Cryptography Glossary

A simple alphabet-number substitution where A = 1, B = 2, C = 3, and so on up to Z = 26. It is not secure, but it is useful for puzzles, teaching basic substitution, and showing how letters can be represented numerically.

An expanded version of the ADFGX cipher introduced later in 1918. It uses a 6x6 Polybius square labelled with A, D, F, G, V, X, allowing the full alphabet and the digits 0-9 to be encrypted directly.

A World War I German field cipher that combines a keyed 5x5 Polybius square with a columnar transposition. Each plaintext letter is first turned into a pair of symbols chosen from A, D, F, G, X, then those symbols are rearranged using a transposition key.

Advanced Encryption Standard. A symmetric block cipher adopted by the U.S. government in 2001, now the most widely used encryption algorithm worldwide. Operates on 128-bit blocks with key sizes of 128, 192, or 256 bits.

American Standard Code for Information Interchange. An older character encoding that represents basic English letters, digits, punctuation, and control characters as numbers. ASCII is important historically because many later encodings and binary-to-text systems were designed around ASCII-compatible text.

A way of representing ASCII characters using hexadecimal byte values. For example, "Hello" becomes 48 65 6C 6C 6F.

Describes text that can safely pass through older or restrictive systems that expect simple printable characters. Base64 is ASCII-safe because its output uses ordinary letters, digits, and a small number of punctuation characters.

A substitution cipher where each plaintext letter is shifted by adding a fixed number modulo the alphabet size. The Caesar cipher is the best-known additive cipher because every letter is moved by the same amount.

A monoalphabetic substitution cipher using the mathematical function E(x) = (ax + b) mod 26, where 'a' must be coprime with 26. Combines multiplication and addition to create the substitution.

The two numbers used in an Affine cipher: usually written as a and b in the formula E(x) = (ax + b) mod 26. The a value controls multiplication, while b controls the final shift.

A well-defined step-by-step procedure for performing a computation or solving a problem. In cryptography, algorithms define how encryption and decryption transform data.

Placeholder names commonly used in cryptography explanations to represent two parties communicating. Traditionally, Alice wants to send a message to Bob, while Eve tries to eavesdrop.

The numerical position of a letter in the alphabet. A1Z26 uses one-based alphabet positions, where A = 1 and Z = 26, while many mathematical ciphers use zero-based positions, where A = 0 and Z = 25.

The process of replacing each letter with the letter in the opposite position of the alphabet. In Atbash, the first letter maps to the last, the second maps to the second-last, and the pattern continues inward: A becomes Z, B becomes Y, C becomes X, and M becomes N.

A general term for ciphers that replace letters according to some rearranged or shifted alphabet. Caesar, Atbash, Keyword, and Affine ciphers are all forms of alphabetical substitution.

An older landline form of Morse code used mainly in the United States. It had different symbol patterns and more complicated timing features than International Morse, and survived for a long time in railway and telegraph contexts.

An ancient Hebrew substitution cipher that reverses the alphabet - A maps to Z, B maps to Y, and so on. It is its own inverse: applying it twice returns the original text.

The process of verifying the identity of a user, system, or message origin. Ensures that the claimed identity is genuine and that data has not been altered in transit.

A polyalphabetic cipher where the key begins with a keyword and then continues using the plaintext itself. This avoids the simple repeated-key pattern of Vigenère, but introduces other weaknesses if enough ciphertext is available.

A JavaScript function that decodes a Base64 string back into binary string data. Like btoa(), it is useful in browser-based tools but must be used carefully when working with Unicode text.

A phrase invented after the fact to explain an existing word or abbreviation. "Save Our Souls" and "Save Our Ship" are backronyms for SOS, not the original meaning of the signal.

The alphabet used in Bacon’s cipher, where each letter is represented by a five-symbol pattern of A and B values. The original Baconian system used a 24-letter alphabet, combining I/J and U/V.

The use of Bacon’s five-symbol patterns to hide a message inside another visible message. Instead of writing A and B directly, the two symbol types might be represented by different fonts, cases, colours, or styles.

A cipher devised by Francis Bacon that represents each letter using a five-character pattern of A and B symbols. It is often associated with hiding messages through differences in typeface, formatting, or other binary distinctions.

Another name for hexadecimal encoding. Base16 represents binary data using sixteen symbols: 0-9 and A-F. Each byte can be shown as exactly two hexadecimal digits.

A binary-to-text encoding system that converts raw bytes into a restricted alphabet of 32 characters. Standard Base32 commonly uses the letters A-Z and the digits 2-7. It is less compact than Base64, but easier to handle in case-insensitive, human-readable, or punctuation-restricted environments.

The character set used by standard Base32: A-Z followed by 2-7. These 32 symbols are enough to represent every possible 5-bit value. The digits 0, 1, 8, and 9 are avoided in the standard alphabet to reduce confusion with letters such as O, I, and B.

A standard processing unit in Base32. Base32 normally takes 5 bytes of input, equal to 40 bits, and splits them into eight 5-bit groups. Each 5-bit group is then represented by one Base32 character.

The equals sign added to the end of some Base32 strings so the output fits the expected 8-character block structure. Padding does not encrypt or hide data; it simply records how the final incomplete group of bytes should be interpreted during decoding.

A modified form of Base32 that changes the alphabet, padding rules, or human-readability features. Examples include standard RFC 4648 Base32, Base32hex, Crockford Base32, and z-base-32.

A variant of Base32 defined in RFC 4648 that uses a different alphabet: 0-9 followed by A-V. Unlike standard Base32, Base32hex preserves numeric sort order more naturally, which makes it useful in some technical systems.

