Core Concepts & Ciphers
Cryptography – The science of hiding and solving secret messages.
Cipher – A method or system for disguising a message.
Plaintext – The original, readable message.
Ciphertext – The encoded or encrypted version of the plaintext.
Encryption / Encipher – Turning readable text (plaintext) into secret form (ciphertext) using a specific key, meant to be reversible only with that key.
Decryption / Decipher – Turning ciphertext back into plaintext using the key that was used to encrypt it.
Key – The secret information used to encrypt or decrypt a message.
Encoding – Changing data into another format for transmission or storage (like Base64), not necessarily secret, and often reversible without a key.
Decoding – Reversing an encoding process (like Base64 decoding), doesn’t require a key and is usually meant to be publicly reversible.
Transcribe – To copy or rewrite something exactly as it appears using a standard alphabet, often used when typing out a cipher made of symbols, for example “#%1 31/” would become “ABC DCE”.
Substitution – Replacing each letter or symbol with something else (e.g., A → X), used in many basic ciphers.
Monoalphabetic – A cipher that replaces each letter with only one possible substitute throughout the message, for example the caesar cipher.
Caesar Shift – A specific type of shift cipher where each letter is shifted by the same fixed number.
Rot – A slang for caesar cipher, rot8 would for example mean a caesar cipher with a shift of 8.
Cryptogram – A monoalphabetic substitution cipher usually with a random alphabet
Polyalphabetic – A cipher that uses multiple alphabets to change how letters are replaced, making it harder to crack, for example the vigenere cipher.
Keyword – An arbitrary selected word used as a key.
Tabula Recta/Vigenère Square – A table with a normal alphabet as a row or column, and an encryption alphabet as the other. Used in Vigenère and similar ciphers.
Autokey – A cipher that uses parts of the plaintext itself as part of the key.
Running-key – A cipher using a long key, often from a book or another text, that changes constantly while encrypting.
One-Time Pad (OTP) – An unbreakable cipher that uses a truly random key the same length as the message, used only once.
Reciprocal – A cipher where encryption and decryption are the same process (like ROT13).
Homophonic – A cipher that uses multiple symbols or letters to represent the same letter to confuse frequency analysis.
Transposition – A cipher that rearranges the letters without changing them.
Permutation – Rearranging elements (like letters or blocks) in a specific order, often to confuse or hide patterns. A transposition cipher will usually have a permutation key.
Polygrammic / Polygraphic – Ciphers that work on chunks of multiple letters at once, like digraphs (2 letters) or trigraphs (3 letters), instead of single letters.
Monome/Mononome – A cipher unit that represents a single letter or symbol (like basic substitution of one letter at a time).
Triliteral – A symbol or cipher element made of 3 letters (often used in historical codes).
Anagram – Rearranging letters of a word/phrase to form a new one (e.g., "silent" → "listen").
A1Z26 – A cipher where the letters of the alphabet become their corresponding letter position. So A → 1 and Z → 26.
Polybius Square – A 5x5 grid used to convert letters to pairs of numbers or coordinates. With a normal polybius square 11 is A 12 is B. 5x5 squares can hold 25 letters, and the english alphabet has 26 so a polybius square alphabet usually discludes “J”.
Classical Cryptography – Traditional cipher methods before computers, like Caesar, Vigenère, or Playfair.
Modern Cryptography – Computer-era encryption based on algorithms and math, like AES and RSA.
AES – A strong, modern encryption standard often used in secure communication (Advanced Encryption Standard).
Block Cipher – A cipher that encodes fixed-size blocks of text instead of letter by letter.
Block – A group of characters treated together in block ciphers.
Block Size – The number of bits or characters in each block (e.g. 128 bits).
Stream Cipher – A cipher that encrypts one bit or character at a time in a continuous stream.
Initialization Vector (IV) – A random value added to encryption to make it unique, even for the same message.
Padding – Extra characters added to a message to hide its true length or align it for encryption.
RSA – A common public-key encryption system used for secure data exchange.
PGP – “Pretty Good Privacy”; a tool that uses public-key cryptography to secure emails and files.
Public Key – A key you share with others so they can encrypt messages to you or verify your signature.
Private Key – A secret key only you keep, used to decrypt messages or create digital signatures.
Signature – A special code made with your private key to prove a message came from you and wasn't changed.
Post-Quantum Cryptography – Cryptography designed to resist attacks from quantum computers, which could at some point break current encryption methods.
Analysis, Tools & Hidden Data
Frequency – How often each letter or symbol appears in a text.
Frequency Analysis – Looking at letter/symbol frequencies to guess the plaintext or cipher used. Can also be used to figure out what language the plaintext might be in.
Index of Coincidence (IoC) – A measure of how likely letters repeat in a text; helps detect cipher type or key length.
Repetition – Repeated letters or chunks in a cipher that might reveal patterns or key length.
Shannon Index – A formula measuring how random or predictable a text is; higher means more randomness.
