encryption Standard (DES) vulnerable to exhaustive keysearch?

<p>Advancements in computing power (D)</p> Signup and view all the answers

What is the maximum key length requirement for Advanced Encryption Standard (AES)?

<p>256 bits (B)</p> Signup and view all the answers

What is the order of the operations used for ENCRYPTION in Advanced Encryption Standard (AES)?

<p>Substitute Bytes, Shift Rows, Mix Columns, Add Round Key (D)</p> Signup and view all the answers

What is the order of the INVERSE operations used for decryption in Advanced Encryption Standard (AES)?

<p>Inverse Substitute Bytes, Inverse Shift Rows, Inverse Mix Columns, Add Round Key (A)</p> Signup and view all the answers

Flashcards

What is Confusion?

Spreading information so minor plaintext changes cause large ciphertext changes.

Why is DES insecure?

Its key length is too short; computing power enables exhaustive keysearch.

AES Block Length?

128 bits

Max AES Key Length?

256 bits

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Mix Columns Purpose?

Mixes columns of the state matrix.

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Add Round Key Purpose?

To XOR the round key with the current block.

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Shift Rows Purpose?

Shifts rows cyclically.

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SubBytes Purpose?

Substitutes bytes using a fixed table.

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DES vs Triple DES?

Triple DES uses three different keys for encryption and decryption.

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Key Expansion Purpose?

To generate the round keys from the original key.

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Advantage: Variable Key Length?

It makes the cipher more secure.

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Purpose of Confusion?

To hide the relation between plaintext and key.

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Purpose of Diffusion?

To spread the information in the plaintext.

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Substitutions & Permutations?

To achieve confusion and diffusion.

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DES Key Length?

56 or 40 bits

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DES Vulnerability?

Advancements in computing power

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AES Max Key Length?

256 bits

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AES Encryption Order?

Substitute Bytes, Shift Rows, Mix Columns, Add Round Key

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AES Decryption Order?

Inverse Substitute Bytes, Inverse Shift Rows, Inverse Mix Columns, Add Round Key

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Study Notes

Symmetric Block Cipher Design Principles

  • In modern symmetric block cipher design, the principles of confusion and diffusion are used to ensure security.

AES (Advanced Encryption Standard)

  • The block length of AES is 128 bits.
  • The maximum key length for AES is 256 bits.
  • The purpose of the Mix Columns step is to provide diffusion by mixing the columns of the state matrix.
  • The purpose of the Add Round Key step is to add the round key to the state matrix.
  • The purpose of the Shift Rows step is to provide diffusion by shifting the rows of the state matrix.
  • The purpose of the SubBytes step is to provide confusion by substituting the bytes of the state matrix.

DES (Data Encryption Standard)

  • The main reason DES is no longer considered secure is that its key length (56 bits) is too short and can be broken by exhaustive key search.
  • The key length requirement for DES is 56 bits.
  • What made DES vulnerable to exhaustive key search is its short key length.

Triple DES

  • The main difference between DES and Triple DES is that Triple DES uses three iterations of the DES algorithm with three different keys.

Key Management

  • The purpose of the key expansion algorithm in AES is to expand the user-provided key into a series of round keys.
  • The main advantage of using a variable key length in symmetric block ciphers is that it provides more flexibility and security.

AES Operations

  • The order of the operations used for encryption in AES is: Add Round Key, SubBytes, Shift Rows, Mix Columns.
  • The order of the inverse operations used for decryption in AES is the reverse of the encryption operations.

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