Dictionary · Decipher reference

Dictionary

Short definitions for the cipher terms, notation, and aliases used across Decipher.

Also found as glossary · terms · definitions

Notation

Symbol	Usual meaning
Pᵢ, Cᵢ, Kᵢ	plaintext, ciphertext, and key or keystream unit at position i
N	number of units in the selected sample
m	alphabet or inventory size
fₓ or nₓ	observed count of unit x
pₓ or p̂ₓ	probability or observed proportion of unit x
k	lag, shift, or other displacement, as defined on the page
H	entropy in bits when the logarithm has base 2
χ²	chi-square statistic
a mod m	the remainder class of a modulo m

A page can give a symbol a more specific meaning. It also says whether positions begin at zero or one when that changes the calculation.

A

Aligned pair Patterns and pairs: An ordered pair formed from units in two sources after applying an offset.
Alphabet Alphabets and symbol values: The ordered set of symbols available to a cipher or analysis.
Alphabet square Grids, squares, and alignments: A rectangular, usually square, arrangement that assigns positions or coordinates to an inventory of symbols.
Aristocrat Challenge Formats: A simple-substitution Challenge Format that retains word divisions and punctuation.
Autocorrelation Coincidence at lag: A numerical correlation between a sequence and a lagged copy. It is not an alias for Decipher's categorical coincidence at lag.NIST, Autocorrelation

B

Baconian Baconian: A biliteral system that represents letters with groups drawn from two classes, conventionally A and B.
Beaufort Beaufort: A periodic polyalphabetic cipher using the relation described on Decipher's Beaufort page.
Biliteral Biliteral systems: Using two distinguishable classes or symbols; the structural idea behind Baconian encodings.

C

Caesar cipher Caesar: Also Caesar shift · shift cipher; A substitution in which every plaintext letter is shifted by the same amount.
Character frequency Frequencies: The count or proportion of each selected character unit in a sample.
Chi-square Chi-square against uniform: Also χ² · chi squared; A statistic that sums squared observed-minus-expected differences divided by expected counts. Decipher's current expectation is uniform.NIST, Chi-square goodness of fit
Cipher: A rule-governed transformation of plaintext material, normally controlled by a key. Historical use overlaps with code.Friedman, Codes and Ciphers NIST, Cipher
Cipher alphabet Substitution: The ciphertext symbols associated with a plaintext alphabet in a substitution system.
Cipher Method: Decipher's defined reversible transformation, separate from text preparation and Challenge Format.
Cipher family: A broad class of related systems, such as substitution or transposition; less exact than a Decipher Cipher Method.
Ciphertext: The material produced by applying the cipher transformation to prepared plaintext material.
Code: In Decipher's working distinction, a system that replaces words, phrases, or other units with code groups rather than transforming a regular alphabet unit by unit.Friedman, Codes and Ciphers NSA, Basic Cryptologic Glossary
Coincidence: An equality between selected units or positions.
Coincidence at lag Text statistics: Also lag coincidence; The proportion of positions whose unit equals the unit k positions later.
Column permutation key Columnar transposition: An ordering that says how grid columns are rearranged or read.
Contact Contacts and reversals: A unit immediately to the left or right of another unit under the selected boundary rules.
Coordinate Squares and pairs: A label for a position in a grid or square, often written as a row-column pair.
Crib Cribs and hypotheses: Also probable word; Plaintext suspected or known to occur at some position in a ciphertext. A crib is not necessarily confirmed known plaintext.
Cryptanalysis Cryptanalysis: The practice of recovering or testing information about a cryptographic system, key, or message without simply following the intended keyed decryption procedure.

D

Decoding: Reversing an encoding or code convention. Historical writing may use the word more broadly.
Decryption: Applying the inverse of a cipher transformation with the required method and key.
Digram Overlapping groups: Also bigram; A sequence of two adjacent selected units.
Distribution Frequencies: The counts or proportions of the values taken by a selected unit.
Distinct units Text statistics: The number of different units observed in a sample.

E

Encoding: Representing material in another form according to a convention; not necessarily encryption and not necessarily secret.
Encryption: Applying a cipher transformation to prepared plaintext material.
Entropy Shannon entropy: Also Shannon entropy; For Decipher's text statistic, the Shannon entropy of the observed unit proportions. It measures the distribution and discards order.Shannon, Mathematical Theory of Communication

F

Filler: Also filler character · filler letter; Material inserted to satisfy or disguise a cipher's structural requirements. Its exact job depends on the cipher.
Fixed point: A unit that a transformation maps to itself.
Frequency: A count. Informal writing may also mean relative frequency, so Decipher uses count or proportion when the distinction matters.
Frequency analysis Frequencies: Analysis of the counts and distributions of units or groups in ciphertext.