A binary-to-text encoding system that converts raw bytes into readable text characters. Base64 is commonly used when binary data needs to travel through text-based systems such as email, HTML, JSON, CSS, URLs, and web APIs. It is not encryption: anyone can decode it.

The set of characters used by standard Base64: A-Z, a-z, 0-9, plus + and /. These 64 characters are enough to represent every possible 6-bit value. The = character may appear as padding, but it is not part of the main Base64 alphabet.

An image represented as a Base64 string instead of being stored as a separate image file. Base64 images are often used in data URIs, small icons, embedded HTML examples, and email content.

The = character added to the end of some Base64 strings so the final output has the correct length. Padding usually appears as one or two equals signs. It does not hide information or make the data more secure.

A URL-safe version of Base64 that replaces + with - and / with _. This makes the encoded text safer to use in URLs, filenames, web tokens, and other places where ordinary Base64 characters might cause problems.

A polyalphabetic cipher related to Vigenère, but with a different encryption formula. It uses a keyword to generate changing shifts and has the unusual property that encryption and decryption are effectively the same operation.

A fractionating cipher invented by Félix Delastelle. It uses a Polybius square to turn letters into coordinates, then mixes row and column numbers before converting them back into ciphertext letters.

A two-character sequence, such as TH, HE, IN, or ER. Bigram frequencies are useful in cryptanalysis because natural languages contain common letter pairs that can survive in disguised form.

A cipher that represents each letter using two possible symbol types. Bacon’s cipher is the classic biliteral cipher because every letter is encoded as a pattern made from only A-type and B-type marks.

A way of writing data using only 0s and 1s. Computers store information in binary, while encodings such as Base64, hexadecimal, and A1Z26 provide more convenient human-readable representations.

A method of converting raw binary data into ordinary text characters. Base64, Base32, and hexadecimal are binary-to-text encodings. They are useful when data must pass through systems designed for text rather than raw bytes.

A symmetric cipher that operates on fixed-size blocks of data, typically 64 or 128 bits. AES and DES are block ciphers. The message is divided into blocks, each encrypted separately.

An attack that tries every possible key until the correct one is found. The simplest attack method: always succeeds eventually, but is impractical for large key spaces.

A group of eight bits. Text, images, encrypted data, and files are commonly handled as bytes before being encoded into formats such as Base64, Base32, or hexadecimal.

A JavaScript function that encodes binary string data into Base64. It is commonly seen in browser-based examples, although modern tools often need extra handling for Unicode text such as emojis and non-English characters.

A substitution cipher where each letter is shifted by a fixed number of positions in the alphabet. Named after Julius Caesar, who used a shift of 3 for military correspondence.

The fixed number of alphabet positions used in a Caesar cipher. A shift of 3 turns A into D, B into E, and so on. ROT13 is a Caesar shift of 13.

A possible decrypted version of a ciphertext produced during an attack. In Caesar cracking, each possible shift produces a candidate plaintext, and the most readable or best-scoring candidate is treated as the likely solution.

One possible shift value tested when attacking a Caesar or ROT-style cipher. Since there are only 26 possible Caesar shifts, a tool can try every candidate shift and rank the results by how closely they resemble normal language.

A properly normalised form of Base32 output according to a particular standard. In standard RFC 4648 Base32, canonical output usually means uppercase letters, the standard A-Z and 2-7 alphabet, and correct padding where required.

The process of converting text to a consistent letter case before processing it. Many classical cipher tools convert input to uppercase so that A-Z alphabet rules can be applied cleanly.

An encoding designed so uppercase and lowercase forms can be treated as equivalent. Base32 is often convenient in case-insensitive environments because its standard form uses uppercase letters and digits, unlike standard Base64, which distinguishes uppercase and lowercase letters.

A system for converting written characters into numbers or bytes. ASCII and UTF-8 are character encodings. Character encoding matters because the same visible text must become bytes before it can be Base64-encoded.

A statistical score used to compare observed letter counts with expected letter counts. In Caesar cracking, the shift with the lowest chi-squared score is often the best candidate because its decrypted text most closely matches ordinary English letter frequencies.

An attack model where the cryptanalyst can choose arbitrary plaintexts and obtain their corresponding ciphertexts. More powerful than a known-plaintext attack.

The alphabet used to produce ciphertext. In a substitution cipher, the plaintext alphabet and cipher alphabet define which letters replace which.

A tool or method that attempts to suggest which cipher may have produced a given ciphertext. It usually looks at clues such as alphabet size, symbol set, spacing, repeated patterns, and frequency distribution.

A physical or visual device made of rotating alphabets, often used to perform Caesar or Vigenère-style shifts. Cipher wheels help show how substitution ciphers move letters around the alphabet.

The encrypted output of a cipher: the unreadable result of applying an encryption algorithm to plaintext. Can only be turned back into plaintext with the correct key.

An attack where the cryptanalyst has access only to the ciphertext. The weakest attack model, but sufficient to break simple ciphers like the Caesar cipher.

Input text after unsupported characters, spacing, punctuation, or letter case have been adjusted for a particular tool. Classical ciphers often clean input because they usually operate on alphabetic letters rather than arbitrary Unicode text.

A keyword after duplicate letters, spaces, punctuation, or unsupported characters have been removed. Keyword ciphers and keyed squares often clean the keyword before building the substitution alphabet or grid.

Readable, unencrypted information. Often used similarly to plaintext, though cleartext can also refer to sensitive data stored or transmitted without encryption.

A system of replacing entire words or phrases with other words, numbers, or symbols. Unlike ciphers, which work on individual characters, codes operate on semantic units.

