Cryptography Glossary
Every term a codebreaker needs to know
72 terms
A
Algorithm
MathematicsA precise, step-by-step procedure for performing an operation. In cryptography, the algorithm defines how plaintext is transformed into ciphertext and back. Unlike a key, the algorithm itself is usually public — security depends on the key, not the secrecy of the method (Kerckhoffs's Principle).
Asymmetric Encryption
Modern Cryptography · 1976A system using two mathematically linked keys: a public key for encryption and a private key for decryption. Anyone can encrypt a message to you, but only you can decrypt it. RSA (1977) and Diffie-Hellman (1976) are the foundational asymmetric systems.
See also: Public Key, Key PairAvalanche Effect
Claude Shannon · 1949The property where a small change in the input (flipping one bit of plaintext or key) causes a large, unpredictable change in the output. Good ciphers exhibit strong avalanche: changing one bit should flip roughly half the output bits. AES achieves full avalanche within 2 rounds.
B
Block Cipher
Modern CryptographyA cipher that operates on fixed-size groups (blocks) of bits. AES uses 128-bit blocks. The same plaintext block always produces the same ciphertext block under the same key — unless a mode of operation like CBC or GCM is used to introduce variation.
See also: Stream Cipher, Mode of OperationBrute Force Attack
CryptanalysisTrying every possible key until the correct one is found. The time required grows exponentially with key length: a 56-bit key (DES) requires at most 2⁵⁶ attempts (~72 quadrillion); a 256-bit key (AES-256) requires 2²⁵⁶ — more than the atoms in the observable universe.
C
Cipher
Arabic: sifr (zero) · Latin: cifraAn algorithm for performing encryption or decryption — a system of rules for transforming readable text (plaintext) into an unreadable form (ciphertext) and back. Differs from a code, which replaces entire words or phrases rather than individual letters.
Ciphertext
Core ConceptThe encrypted output produced by a cipher. Without the correct key, ciphertext should be indistinguishable from random noise. The goal of cryptanalysis is to recover the plaintext from the ciphertext without knowledge of the key.
See also: PlaintextConfusion
Claude Shannon · 1949The property that makes the relationship between the key and ciphertext as complex as possible. Each bit of ciphertext should depend on many bits of the key. Substitution operations (S-boxes in AES) provide confusion. Paired with diffusion, it forms the foundation of modern cipher design.
See also: DiffusionCrib
Bletchley Park · WWIIA known or guessed piece of plaintext used to help break a cipher. At Bletchley Park, weather report formats ("WETTER") and standard military phrases ("HEIL HITLER") served as cribs against Enigma. Modern equivalent: known-plaintext attack.
Cryptanalysis
Greek: kryptós (hidden) + analysisThe study of breaking ciphers — recovering plaintext or keys without authorized access. Techniques range from frequency analysis (850 AD) to differential cryptanalysis (1990) to side-channel attacks. The discipline that keeps cryptography honest.
Cryptography
Greek: kryptós (hidden) + graphein (to write)The practice and study of techniques for securing communication in the presence of adversaries. Encompasses encryption, authentication, integrity verification, and non-repudiation. Cryptology is the umbrella field covering both cryptography and cryptanalysis.
D
Decryption
Core ConceptThe reverse of encryption: transforming ciphertext back into readable plaintext using the correct key. In symmetric ciphers, the same key encrypts and decrypts. In asymmetric ciphers, the private key decrypts what the public key encrypted.
Diffusion
Claude Shannon · 1949The property that spreads the influence of each plaintext bit across many ciphertext bits. Changing one letter of input should change many letters of output. Transposition and mixing operations (MixColumns in AES) provide diffusion. Paired with confusion, the two concepts underpin all modern encryption.
See also: Confusion, Avalanche EffectDigraph
Linguistics / CryptographyA pair of letters treated as a single unit. The Playfair cipher encrypts digraphs rather than individual letters, which disrupts single-letter frequency analysis. Common English digraphs: TH, HE, IN, ER, AN.
E
Encryption
Core ConceptThe process of converting plaintext into ciphertext using a cipher and a key. The result is unintelligible without the corresponding decryption key. Encryption protects confidentiality — one of the three pillars of information security (CIA triad).
Entropy
Information Theory · Claude Shannon · 1948A measure of randomness or uncertainty in a system. In cryptography, higher entropy means more unpredictability. A key with n bits of entropy has 2ⁿ equally likely values. A strong 256-bit key has 256 bits of entropy; a password like "password123" has far fewer, regardless of length.
