Cipher Deck

Confidential AND authenticated message (the full pipeline)

1. Sender hashes the message and signs the hash with their own private key (authentication).
2. Sender generates a random one-time symmetric session key.
3. Sender encrypts the message + signature with that symmetric key (fast, provides confidentiality).
4. Sender encrypts the symmetric key itself with the receiver's public key (secure key exchange).
5. Sender transmits the encrypted message bundle + the encrypted symmetric key.
6. Receiver decrypts the symmetric key with their own private key, then uses it to decrypt the message, then verifies the signature with the sender's public key.

ECB — Electronic Codebook

Each block is encrypted independently with the same key. No IV, no chaining, so identical plaintext blocks always produce identical ciphertext blocks. Weakest mode — patterns in the plaintext leak through.

CBC — Cipher Block Chaining

The IV is XORed with the first plaintext block before encryption. The resulting ciphertext block is then XORed with the next plaintext block before it's encrypted, and so on — each block's encryption depends on the one before it.

CFB — Cipher Feedback

The IV is encrypted (not the plaintext). That encrypted output is XORed with the plaintext block to make the ciphertext. That same ciphertext block is then fed back in and encrypted to produce the keystream for the next block. Turns a block cipher into a self-synchronizing stream cipher.

OFB — Output Feedback

The IV is encrypted, and that output is XORed with the plaintext block to make ciphertext — but unlike CFB, the encrypted output itself (not the ciphertext) is fed back and re-encrypted to generate the next block's keystream. The keystream can be precomputed independent of the data.

CTR — Counter

A nonce combined with an incrementing counter is encrypted, and the result is XORed with the plaintext block. Every block is completely independent, which makes CTR parallelizable and fast — no chaining dependency at all.

Caesar cipher

Simple substitution — every letter is shifted a fixed number of positions through the alphabet.

Vigenère / polyalphabetic cipher

Uses a repeating keyword to pick a different Caesar shift for each letter. Broken by the Kasiski examination, which looks for repeated sequences in the ciphertext — the distance between repeats reveals likely key lengths.

Playfair cipher

Encrypts pairs of letters (digraphs) using a 5×5 matrix built from a keyword (I and J share a cell).

Bifid cipher

Plots each letter onto coordinates in a Polybius square (mapping letters to numeric values), then fractionates and recombines those coordinates to diffuse a single letter's information across the ciphertext.

Brute force

Systematically tries every possible key until the correct one is found.

Differential cryptanalysis

Studies how differences in plaintext input propagate into differences in the resulting ciphertext, to recover information about the key.

Linear cryptanalysis

Builds linear approximations that describe the cipher's behavior well enough to recover key bits statistically.

Algebraic attack

Models the cipher as a system of equations (often Boolean) and solves the system to recover the key.

Rainbow table

Precomputed table of hash chains used to reverse hashed passwords quickly instead of brute forcing each one.

Birthday attack

Exploits the birthday paradox to find hash collisions faster than brute force would suggest.

SHA-1

Produces a 160-bit hash value.

LM hash / NTLM (as tested in this course)

The password is padded with null characters up to 14 characters, then the padded value is encrypted with DES to produce the hash. This padding-to-14 weakness is a big reason legacy LAN Manager–style hashing is considered broken.

Chaskey

A lightweight MAC algorithm built around a 128-bit permutation.

Hashing digital evidence

Investigators hash evidence at collection time so they can later prove the data was never altered — any change to the evidence produces a completely different hash value, demonstrating integrity in court.

PEM (Privacy Enhanced Mail)

Base64, text-based encoding format used for certificates and keys — the ".pem" file you see everywhere.

Key / certificate lifecycle

OCSP (Online Certificate Status Protocol)

Lets a client check in real time whether a specific certificate has been revoked, instead of downloading a full CRL.

TRNG (True Random Number Generator)

Generates randomness from unpredictable physical phenomena (thermal noise, radioactive decay, etc.), as opposed to a deterministic pseudo-random algorithm.

Clipper chip

1990s NSA-backed encryption chip that used the Skipjack algorithm with a built-in government key-escrow backdoor.

Forward secrecy

Ensures that if a long-term private key is ever compromised, past session traffic still can't be decrypted, because each session used its own ephemeral key (typically via ephemeral Diffie-Hellman) that was never derived from the long-term key.

Homomorphic encryption

Allows computations to be performed directly on encrypted data, producing an encrypted result that matches what you'd get computing on the plaintext — without ever revealing the plaintext.

Mobile network stream ciphers

2G GSM networks used A5/1 and the deliberately weaker export-grade A5/2 for over-the-air voice/data encryption. 3G (3GPP) networks moved to KASUMI, also known as A5/3.

Key length vs. performance vs. security

As key length increases: security improves, but performance (encryption/decryption speed) decreases.

Prime numbers

A number greater than 1 that is only evenly divisible by 1 and itself — the foundation of RSA and other asymmetric algorithms.

Blockchain basics

A blockchain is a distributed, append-only ledger — each block references and cryptographically hashes the block before it, so altering old data breaks every block after it. Bitcoin adds a new block roughly every 10 minutes, and as the chain grows longer the mining reward decreases over time (block reward halving), even though total network difficulty tends to rise.

Bitcoin vs. Ethereum

Bitcoin uses proof-of-work mining (solving hash puzzles) purely to move currency. Ethereum also runs a blockchain but adds smart contracts — self-executing code stored on-chain — and has since moved from proof-of-work to proof-of-stake, where validators are chosen based on coins they lock up rather than computing power spent.