For a simple one-round block cipher with an S-box, if the input difference to the S-box is 0x1 and the output difference is 0x2 with probability 1/4, what does this pair (0x1 → 0x2) represent?

A one-round differential characteristic for that S-box.

The key schedule of the cipher.

A linear approximation of the S-box.

A security analyst is trying to break a cipher that uses a different substitution alphabet for each plaintext position (a polyalphabetic cipher). The analyst has a large amount of ciphertext. Which approach is most likely to succeed?

Determine the key length using the Kasiski examination or index of coincidence, then perform frequency analysis on each key-position group

Apply single-letter frequency analysis directly to the ciphertext

Try all possible keys by brute force

Use differential cryptanalysis to recover the key

EC-CouncilCertified Encryption Specialist

Domain 5Objective 1

Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 5)

Part of the Cryptanalysis domain, which makes up ~14% of our current practice bank. EC-Council does not publish an official question count, but from its 120-minute exam (~50–80 total, ~7–11 in this domain), expect 2–4 from this objective — we provide 29 practice questions to prepare you well beyond it. (estimate)

29questions here

6free pages

6concepts

Questions 21–25

21application · medium
An analyst is decrypting a monoalphabetic substitution ciphertext. The analyst has determined that the ciphertext symbol 'B' appears most frequently, and the symbol 'C' appears next most frequently. The analyst also notices that the digraph 'BC' appears very often. Based on English language statistics, what is the most likely plaintext for the digraph 'BC'?
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22application · medium
An analyst is constructing a differential characteristic for a 3-round SPN cipher. The analyst has the following round differential probabilities: round 1: 1/4, round 2: 1/8, round 3: 1/2. What is the probability of the full 3-round characteristic?
Select an answer first
23foundation · easy
For a simple one-round block cipher with an S-box, if the input difference to the S-box is 0x1 and the output difference is 0x2 with probability 1/4, what does this pair (0x1 → 0x2) represent?
Select an answer first
24application · easy
An analyst is analyzing a ciphertext and finds that the most common digraph is 'QW'. In English, the most common digraphs are 'TH', 'HE', 'IN', 'ER', and 'AN'. What is the most likely plaintext digraph for 'QW'?
Select an answer first
25application · medium
A security analyst is trying to break a cipher that uses a different substitution alphabet for each plaintext position (a polyalphabetic cipher). The analyst has a large amount of ciphertext. Which approach is most likely to succeed?
Select an answer first

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Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 5)

Questions 21–25

An analyst is constructing a differential characteristic for a 3-round SPN cipher. The analyst has the following round differential probabilities: round 1: 1/4, round 2: 1/8, round 3: 1/2. What is the probability of the full 3-round characteristic?

For a simple one-round block cipher with an S-box, if the input difference to the S-box is 0x1 and the output difference is 0x2 with probability 1/4, what does this pair (0x1 → 0x2) represent?

An analyst is analyzing a ciphertext and finds that the most common digraph is 'QW'. In English, the most common digraphs are 'TH', 'HE', 'IN', 'ER', and 'AN'. What is the most likely plaintext digraph for 'QW'?

A security analyst is trying to break a cipher that uses a different substitution alphabet for each plaintext position (a polyalphabetic cipher). The analyst has a large amount of ciphertext. Which approach is most likely to succeed?

Keep studying — Cryptanalysis