The number of ways matching letters can be selected from a text. It is used inside the index of coincidence formula to measure how uneven or language-like a letter distribution is.

The sequence in which columns are read during a columnar transposition cipher. Usually determined by alphabetising the letters of a keyword.

The number of columns used in a transposition grid. In columnar transposition, the column width is usually determined by the length of the keyword.

A transposition method in which text is written into rows beneath a keyword and then read out column by column according to the alphabetical order of that keyword. It is the second stage of the ADFGX and ADFGVX ciphers.

A radio signalling method often associated with Morse code. In CW, a radio carrier is switched on and off to create short and long signals that can travel effectively even under weak or noisy conditions.

A two-symbol reference that identifies the row and column of a letter in a Polybius square. In ADFGX, a plaintext letter such as E might become a pair like XF depending on where it sits in the keyed grid.

Two numbers are coprime if they have no common factor other than 1. In the Affine cipher, the multiplier a must be coprime with 26; otherwise, the cipher cannot be reliably reversed.

A word, phrase, or message fragment that a codebreaker suspects appears in the plaintext. Cribs are especially valuable in attacks on historical ciphers because they provide likely footholds for reconstructing keys or message structure.

A human-friendly Base32 variant designed by Douglas Crockford. It avoids confusing characters, treats some similar-looking symbols as equivalent, and is often used for identifiers, codes, and strings that people may need to read, type, or speak aloud.

The study of methods for breaking cryptographic systems: recovering plaintext from ciphertext without knowledge of the key. Includes techniques like frequency analysis and differential cryptanalysis.

A short encrypted puzzle, usually made with a simple substitution cipher. Cryptograms are often solved using frequency analysis, word patterns, and common letter combinations.

The science and art of secure communication. From the Greek 'kryptos' (hidden) and 'graphein' (to write) - the practice of encoding messages so only authorized parties can read them.

The umbrella field encompassing both cryptography, which makes ciphers, and cryptanalysis, which breaks ciphers. Sometimes used interchangeably with cryptography.

Data Encryption Standard. A symmetric block cipher developed by IBM and adopted as a U.S. federal standard in 1977. Uses a 56-bit key on 64-bit blocks. Now considered insecure due to its short key size.

The long signal unit in Morse code. In standard International Morse timing, a dash lasts three times as long as a dot.

A way of embedding data directly inside HTML or CSS instead of linking to a separate file. Base64 is often used in data URIs to include small images, icons, fonts, or files directly inside a web page.

The process of converting encoded data back into its original form. Base64 decoding reverses Base64 encoding and reconstructs the original bytes. Decoding is different from decryption because no secret key is required.

The process of converting ciphertext back into plaintext using the appropriate key and algorithm. The reverse operation of encryption.

A situation where a tool cannot produce a valid decoded or decrypted result. This can happen because the input is malformed, the wrong key was used, padding is incorrect, or the chosen cipher settings are not compatible with the ciphertext.

The key used to reverse encryption and recover plaintext from ciphertext. In symmetric ciphers it is usually the same as the encryption key; in asymmetric cryptography it may be different.

A number calculated from a square matrix. In the Hill cipher, the determinant helps decide whether a key matrix can be inverted modulo 26. If the determinant has no modular inverse, the key matrix cannot be used for proper decryption.

A key exchange protocol from 1976 that allows two parties to establish a shared secret over an insecure channel. Considered the first public-key protocol and a major turning point in cryptography.

A cryptographic scheme that verifies the authenticity and integrity of a digital message. Uses public-key cryptography: the sender signs with their private key, and anyone can verify with the public key.

A pair of letters treated as a unit. In classical cryptography, many systems work with digraphs rather than single letters, either for encryption itself or for frequency analysis of common two-letter patterns such as TH, HE, or ER.

The process by which a cipher makes letter frequencies more even. Polyalphabetic ciphers such as Vigenere flatten the obvious peaks found in ordinary English, making simple frequency analysis less effective.

The short signal unit in Morse code. Morse timing is usually measured relative to the length of one dot.

A stronger form of transposition cipher where the message is rearranged twice, often using two different keys. It is harder to solve than a single columnar transposition.

The process of removing repeated letters from a keyword while keeping the first occurrence of each letter. This is commonly used when building keyed alphabets or keyed Polybius squares.

A communication system that sends messages over wires using electrical signals. Morse code became one of the most important signalling languages of the electric telegraph era.

The short gap between dots and dashes inside a single Morse character. It is usually one timing unit long and helps distinguish the separate parts of a letter.

To convert plaintext into ciphertext using a cipher. The word is often used in classical cryptography as an older or more formal alternative to “encrypt”.

Text produced by an encoding process. A Base64 encoded string may look like random letters, numbers, and symbols, but it is only a different representation of the original data, not a secret message.

The process of converting data from one representation to another. Morse code, Base64, hexadecimal, and A1Z26 are encodings. Encoding is reversible but does not provide secrecy by itself.

Encoding changes how data is represented so it can be stored or transmitted more conveniently. Encryption changes data so it cannot be understood without a key. Base64 is encoding, not encryption.

The process of converting plaintext into ciphertext using a cipher and a key. Protects the confidentiality of data by making it unreadable without the key.

The key used to transform plaintext into ciphertext. In classical ciphers this may be a shift, keyword, square, rail count, or matrix. In modern systems it is usually a string of bits.

A reference table showing the approximate frequency of each letter in English. Cryptanalysis tools use these tables to compare ciphertext or candidate plaintext against normal English letter patterns.