Unigram, Bigram / Digram / Bigraf, Trigram – These refer to 1-letter, 2-letter, and 3-letter chunks of text. Used in frequency analysis to find common patterns like "TH" or "ING".
Isogram / Heterogram – A word or phrase where no letter repeats (e.g., "lumberjack").
Cryptoanalysis – The art of breaking ciphers and uncovering hidden messages without knowing the key.
Crib – A guessed or known word or phrase that helps break a cipher (like knowing "the" appears in English, or knowing the person always signs their encrypted messages with their name.).
Cribbing – The act of searching for or using cribs to crack encryption.
Dragging / Sliding – A technique in cribbing where you move the guessed word across the ciphertext to find a match.
Brute Force – Trying every possible key or solution until the correct one is found.
Collision – When two different inputs give the same output (e.g., in a hash), which can be exploited or hint at a weakness.
Hash – A function that turns data into a fixed string. It’s one-way: you can't turn it back into the original text unless you try to hash your own data until something matches the hash you’re cracking.
Salt – Random data added to a password before hashing it, so that two users with the same password get different hashes.
Checksum – A short code generated from data to verify it’s still the same data and hasn’t changed or been corrupted.
Steganography – Hiding information in plain sight, like in images, sound files, or even spaces and formatting.
Whitespace – Invisible characters (spaces, tabs, etc.) that can be used to hide messages in steganography.
Noise – Random or irrelevant data added to confuse, distract, or hide the real signal. Sometimes noise can be an unwanted side product.
Trailing – Data that comes after the intended message, possibly hiding extra info or noise. “Hello World ” has a trailing whitespace.
Obfuscation – Deliberately making something harder to understand without truly hiding it.
Red Herring – A deliberate distraction in puzzles to mislead you away from the real solution.
SSTV – Slow-Scan Television; a way of sending images over radio that can be encoded with hidden info. You can often hear if an audio message is SSTV.
Metadata – Data about data, like the author of a file or the time an image was created within the file; sometimes holds hidden clues.
LSB (Least Significant Bit) – The smallest bit of data, often used in image/audio steganography to hide secret bits.
Bitplane – A layer of bits across an image; changing a single bitplane can hide or reveal hidden patterns.
Fourier Transformation – Converts a signal (like sound) into frequencies; useful to find patterns or hidden messages.
Spectral Analysis – Looking at the frequency content of a signal (e.g., audio or image), often used to detect hidden info.
Spectrogram – A visual map of frequencies over time, often used in puzzles to hide text or imagery.
Algorithm – A step-by-step method to encrypt or decrypt data, like a recipe computers follow.
Bit – A single 0 or 1; the smallest unit of digital data.
Byte – A group of 8 bits; often represents a single character in text or a chunk of data.
Numeral – A symbol or number (like 7 or X) used in a numeric system, sometimes important in ciphers.
Base – A numbering system (like base-10 = decimal, base-2 = binary or base-64). There are often used to encode raw data.
Raw Data – Unprocessed information, if viewed without an encoding it will often look gibberish.
Binary – A language of 0s and 1s. Computers and some puzzles use it as a base form of data.
ASCII – An older character standard using numbers 0–127 to represent English letters and symbols.
Unicode – A standard for representing characters from almost every language (like emojis, Arabic, etc.).
Boolean – A true/false value, often used in logic puzzles or computer-based cipher tools.
Logical Operator – Tools like AND, OR, XOR, NOT used in logic and bitwise encryption schemes.
Modulo – A math operation that gives the remainder (often used in Caesar or Vigenère ciphers for wrapping around alphabets).
Bit Shift – Moving bits left or right, often used in ciphers or encoding to change values subtly.
Regex (Regular Expression) – A tool for searching or matching patterns in text. For example [A-Z0-9] would match all characters that are either A to Z or 0 to 9.
Hill Climbing – A guessing algorithm that tweaks a solution bit by bit, keeping changes that improve it (like trying small key changes to decrypt something better).
Esolang – Short for "esoteric language," like Brainfuck or Befunge — weird programming languages sometimes used in puzzles.
Mnemonic key – A key constructed as to be easily remembered.
Matrix – A grid of letters or numbers, often used in ciphers like the Hill cipher to transform text mathematically.
Periodic/Aperiodic – Periodic means the cipher repeats in a pattern (e.g. every 3 letters); aperiodic means it doesn’t follow a strict repeat.
Idiomorphic – means that a plaintext or cipher has a noticeable pattern — especially where and how often letters repeat. It looks “structured” or “not random” because of those repeating parts. Think of it like this: If you see something like "ABABAB", that is idiomorphic because there’s a clear repeating pattern.
Idiome – A recognizable style or structure in ciphertext (like a cipher’s “accent” or common traits).
Prefix – A piece of data or text added at the beginning of a message. “+1” uses “+” as a prefix.
Suffix – A piece of data or text added at the end of a message. “1+” uses “+” as a suffix.
Separator – A symbol or space that splits sections of a message like a space or colon.