Symmetric Cryptography and Hashes

Free EC-Council Certified Encryption Specialist practice — 6 questions on Symmetric Cryptography and Hashes, with explanations. No sign-up. Full 12-question mixed test →

Question 1 of 6 · Symmetric Cryptography and Hashes

A streaming video service encrypts video segments so that any client can seek to an arbitrary point in the stream and decrypt only the segment it needs, without decrypting or even receiving any preceding segments. Which block cipher mode of operation BEST supports this requirement?

In CTR mode, the keystream block for position i is E(nonce || counter=i), computed entirely independently of every other block. A client can jump straight to segment 1000 and generate the correct keystream with no data from any other segment, giving true random access.

Question 2 of 6 · Symmetric Cryptography and Hashes

DES has a small set of keys for which the key schedule generates the identical 48-bit round subkey for all 16 rounds, meaning encryption with such a key is its own inverse. How many of these 'weak keys' exist among DES's key space?

DES has exactly 4 weak keys — those where the two 28-bit C and D halves are all zeros, all ones, or one half all zeros and the other all ones — causing every round to produce the same subkey, so E(k)=D(k).

Question 3 of 6 · Symmetric Cryptography and Hashes

The AES (Rijndael) S-box provides strong resistance to differential and linear cryptanalysis primarily because it is constructed by combining which two mathematical operations over GF(2^8)?

The Rijndael S-box first computes the multiplicative inverse of each byte in GF(2^8) (with 0 mapped to itself), then applies a fixed affine transformation over GF(2). The inverse function gives strong nonlinearity, and the affine transform prevents the S-box from having a simple algebraic description, together giving high resistance to differential and linear attacks.

Question 4 of 6 · Symmetric Cryptography and Hashes

A developer must derive a strong 256-bit encryption key from a user-supplied passphrase, and the derivation process must be deliberately slow and tunable to resist brute-force and dictionary attacks. Which construction is designed for this purpose?

PBKDF2 (Password-Based Key Derivation Function 2) is explicitly designed for key stretching: it repeatedly applies an underlying PRF (typically HMAC) with a salt over many iterations, deliberately increasing computational cost to slow down offline brute-force and dictionary attacks.

Question 5 of 6 · Symmetric Cryptography and Hashes

Per current OWASP password storage guidance, when PBKDF2-HMAC-SHA256 is used for password hashing, what is the minimum recommended iteration count?

OWASP's Password Storage Cheat Sheet recommends a minimum of 600,000 iterations for PBKDF2-HMAC-SHA256 to keep offline cracking sufficiently expensive given modern hardware.

Question 6 of 6 · Symmetric Cryptography and Hashes

WEP encrypts wireless frames using RC4 with a 24-bit initialization vector (IV) prepended to a static key. Researchers demonstrated the Fluhrer-Mantin-Shamir (FMS) attack, allowing full key recovery after capturing enough frames. Which weakness in RC4 made this attack possible?

The FMS attack exploits the fact that RC4's KSA generates certain 'weak' IV/key combinations where the first bytes of the keystream leak statistical information correlated with the key bytes. By capturing many frames using weak IVs and analyzing the first keystream byte, attackers could recover the static key.

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