An electromechanical rotor cipher machine used by Nazi Germany during WWII. Considered virtually unbreakable, it was famously cracked by Polish and British cryptanalysts at Bletchley Park.

The number of times a letter would be expected to appear in a text of a given length if the text followed normal language frequencies. For example, if E is expected to appear about 12.7% of the time, a 200-letter English text would be expected to contain about 25 Es.

A misleading result where a tool suggests a possible decryption or cipher type that appears plausible but is not actually correct. False positives are more likely when the ciphertext is short, unusual, non-English, or not produced by the assumed cipher.

A cipher designed for practical military use in the field rather than for formal diplomatic correspondence. Field ciphers must balance security with speed, simplicity, and reliability under operational pressure.

An extra letter inserted to make a cipher’s rules work. In Playfair, a filler such as X is often placed between repeated letters in a pair or added to the end of a message with an odd number of letters.

An operator's distinctive Morse sending style. Skilled listeners could sometimes recognise individual operators from their timing, rhythm, and spacing.

An encoding system where each symbol or character is represented using five bits. Bacon’s cipher and Base32 both connect naturally to the idea of five-bit representation.

A 5x5 grid used to arrange 25 letters. Playfair, Polybius, Bifid, Tap Code, and ADFGX-style systems often use a five-by-five square, usually with I and J sharing one cell.

A substitution rule where every letter is moved by the same number of positions in the alphabet. Caesar and ROT13 are fixed-shift ciphers.

A cipher that splits each plaintext letter into smaller components before further processing. ADFGX is a fractionating cipher because each plaintext letter becomes a two-symbol coordinate pair before transposition.

The process of breaking a plaintext unit into multiple parts so its original structure becomes harder to detect. In ADFGX, fractionation occurs when each letter is converted into a coordinate pair and those pair elements are later separated by transposition.

The study of letter frequency patterns in ciphertext to break substitution ciphers. In English, 'E' is the most common letter, making it a natural starting point for analysis.

The overall pattern of how often each letter or symbol appears in a text. In English, the frequency distribution is uneven, with letters such as E, T, A, O, I, and N appearing much more often than rare letters such as Q, X, and Z.

A noticeably high point in a frequency distribution. Simple substitution ciphers preserve frequency peaks: if E is common in the plaintext, its substitute will usually be common in the ciphertext.

A table listing each symbol in a text alongside its count and percentage. Frequency tables are often the first output of a frequency-analysis tool and provide the starting point for attacking substitution ciphers.

The German signal officer generally credited with devising the ADFGX cipher and its later ADFGVX variant during World War I. His design aimed to give German radio traffic stronger protection against interception.

The French cryptanalyst most closely associated with breaking German ADFGX and ADFGVX traffic during World War I. His work became one of the most famous codebreaking achievements of the war.

The German telegraph official whose revisions helped simplify earlier Morse systems and strongly influenced the development of International Morse code.

A measure of how closely one distribution matches another. In frequency analysis, goodness of fit describes how closely a candidate plaintext matches expected English letter frequencies. Chi-squared scoring is one way to measure this.

A cipher that uses a square or rectangular grid to locate, substitute, or rearrange letters. Polybius, Playfair, Bifid, ADFGX, and ADFGVX all rely on grid-based thinking.

The row-and-column positions used to locate a character inside a cipher grid. Polybius-style systems turn letters into grid coordinates before further encoding or rearrangement.

A one-way function that maps data of arbitrary size to a fixed-size output called a hash. Used for data integrity verification and password storage. Not reversible - cannot recover input from output.

A visual signalling device that sends messages using flashes of reflected sunlight. Like lamps and flags, it belongs to the wider history of non-electrical signalling systems.

The process of converting hexadecimal text back into the original bytes or readable text. For example, 48 65 6C 6C 6F hex-decodes to Hello when interpreted as ASCII or UTF-8 text.

A single hexadecimal character from 0-9 or A-F. Each hex digit represents 4 bits, also called one nibble. Two hex digits together usually represent one byte.

The process of converting text or raw bytes into hexadecimal form. Each byte becomes two hexadecimal digits, making the data easier to inspect, copy, compare, or transmit through text-based systems.

Two hexadecimal digits that represent one byte. In hex-encoded text, each byte is usually written as a hex pair such as 41, 7A, or FF.

A sequence of hexadecimal digits used to represent bytes as readable text. For example, the text Hello can be represented as the hex string 48656C6C6F.

A base-16 numbering system using the symbols 0–9 and A–F. Hexadecimal is widely used to display bytes, hashes, colours, memory addresses, and cryptographic fingerprints.

A message concealed inside another visible message, image, layout, pattern, or signal. Bacon’s cipher can be used this way when the A/B symbols are disguised as differences in typography or formatting.

A mathematical substitution cipher invented by Lester S. Hill in 1929. It encrypts blocks of letters using matrix multiplication modulo 26, making it one of the first classical ciphers to use linear algebra.

An encoding designed with human copying, reading, typing, or speaking in mind. Base32 is more human-friendly than Base64 in some contexts because it avoids lowercase dependence and punctuation characters such as + and /.

A convention used in many 5x5 Polybius-square ciphers where I and J share a single cell so the alphabet fits into 25 spaces. This means context may be needed during decryption to tell which letter was originally intended.

A statistical measure of how likely two randomly selected letters from a text are to be the same. It is useful for detecting whether a cipher is monoalphabetic, polyalphabetic, or close to random.

A message captured in transit by someone other than its intended recipient. In wartime radio communication, interception was common, which is why stronger ciphers became necessary.

The standardised form of Morse code used internationally, especially in radio communication. It represents letters, numbers, punctuation, and procedural signals using dots, dashes, and timed gaps.