F
Fractionation
19th Century · DelastelleA technique that breaks each plaintext letter into multiple components (e.g., coordinates on a grid), manipulates the components separately, then recombines them. Used in Bifid, Trifid, and ADFGVX ciphers. The ancestor of modern bit-level diffusion.
Frequency Analysis
Al-Kindi · Baghdad · ~850 ADThe study of letter (or symbol) frequency in ciphertext. In English, E appears ~12.7% of the time, T ~9.1%, A ~8.2%. If a cipher preserves these frequencies — as all monoalphabetic substitutions do — counting letter occurrences reveals the key. The first systematic cryptanalysis technique ever described.
H
Hash Function
Modern CryptographyA one-way function that maps input of any size to a fixed-size output (digest). Cryptographic hash functions must be: (1) preimage-resistant (can't reverse), (2) collision-resistant (can't find two inputs with the same hash), (3) avalanche-exhibiting (small input change → completely different hash). SHA-256 produces a 256-bit digest.
Homophonic Substitution
15th–16th CenturyA substitution cipher where each plaintext letter can map to multiple different ciphertext symbols, roughly proportional to the letter's frequency. E might map to 13 different symbols, Z to 1. This flattens frequency distributions, making frequency analysis much harder — but not impossible.
I
Index of Coincidence (IC)
William Friedman · 1922The probability that two randomly chosen letters from a text are the same. English text has IC ≈ 0.0667; random text ≈ 0.0385. Used to estimate whether a cipher is monoalphabetic or polyalphabetic, and to determine the key length in Vigenère-type ciphers.
Initialization Vector (IV)
Modern CryptographyA random or pseudo-random value used to ensure that encrypting the same plaintext with the same key produces different ciphertext each time. Required by block cipher modes like CBC and GCM. The IV is not secret but must not be reused with the same key.
K
Kasiski Examination
Friedrich Kasiski · 1863A technique for determining the key length of a Vigenère cipher. Find repeated sequences in the ciphertext, measure the distances between them, and compute the GCD of those distances. The GCD is likely the key length (or a multiple). Once you know the key length, each column becomes a simple Caesar cipher.
Kerckhoffs's Principle
Auguste Kerckhoffs · 1883"A cryptosystem should be secure even if everything about the system, except the key, is public knowledge." The opposite of security through obscurity. Modern corollary: AES, RSA, and SHA are all published algorithms — their security relies entirely on the secrecy and strength of the keys.
Key
Core ConceptThe secret value that parameterizes a cipher. The same algorithm with different keys produces different ciphertexts from the same plaintext. Key length determines the brute-force difficulty: doubling the key length squares the search space. A 128-bit key has 2¹²⁸ possible values.
Key Pair
Asymmetric Cryptography · 1976A matched set of public key and private key in an asymmetric cryptosystem. The public key is shared openly; the private key is kept secret. Data encrypted with one can only be decrypted with the other. Used for encryption (RSA), digital signatures (ECDSA), and key exchange (DH).
Keyspace
CryptanalysisThe set of all possible keys for a given cipher. Caesar cipher: 25 keys. Monoalphabetic substitution: 26! ≈ 4×10²⁶ keys. AES-256: 2²⁵⁶ keys. A large keyspace is necessary but not sufficient for security — frequency analysis breaks monoalphabetic despite its enormous keyspace.
M
Mode of Operation
Modern Block CiphersA method for applying a block cipher to messages longer than one block. ECB (Electronic Codebook) encrypts each block independently — insecure because identical blocks produce identical ciphertext. CBC (Cipher Block Chaining) XORs each block with the previous ciphertext block. GCM provides both encryption and authentication.
Monoalphabetic Cipher
AncientA substitution cipher using a single fixed alphabet mapping. Each plaintext letter always maps to the same ciphertext letter. Caesar, Atbash, and keyword substitution are all monoalphabetic. Vulnerable to frequency analysis because letter frequencies are preserved exactly.
See also: Polyalphabetic CipherN
Nonce
Number used onceA value that must never repeat in a given context. Similar to an IV but with a stricter uniqueness requirement. In AES-GCM, reusing a nonce with the same key completely breaks authentication and can leak plaintext via XOR of ciphertext streams.
Nomenclator
Renaissance · 15th–18th CenturyA hybrid system combining a simple substitution alphabet with a codebook of common words and names. Used extensively in diplomatic correspondence for centuries. More secure than pure substitution but still vulnerable to frequency analysis of the substitution portion.
O
One-Time Pad (OTP)
Gilbert Vernam · 1917 / Claude Shannon · 1949The only cipher proven to be unbreakable — if the key is truly random, at least as long as the message, used only once, and kept secret. Shannon proved this mathematically in 1949. Impractical for most uses because the key distribution problem is equivalent to the message distribution problem.
