EUC-EO flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Kerckhoffs's principle

A cryptosystem should remain secure even if everything about it except the key is public. Security depends on key secrecy, not algorithm secrecy.

Confusion and diffusion

Confusion makes the relationship between key and ciphertext complex, typically via substitution. Diffusion spreads plaintext influence across the ciphertext, typically via permutation and mixing.

One-time pad condition

The pad must be truly random, at least as long as the message, used exactly once, and kept secret. Violating any condition, especially reuse, breaks its proven security.

Stream cipher keystream reuse

XORing two ciphertexts encrypted with the same keystream yields the XOR of the plaintexts, which often allows recovery through crib-dragging and statistics.

AES key and block sizes

AES uses a fixed 128-bit block with 128-, 192-, or 256-bit keys and 10, 12, or 14 rounds respectively. It replaced DES as the standard block cipher.

ECB mode weakness

Electronic Codebook encrypts each block independently with the same key, so identical plaintext blocks produce identical ciphertext blocks, leaking structure and patterns.

CBC IV requirements

CBC needs a unique, unpredictable initialization vector per encryption under a given key. A predictable or reused IV enables chosen-plaintext attacks and leaks equality of first blocks.

GCM nonce reuse

Reusing a GCM nonce with the same key reveals the authentication key and the XOR of plaintexts, destroying both confidentiality and integrity guarantees.

Meet-in-the-middle attack

Against double encryption, the attacker computes and stores intermediate values from both directions, finding the key pair in roughly 2^(k+1) work instead of 2^(2k).

RSA hard problem

RSA security relies on the difficulty of factoring the modulus, the product of two large primes. Recovering the primes yields the private exponent.

Diffie-Hellman limitation

Diffie-Hellman establishes a shared secret over an open channel but does not authenticate participants, so it is vulnerable to man-in-the-middle unless combined with signatures or certificates.

Certificate revocation checking

CRLs publish revoked serial numbers periodically, while OCSP queries status in real time. OCSP stapling lets the server present a signed, cached response.

Certificate key usage extension

The key usage and extended key usage extensions restrict what a certified key may do, such as digital signature, key encipherment, server authentication, or code signing.

Hash collision resistance

Collision resistance means finding any two distinct inputs with the same digest is infeasible. By the birthday bound, an n-bit hash offers about n/2 bits of collision resistance.

HMAC construction

HMAC hashes the key XORed with an inner pad concatenated with the message, then hashes that result with the key XORed with an outer pad.

Length extension attack

Given hash(secret||message), an attacker can compute hash(secret||message||padding||extra) for Merkle-Damgård hashes without knowing the secret. HMAC prevents this.

Password salting purpose

A unique random salt per password prevents precomputed rainbow tables and hides identical passwords. It does not slow guessing, so a slow KDF is also required.

Digital signature process

The signer hashes the message and transforms the digest with a private key. The verifier recomputes the digest and checks the signature with the public key.

ECDSA nonce reuse failure

Reusing the same random k for two ECDSA or DSA signatures lets anyone solve for the private key from the two signatures and the known messages.

Certificate chain validation

Validation verifies each signature up to a trusted root, checks validity dates, key usage, basic constraints, name matching, and revocation status.

Certificate signing request

A CSR contains the subject's public key and identity attributes and is signed with the matching private key to prove possession before a CA issues a certificate.

Envelope encryption

A data key encrypts the data, and a key-encryption key encrypts the data key. This limits exposure and simplifies rotation of the root key.

Perfect forward secrecy

Ephemeral per-session key agreement ensures that compromise of a long-term private key does not expose previously recorded session keys or past traffic.

Hardware security module

An HSM is tamper-resistant hardware that generates, stores, and uses keys internally so private key material never appears in general-purpose memory.

Key zeroization

Zeroization securely erases key material by overwriting or destroying the storage medium, ensuring deleted keys cannot be recovered by forensic analysis.

TLS 1.3 key changes

TLS 1.3 removes RSA key transport and static Diffie-Hellman, mandates forward secrecy, and completes the handshake in one round trip.

IPSec ESP versus AH

ESP provides confidentiality plus integrity and authentication, while AH provides integrity and authentication of the packet header but no encryption.

SSH authentication layers

SSH authenticates the server with a host key and the client with a password or user key pair, after a key exchange that establishes session encryption.

S/MIME versus PGP trust

S/MIME relies on hierarchical X.509 certificates from CAs, while PGP uses a decentralized web of trust where users sign each other's public keys.

LSB steganography

Least significant bit embedding replaces low-order bits of carrier samples with message bits. It is high capacity and imperceptible but fragile to compression and resizing.

Steganalysis chi-square test

The chi-square attack detects LSB embedding by testing whether the frequency pairs of adjacent pixel values have been equalized, a statistical artifact of embedding.

Chosen-ciphertext attack

The attacker submits chosen ciphertexts to a decryption oracle and observes results. Adaptive versions model padding oracles and Bleichenbacher attacks on RSA.

Side-channel attack

Side-channel attacks infer keys from physical leakage such as timing, power consumption, cache behavior, or electromagnetic emissions rather than from mathematical weakness.