A key that cannot be used with the selected cipher. For example, an Affine cipher multiplier that is not coprime with 26 is invalid, and a Hill cipher matrix without a modular inverse cannot decrypt reliably.

A matrix that reverses the action of another matrix. In the Hill cipher, the key matrix must have an inverse modulo 26 so the ciphertext can be decrypted.

The deliberate interference with a communication signal to make it harder to receive, understand, or trust. Jamming is not encryption, but it belongs to the wider world of secure communication because it attacks the channel rather than the message.

A method for breaking polyalphabetic ciphers, especially Vigenere, by finding repeated sequences of characters in the ciphertext. The distances between repetitions can reveal the key length.

A cipher's security should depend solely on the secrecy of the key, not the algorithm. Published by Auguste Kerckhoffs in 1883 - a cornerstone of modern cryptographic design.

A piece of information, usually a string of bits, that controls the operation of a cryptographic algorithm. The same algorithm with different keys produces different ciphertexts.

The number of characters, numbers, columns, or symbols in a key. In Vigenère-style ciphers, key length affects repeated patterns. In columnar transposition, key length determines the number of columns.

The matrix used as the key in the Hill cipher. It must be invertible modulo 26; otherwise, decryption will not work reliably.

The unsafe practice of using the same key repeatedly across multiple messages. Key reuse often gives cryptanalysts patterns to compare and exploit, especially in classical and stream-based systems.

The total number of possible keys for a given cipher. A larger key space makes brute-force attacks harder. A 256-bit key has 2^256 possible keys - an astronomically large number.

A square grid built from a keyword and an alphabet. Playfair, Polybius, Bifid, ADFGX, and ADFGVX systems can all use key squares to control how letters or symbols are converted.

An alphabet rearranged using a keyword and then completed with the remaining unused letters. Keyed alphabets are often used to build Polybius squares and substitution tables in classical ciphers.

A sequence of key values applied across a message. In Vigenère, the repeated keyword produces a simple keystream. In Autokey, the keystream begins with a keyword and then continues with message text.

A substitution cipher that builds a mixed alphabet from a keyword followed by the remaining unused letters. It is stronger than a plain Caesar shift but still vulnerable to frequency analysis.

An attack where the cryptanalyst has access to both the plaintext and its corresponding ciphertext. The Lorenz cipher in WWII was broken using this approach.

The distinctive statistical pattern produced by a natural language. Letter frequencies, common bigrams, common trigrams, word lengths, and repeated patterns all form part of a language's fingerprint.

A group of letters processed together as one unit. Hill cipher works on blocks of letters, while Playfair works on two-letter blocks.

The rate at which each letter appears in a language or text. In English, E, T, A, O, I, and N are common, which helps break many substitution ciphers.

A square or rectangular arrangement of letters used by a cipher. Letter grids make it possible to convert letters into positions, pairs, or movement rules rather than simple one-letter substitutions.

The frequency of two-letter combinations in a language. Common English pairs such as TH, HE, AN, IN, and ER are useful in solving digraph ciphers and substitution puzzles.

A communication method that requires sender and receiver to be visible to each other. Semaphore flags, lamps, beacons, and heliographs are examples.

An uneven frequency distribution with clear peaks and troughs. Ordinary English is lumpy because some letters appear much more often than others. Simple substitution ciphers usually preserve this lumpiness.

Multipurpose Internet Mail Extensions. A standard that allowed email to carry attachments, images, different character sets, and non-plain-text content. Base64 became widely used because MIME needed a reliable way to represent binary attachments as text.

A form of Base64 commonly used in email systems. MIME Base64 often splits long encoded output into fixed-length lines so messages can pass safely through older mail systems.

The property that determines whether a Hill cipher key matrix can be reversed. A matrix must be invertible modulo 26 for the ciphertext to be decrypted back into the original plaintext.

A method of combining rows and columns of numbers. In the Hill cipher, plaintext letter numbers are grouped into vectors and multiplied by a key matrix modulo 26.

A reversed version of the normal alphabet used as a substitution alphabet. In the Atbash cipher, the plaintext alphabet A-Z is paired with a mirror alphabet Z-A, so A maps to Z, B maps to Y, C maps to X, and so on. This mirror structure is what makes Atbash simple to understand and easy to reverse.

An alphabet whose letters have been rearranged rather than left in ordinary A-Z order. Mixed alphabets are used to make substitution systems less predictable and more resistant to straightforward analysis.

Arithmetic where numbers wrap around after reaching a fixed modulus. Classical ciphers often use modulo 26 arithmetic because the English alphabet has 26 letters.

A number that reverses multiplication under modular arithmetic. In the Affine cipher, decryption requires the modular inverse of the multiplier a. In the Hill cipher, matrix inversion also depends on modular inverses.

Arithmetic performed with wraparound at 26. In alphabet-based ciphers, A–Z are often represented by 0–25, and calculations are reduced modulo 26.

A substitution cipher where each letter is always replaced by the same corresponding letter. Vulnerable to frequency analysis because letter frequencies are preserved.

An encoding and signalling system that represents letters, numbers, punctuation, and procedural signals using short and long pulses. These pulses can be sent as sound, electrical current, radio, light, or physical taps.

The rhythm system that defines Morse code. A dot is one timing unit, a dash is three units, gaps between parts of a letter are one unit, gaps between letters are three units, and gaps between words are seven units.

A substitution cipher that multiplies each plaintext letter value by a fixed number modulo 26. It can be seen as part of the Affine cipher where the additive shift is zero.