P
Plaintext
Core ConceptThe original, readable message before encryption. Denoted P or M in formulas. The goal of encryption is to protect plaintext from unauthorized readers. The goal of cryptanalysis is to recover it.
See also: CiphertextPolyalphabetic Cipher
Leon Battista Alberti · 1467A cipher that uses multiple substitution alphabets, switching between them according to a key. The Vigenère cipher is the classic example: each key letter selects a different Caesar shift. This disrupts single-letter frequency analysis, but patterns at the key-length interval can still be detected (Kasiski/Friedman).
See also: Monoalphabetic CipherPolybius Square
Polybius · ~200 BCA 5×5 grid containing 25 letters (I and J share a cell) used to convert letters into two-digit coordinates. The foundation of many fractionation ciphers (Bifid, Trifid, ADFGX, Nihilist) where coordinates are manipulated separately before recombination.
Public Key
Diffie-Hellman · 1976The freely shared half of an asymmetric key pair. Anyone can use your public key to encrypt a message to you or to verify your digital signature. The mathematical relationship between public and private keys must be computationally infeasible to reverse (factoring for RSA, discrete log for DH/ECDH).
R
Rotor
Enigma Machine · 1920sA wired disk that performs a substitution cipher by routing electrical signals through scrambled connections. Multiple rotors in series create a polyalphabetic cipher. The rotors step after each keypress, changing the substitution — giving Enigma ~17,576 initial rotor positions per rotor order.
Round
Modern Block CiphersOne complete iteration of a cipher's internal transformation. AES uses 10, 12, or 14 rounds depending on key size. Each round applies the same sequence of operations (SubBytes, ShiftRows, MixColumns, AddRoundKey). More rounds = more security, more computation.
S
S-box (Substitution Box)
Modern Block CiphersA lookup table that performs nonlinear substitution in block ciphers. In AES, each byte of state is replaced via a 256-entry S-box derived from finite field inversion. The S-box provides confusion — the nonlinearity that makes the cipher resistant to linear and differential cryptanalysis.
Steganography
Greek: steganos (covered) + graphein (to write)Hiding the existence of a message, rather than hiding its meaning. Invisible ink, microdots, messages hidden in image pixels. Complementary to cryptography: steganography conceals the fact that communication is happening; cryptography protects the content if it's discovered.
Stream Cipher
Modern CryptographyA cipher that encrypts one bit (or byte) at a time by XORing plaintext with a pseudorandom keystream generated from a key and nonce. Faster than block ciphers for streaming data. ChaCha20 is the most widely deployed modern stream cipher (used in TLS, WireGuard).
See also: Block CipherSubstitution Cipher
AncientAny cipher that replaces each element of the plaintext with another element. Monoalphabetic substitution uses one mapping; polyalphabetic uses multiple. Homophonic maps each letter to several possible symbols. The oldest and most intuitive class of ciphers.
Symmetric Encryption
All classical ciphers · Modern: AESA system where the same key is used for both encryption and decryption. All classical ciphers are symmetric. Modern examples: AES, ChaCha20. The key must be shared securely between parties — the fundamental key distribution problem that asymmetric cryptography was invented to solve.
T
Tabula Recta
Johannes Trithemius · 1508A 26×26 grid where each row is the alphabet shifted by one position. The foundational tool for Vigenère-type ciphers: find the plaintext letter's column, the key letter's row, and the intersection is the ciphertext letter. Also called a Vigenère square.
Transposition Cipher
Ancient / MilitaryA cipher that rearranges the positions of plaintext characters without changing them. Rail Fence, Columnar, and Route ciphers are all transposition methods. Letter frequencies are completely preserved (unlike substitution), so frequency analysis is useless — but anagramming and pattern analysis can break them.
V
Vigenère Cipher
Giovan Battista Bellaso · 1553 (misattributed to Vigenère)A polyalphabetic substitution cipher using a repeating keyword. Each key letter selects a different Caesar shift. Called "le chiffre indéchiffrable" for 300 years until Babbage (~1854) and Kasiski (1863) independently broke it by exploiting the repeating key pattern.
X
XOR (Exclusive OR)
Boolean Logic / CryptographyA binary operation: 0⊕0=0, 0⊕1=1, 1⊕0=1, 1⊕1=0. The workhorse of modern cryptography because it's perfectly balanced (output is equally likely 0 or 1), self-inverse (A⊕B⊕B=A), and extremely fast in hardware. The One-Time Pad and all stream ciphers use XOR as their core operation.