SHA-1 Hash Function Overview

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Questions and Answers

What is the range of indices for the first set of values in the SHA-1 message schedule?

  • 1 to 15
  • 0 to 15 (correct)
  • 0 to 20
  • 1 to 16

How many rounds does the SHA-1 algorithm process each message block?

  • 40
  • 10
  • 20 (correct)
  • 80

In the SHA-1 message schedule, what is computed for the indices greater than 15?

  • A single value derived from $W_{j-16}$
  • $W_{j-3}$ xor with the sum of the previous three values
  • The direct copy of $W_{j-8}$
  • A combination of four previous words using XOR operations (correct)

What is the size of the message block used in SHA-1?

<p>512 bits (A)</p>
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Which of the following correctly describes the output word $W_j$ when $j$ is less than or equal to 15?

<p>It is initialized directly from the 512-bit message block. (C)</p>
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What is the main structure used by the SHA-1 algorithm to process message blocks?

<p>Merkle Damgård scheme (C)</p>
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How does the SHA-1 algorithm use previous hash values in its process?

<p>The previous hash value acts as a key for generating the next hash value. (D)</p>
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What is the role of the padding and length sequence in the SHA-1 algorithm?

<p>It ensures that the message blocks are of equal size. (B)</p>
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What is the final output of the SHA-1 algorithm referred to as?

<p>Message digest (D)</p>
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Which of the following statements about SHA-1's hash value is true?

<p>SHA-1 generates a larger hash value compared to its predecessors. (A)</p>
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Flashcards

SHA-1 message schedule

A process that generates 80 32-bit words (W0 to W79) from a 512-bit message block used in the SHA-1 algorithm.

Message Block

A 512-bit chunk of input data processed during each stage of SHA-1.

W(j)

A 32-bit word generated in the SHA-1 message schedule formula.

Round

A repeating step in the processing of a message block within a cryptographic hash function.

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SHA-1 Algorithm Stages

The four cyclical stages of the SHA-1 process that use 20 rounds to process message blocks.

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Message Schedule Formula

The computation for generating the 32-bit words W(j), used in each round, based on previous words in the message schedule

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Word Wj (j>15) calculation

Calculating a word from previous words within the message schedule(W(j-16), W(j-14), W(j-8), and W(j-3)).

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Hash Function

Transforms input data into fixed-size output data (hash).

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SHA-1

A cryptographic hash function that produces a message digest.

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Message Digest

Fixed-size output of a hash function, representing the input message.

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Merkle-Damgård Scheme

The overall structure of the SHA-1 algorithm, using a compression function.

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Compression Function (SHA-1)

Processes input data (previous hash & block) to transform to a new hash value.

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Hash Value

The result of a hash function's computation; a fixed-size string of bits.

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Message Block

A portion of the input message processed in each step of the algorithm.

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Iterative Nature of SHA-1

Each block's hash value is dependent on the previous.

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Padding and Length Sequence

Added to the end of the message to ensure fixed-size block inputs for the algorithm.

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Study Notes

SHA-1 Hash Function

  • SHA-1 produces a 160-bit message digest.
  • Preprocessing is needed before hash computation. The message is padded to fit a multiple of 512 bits. The initial value H₀ is set to a predefined constant.
  • The compression function processes messages in 512-bit chunks. The function consists of 80 rounds, divided into four stages of 20 rounds each.
  • Padding: The length of the message (l) is added to the padded message.
  • A message schedule computes 80 32-bit words (W₀, W₁, ..., W₇₉) from the 512-bit message block.
  • Wᵢ = { (Wᵢ₋₁₆ ⊕ Wᵢ₋₁₄ ⊕ Wᵢ₋₈ ⊕ Wᵢ₋₃) <<< 1 } for 16 ≤ j ≤ 79
  • Five working registers (A, B, C, D, E) of size 32 bits.
  • The hash value Hᵢ consists of five 32-bit words: H(0), H(1), H(2), H(3), H(4)
  • The hash value initially holds the initial value H₀.

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