A sequence of n characters. A bigram is a 2-gram, a trigram is a 3-gram, and a quadgram is a 4-gram. N-gram patterns are useful in cryptanalysis because natural languages contain repeated character sequences.

A group of 4 bits. One hexadecimal digit represents exactly one nibble, which is why hexadecimal is so convenient for displaying binary data.

A cipher that does not require a secret key to encrypt or decrypt a message. Atbash is a no-key cipher because its rule is fixed: the alphabet is always reversed. This makes it easy to use and teach, but very weak for secrecy because anyone who recognises the pattern can reverse it immediately.

A form of Base32 where the trailing equals signs are omitted. Some systems allow Base32 without padding because the original length can be inferred or is already known from context. This can make strings shorter and cleaner, but the decoder must know how to handle the missing padding.

A character that is not part of the normal A-Z alphabet, such as a space, digit, punctuation mark, emoji, or accented letter. Classical cipher tools often need options for whether to remove, preserve, or ignore these characters.

A meaningless character inserted into a ciphertext to confuse analysis or disguise message length. Nulls were sometimes used in historical cipher systems and puzzle ciphers.

A cipher or encoding that represents letters using numbers. A1Z26 is the simplest example, where letters are replaced by their alphabet positions.

The actual number of times a letter or symbol appears in a specific text. Frequency analysis compares observed counts with expected counts to judge whether a piece of text resembles normal language.

A numbering system that starts counting at 1. A1Z26 uses one-based indexing because A = 1, B = 2, and Z = 26.

An encryption method using a random key that is as long as the message and used only once. Proven to be mathematically unbreakable if the key is truly random and never reused.

A special URI format used to set up authenticator apps. It often contains a Base32-encoded secret key, along with information such as the account name, issuer, and algorithm. QR codes for two-factor authentication commonly store otpauth URIs.

A character added to encoded data to make the final output fit a required length or block size. In Base64, the equals sign is used as padding.

A rule for grouping letters into pairs before encryption. Playfair uses pairing rules to split text into digraphs, handle repeated letters, and deal with odd-length messages.

An incomplete recovery of plaintext where only some letters, words, or fragments have been solved. In substitution ciphers, frequency analysis often produces a partial decryption before the full message becomes clear.

The final block of input bytes when there are not enough bytes to fill a complete Base32 block. Because Base32 normally works with 5-byte chunks, shorter final chunks require padding or special handling during decoding.

To convert percent-encoded URL sequences back into readable characters. For example, %20 becomes a space. This is useful for understanding how text survives inside URLs.

To convert characters into % followed by hexadecimal values so they can safely appear inside a URL. Spaces, punctuation, and non-ASCII characters often need percent encoding.

The length of a repeating key or cycle in a polyalphabetic cipher. In Vigenère, the period is usually the length of the keyword.

A readable text version of data that may originally have been binary. Base64 creates a plain text representation of bytes, which makes the data easier to copy, paste, transmit, or embed in other text formats.

The original, readable message before encryption, or the decrypted output after decryption. Also called cleartext.

The alphabet of readable input letters before encryption. In substitution ciphers, the plaintext alphabet is mapped to a cipher alphabet.

The measurable patterns found in ordinary readable language, including letter frequencies, common word lengths, repeated words, bigrams, and trigrams. Weak ciphers are vulnerable when they preserve too much plaintext structure in the ciphertext.

A block of plaintext letters represented as a column or row of numbers. In the Hill cipher, plaintext vectors are multiplied by the key matrix to produce ciphertext vectors.

A digraph substitution cipher invented by Charles Wheatstone in 1854. Encrypts pairs of letters using a 5x5 key square. Considerably harder to break than single-letter substitution ciphers.

A cipher that uses multiple substitution alphabets, rotating through them during encryption. Makes frequency analysis much harder. The Vigenere cipher is the most famous example.

The row-and-column references used to locate characters in a Polybius square. For example, a letter might be represented as 23 if it appears in row 2, column 3.

A square grid used to convert letters into coordinates. In classical cryptography it is usually 5x5, while ADFGVX uses a 6x6 version so that both letters and digits can be represented.

The secret key in asymmetric cryptography. Must never be shared. Used for decryption and digital signing. Paired with a corresponding public key.

A procedural Morse code signal used to control message flow rather than represent ordinary text. Prosigns can indicate actions such as waiting, ending a message, or correcting an error.

A QR code used to add an account to an authenticator app. It usually contains an otpauth URI, which includes a Base32-encoded secret. Scanning the QR code lets the app store the secret without the user typing it manually.

The openly shared key in asymmetric cryptography. Can be freely distributed. Used for encryption and signature verification. Paired with a corresponding private key.

An encryption system that uses a pair of keys - a public key for encryption and a private key for decryption. Eliminates the need to securely share a secret key. Invented in the 1970s.

A four-letter sequence, such as THAT, TION, THER, or HERE. Quadgram scoring can be used by more advanced substitution-solving tools because four-letter patterns often give stronger evidence than single letters or bigrams.

The Internet standard that defines common Base16, Base32, and Base64 encodings. It specifies the standard Base32 alphabet, padding behaviour, and related variants such as Base32hex.

A special case of the Caesar cipher with a shift of 13. Its own inverse - applying it twice returns the original text. Not secure; commonly used to obscure spoilers or jokes online.

The first practical public-key cryptosystem, introduced in 1977 and named after Rivest, Shamir, and Adleman. Security relies on the difficulty of factoring the product of two large prime numbers.

The number of rows or “rails” used in the Rail Fence cipher. Changing the rail count changes the zigzag pattern and therefore the ciphertext.

