Symmetric Cryptography & Hashes
This domain tests how symmetric primitives are built and selected: block versus stream ciphers, DES/3DES/AES parameters, modes of operation, IV and nonce rules, and hash properties. Questions often turn on a single parameter, such as AES block size or the difference between a hash and a MAC.
Block and stream ciphers
- Block ciphers encrypt fixed-size blocks: DES uses a 64-bit block and a 56-bit key over 16 Feistel rounds; 3DES applies DES encrypt-decrypt-encrypt (EDE). Two-key 3DES gives about 112 bits of strength; three-key 3DES has a 168-bit key but roughly 112 bits of effective security against meet-in-the-middle attacks.
- AES is a substitution-permutation network with a 128-bit block. Key sizes 128/192/256 use 10/12/14 rounds. The block size is always 128 bits, regardless of key size.
- Stream ciphers generate a keystream and XOR it with data; reusing a keystream is catastrophic.
Modes and IVs
- ECB is deterministic and leaks equality of plaintext blocks; it should not be used for real data.
- CBC chains blocks and needs a random, unpredictable IV. The IV is not secret, but it must not repeat.
- CTR turns a block cipher into a stream cipher; a nonce/counter must never repeat under the same key.
- GCM is an AEAD mode, providing confidentiality and integrity in one construction.
Hashes and MACs
A cryptographic hash is one-way, collision-resistant and avalanche-prone. Preimage resistance is harder than collision resistance; collision attacks need about 2^(n/2) work. MD5 and SHA-1 are broken for collision resistance. HMAC combines a hash with a secret key for message authentication. Salts are unique per password and need not be secret; peppers are secret and shared.
Worked example
A payroll table stores salary amounts in fixed blocks. Under ECB, two employees with the same salary produce identical ciphertext blocks, so an attacker who can see the database learns which records share a value even without the key. Under CBC or GCM, the chaining or nonce makes those blocks differ. The problem is the mode, not AES itself.
Traps
- Saying a hash “encrypts” data; it is one-way and not reversible.
- Confusing key length with block size in AES.
- Treating the CBC IV as a secret; it must be unpredictable, not hidden.
- Believing a salt makes a weak password strong; it only defeats precomputation.
- Using ECB because it is simple or parallelizable.