A transposition cipher that writes the message in a zigzag pattern across multiple 'rails' and then reads it off row by row. Easy to break but demonstrates the transposition concept.

The original binary form of data before it is converted into a text representation. Images, files, encrypted messages, compressed data, and ordinary text can all be treated as raw bytes before Base64 encoding.

A cipher where the same operation can be used for both encryption and decryption. Atbash and ROT13 are simple examples; Beaufort also has a reciprocal quality.

One of the Playfair cipher rules. When two letters in a pair form opposite corners of a rectangle inside the key square, each letter is replaced by the letter in the same row but the other corner of the rectangle.

A cipher that uses a keyword repeatedly across the message. Vigenère is the classic example. Repeated keys create patterns that can be attacked using methods such as Kasiski examination.

An alphabet written in the opposite order from normal. Instead of A B C D continuing forward to Z, a reverse alphabet begins Z Y X W and continues backward to A. Atbash uses this reverse alphabet as its cipher alphabet, replacing each letter with the letter in the same position on the reversed row.

A transposition cipher where plaintext is written into a grid and read out using a particular route or path. The Rail Fence and columnar transposition ciphers belong to the broader family of rearrangement systems.

A symbol used to identify a row in a cipher grid. Polybius squares often use numbers as row labels, while ADFGX and ADFGVX use letters such as A, D, F, G, X, and V.

The best-known Morse distress signal: three dots, three dashes, and three dots. It was chosen because it is simple, distinctive, and easy to recognise, not because it originally stood for "Save Our Souls" or "Save Our Ship."

One of the Playfair cipher rules. If both letters in a pair appear in the same column of the key square, each is replaced by the letter below it, wrapping around to the top if necessary.

One of the Playfair cipher rules. If both letters in a pair appear in the same row of the key square, each is replaced by the letter to its right, wrapping around to the left if necessary.

The amount of text available for analysis. Frequency analysis becomes more reliable with larger sample sizes because long texts are more likely to resemble normal language frequencies. Short ciphertexts can produce misleading results.

A rule used to rank possible decryptions. A Caesar cracker might use chi-squared scoring, while a substitution solver might use word matches, n-gram frequencies, or a combination of statistical measures.

An ancient Spartan transposition cipher device. A strip of parchment was wrapped around a rod of a specific diameter; the message could only be read when re-wrapped on a rod of the same size.

A shared secret value used by an authenticator app and a server to generate one-time passcodes. In many TOTP systems, this secret is displayed or transported as Base32 text because Base32 is easier to copy, scan, and type than raw bytes.

A cipher where the same operation can be used for both encryption and decryption. Atbash is self-inverse because each letter pair works in both directions: A becomes Z, and Z becomes A. Applying the Atbash cipher twice returns the original message.

A visual signalling system that uses flags, arms, or mechanical indicators to represent letters or messages. Semaphore helped transmit information before electrical communication became widespread.

A substitution cipher where each letter is shifted by a fixed number of positions in the alphabet. The Caesar cipher is the best-known shift cipher.

A physical carrier of information, such as sound, light, electrical current, radio waves, or touch. Morse code works by turning language into controlled patterns of signal and silence.

A pause between Morse signal elements, letters, or words. Different gap lengths separate parts of a character, complete characters, and whole words.

A cipher where each plaintext letter is replaced by exactly one ciphertext letter according to a fixed alphabet mapping. Stronger than Caesar in key space, but still vulnerable to frequency analysis.

A way of thinking about Caesar, Vigenère, and related ciphers: one alphabet is shifted against another to create substitutions. Cipher wheels and tabula recta tables show this visually.

A method of reading overlapping groups of characters from a text. For example, the text ABCDE contains the bigrams AB, BC, CD, and DE when read with a sliding window of length 2.

The choice to keep word spaces visible in ciphertext. Preserving spaces makes text easier to read but gives attackers useful clues about word lengths and structure.

An attack that exploits measurable patterns in ciphertext rather than simply trying every key. Frequency analysis, index of coincidence, n-gram scoring, and chi-squared testing are all examples of statistical cryptanalysis.

Information unintentionally revealed through patterns in ciphertext or system behaviour. Classical ciphers often leak letter frequencies, while modern systems may leak information through timing, message lengths, repeated values, or implementation mistakes.

The art of hiding the existence of a message, as opposed to cryptography, which hides its meaning. Techniques include invisible ink, microdots, and embedding data in image files.

A message that begins, ends, or is phrased in a predictable way. Repeated military formulas, headers, and standard wording can give codebreakers valuable clues when attacking a cipher.

A symmetric cipher that encrypts data one bit or byte at a time, typically by XORing it with a keystream. Useful for real-time applications where data arrives continuously.

A cipher that replaces each letter with another according to a fixed system. Includes monoalphabetic variants like Caesar and Atbash, and polyalphabetic variants like Vigenere.

The collection of characters that appear in a piece of encoded text or ciphertext. A cipher identifier can use the symbol set to guess whether text looks like Morse, Base32, Base64, hexadecimal, binary, or a classical cipher.

An encryption system where the same key is used for both encryption and decryption. Also called secret-key or shared-key cryptography. AES and DES are symmetric ciphers.

The secret key used in Time-Based One-Time Password systems such as authenticator apps. TOTP secrets are commonly represented in Base32 so they can be shown as text, embedded in QR codes, and entered manually if needed.

A square table of shifted alphabets used to perform Vigenère-style encryption. Each row represents a Caesar shift, and the key selects which row is used.

A signalling system that represents letters using pairs of taps, usually based on a 5x5 Polybius square. It is associated with prisoners communicating through walls or pipes.

A pair of tap counts used in Tap Code. The first group usually gives the row of the letter, and the second group gives the column.

The long-distance transmission of written messages using signals rather than physical delivery. Telegraphy may use wires, radio, light, or other signalling methods.

The process of preparing text before encoding, decoding, encryption, or analysis. This can include uppercasing letters, removing accents, replacing unsupported characters, stripping punctuation, or standardising whitespace.

A file or data format designed mainly around readable text characters. HTML, CSS, JSON, XML, and email messages are text-based formats. Base64 is often used when these formats need to include binary data.

The basic unit used to describe Morse code timing. A dot lasts one unit, a dash lasts three units, the gap between letters lasts three units, and the gap between words lasts seven units.

The pitch used when Morse code is played as sound. Morse audio tools often allow the tone frequency to be changed so the signal is more comfortable to hear.

The study of message patterns such as length, timing, routing, repetition, and volume rather than just message content. Even when a cipher is strong, traffic analysis can reveal useful intelligence about how messages are being used.

The reason Base64 is often used: it makes data safer to move through systems that might reject, alter, or misread raw binary bytes. Base64 does not make the data private; it only makes it easier to transport.

A cipher that rearranges the positions of the letters without changing the letters themselves. The Rail Fence cipher and columnar transposition are examples.

A three-character sequence, such as THE, ING, ION, or AND. Trigram frequencies can help identify language patterns in ciphertext.

An encoding designed to avoid characters that have special meaning in URLs. Base64URL is URL-safe because it replaces + and / with - and _.

A widely used character encoding for Unicode text. When ordinary text is Base64-encoded, it is usually first converted into bytes using UTF-8. Base64 works on bytes, not directly on abstract letters or symbols.

A single-character frequency unit. In frequency analysis, unigram analysis means counting individual letters rather than pairs, triples, or longer sequences.

A character that a particular cipher or encoding mode cannot process directly. For example, a strict Caesar cipher may support only A-Z letters, while a Base64 tool can operate on arbitrary UTF-8 bytes.

An alphabet that uses capital letters rather than relying on both uppercase and lowercase forms. Standard Base32 uses an uppercase alphabet, which helps it survive systems where letter case may be changed or ignored.

A close relative of the Beaufort cipher with a slightly different encryption formula. It belongs to the same family of Vigenère-like polyalphabetic ciphers.

A polyalphabetic cipher using a keyword to select different Caesar shifts for each letter position. Called 'le chiffre indechiffrable' for nearly 300 years until broken by Kasiski in 1863.

The transmission of telegraphic messages by radio rather than wire. Its military use during World War I made interception much easier and helped drive the development of ciphers such as ADFGX.

The repeated-letter structure of a word, such as ABBA for “noon” or ABBC for “meet”. Word patterns help solve substitution ciphers when spaces are preserved.

A symbol or pause used to show where one word ends and another begins. In written Morse examples, a slash is often used as a word separator, while real transmission uses a longer timed gap.

A speed measurement for Morse transmission. Higher words-per-minute values make the dots, dashes, and gaps shorter, causing the Morse signal to play faster.

The behaviour in alphabetic ciphers where shifting past Z continues again from A. Caesar, Vigenère, Affine, and ROT13 all rely on alphabetic wraparound.

Exclusive OR - a logical operation that outputs true when exactly one input is true. In cryptography, XOR is used extensively because it is its own inverse: (A XOR B) XOR B = A.

A stream-cipher system that combines conventional cryptographic keys with quantum optical communication. Y-00, also known as the AlphaEta protocol, was proposed as a way to make encrypted signals harder to measure, copy, or attack during transmission.

A numbering system that starts counting at 0. Many mathematical ciphers use zero-based indexing, where A = 0, B = 1, and Z = 25, because this works naturally with modulo 26 arithmetic.

The up-and-down writing path used in the Rail Fence cipher. Plaintext is written diagonally across a number of rails, then read off row by row to produce the ciphertext.

A Base32 variant designed to be easier for people to use. It rearranges the alphabet to reduce confusion and improve readability, especially when encoded strings may be copied, typed, or read aloud.

The standard input block used by Base64. Base64 processes data in groups of 24 bits, usually made from three 8-bit bytes. Those 24 bits are then split into four 6-bit groups, producing four Base64 characters.

A reduced alphabet used by some historical ciphers. Bacon’s original cipher used 24 letters by combining I/J and U/V, while many Polybius-square systems use 25 cells by combining I/J.

A 5x5 grid containing 25 positions. Many classical ciphers use a 25-cell grid by merging I and J so the alphabet fits neatly into a square.

The modern English alphabet from A to Z. Many classical cipher tools use a 26-letter alphabet, especially Caesar, ROT13, Atbash, Affine, Keyword, Vigenère, Beaufort, and Autokey.

A 6x6 grid containing 36 positions. ADFGVX uses a 36-cell grid so that the 26 letters and 10 digits can all be included.

The standard input block size used by Base32. Five bytes make 40 bits, and those 40 bits are split into eight 5-bit groups, producing eight Base32 characters.

A group of five binary digits used inside Base32 encoding. Because five bits can represent 32 possible values, each 5-bit group maps neatly to one character in the Base32 alphabet.

A unit used inside Base64 encoding. Base64 takes binary data and splits it into groups of 6 bits, because 6 bits can represent 64 possible values. Each 6-bit value is then mapped to one character in the Base64 alphabet.

The standard output block produced by Base32 when it encodes a full 5-byte input block. Each 40-bit block becomes eight Base32 characters. Shorter final input blocks may produce fewer real characters followed by padding.