G

Gap Repetitions and periods: Also distance · separation; The difference between reported positions of successive occurrences of a repeated sequence.
Gronsfeld Gronsfeld: A periodic shift cipher whose key is a sequence of decimal digits.
Grouping: Visual separation of ciphertext into blocks. Grouping need not mark linguistic word boundaries.

H

Hypothesis Cribs and hypotheses: A Solver-authored claim retained for testing. It remains provisional even when internally consistent.

I

Index of coincidence Index of coincidence: Also IC · coincidence index; In Decipher, the raw finite-sample probability that two distinct positions selected without replacement contain the same unit.Friedman, Index of Coincidence Campaigne, Index of Coincidence
Inventory: The set of distinct units admitted or observed for a particular operation.

K

Kasiski examination Repetitions and periods: Also Kasiski method · Kasiski test; Using repeated ciphertext sequences and factors of their separations as evidence for a period in a periodic polyalphabetic system.Gaines, Cryptanalysis Military Cryptanalysis, Part II
Key: Information that selects one transformation from a Cipher Method's possible transformations.
Key length Candidate and fundamental period: The number of units in the stated key. A repeated written key can have a shorter fundamental period.
Keyword: A word or phrase used to derive a key, alphabet, square, or ordering; not automatically identical to the derived key.
Known plaintext Cribs and hypotheses: Confirmed plaintext corresponding to some ciphertext. A plausible crib is not known plaintext merely because it fits.

L

Lag Coincidence at lag: The number of positions separating the units compared in a lagged measure.
Letter frequency Frequencies: Frequency analysis whose selected units are letters under a stated alphabet and case-folding rule.

M

Mapping Cribs and hypotheses: A proposed or defined association from one unit to another. A one-to-one mapping does not allow two source units to share one target.
Modular arithmetic Modular arithmetic: Also modulo · mod; Arithmetic on remainder classes.
Monoalphabetic substitution Substitution: A substitution using one fixed plaintext-to-ciphertext alphabet throughout the selected material.

N

N-gram Overlapping groups: Also ngram; A sequence of n adjacent selected units. Decipher's frequency analysis currently supports n equal to 1, 2, or 3.
Null: A symbol or group intended to carry no plaintext content. Historical practice can overlap with filler.

O

Offset: A displacement applied before positions, samples, or aligned sources are compared.
Ordered pair Adjacent and aligned pairs: A pair in which first and second position matter, so AB differs from BA.

P

Padding: Material added to reach a required length or block shape. It is not automatically a cryptanalytic null.
Pair frequency Adjacent and aligned pairs: The count or proportion of an ordered adjacent or aligned pair under stated unit and offset rules.
Patristocrat Challenge Formats: A simple-substitution Challenge Format with word divisions and punctuation removed.
Period Repetitions and periods: The smallest positive displacement over which a repeating process or sequence repeats. A tested column count is only a candidate period.
Periodic phase Periodic columns: Also periodic column · phase; The subsequence of units whose indices share one remainder modulo a candidate period.
Plaintext: Material on the readable side of a cipher relation. It may already have undergone text preparation.
Plaintext Material: Decipher's name for the original input before Cipher-Method-specific text preparation.
Playfair Playfair: A digraphic substitution cipher driven by positions in a keyed square.
Polybius square Polybius: A square that represents symbols by coordinate pairs.
Polyalphabetic substitution Periodic Keys: A substitution in which the operative cipher alphabet changes by position or state.
Proportion: Also relative frequency; A count divided by the stated total.

R

Rank: Position after sorting observed or reference values, normally from greatest to least.
Self-inverse Beaufort: Also reciprocal; A transformation whose encryption and decryption operations are the same under the stated convention.
Repeated sequence Repetitions and periods: A sequence of selected units occurring more than once in a sample.
Reversal Contacts and reversals: An ordered pair observed in both directions, such as AB and BA.
Row order and column order Regular grids: Also row order · column order; Permutations specifying how grid rows or columns are arranged or read.

S

Sample Choose the sample first: The units actually supplied to a statistic after filtering, segmentation, phasing, or blocking.
Shift: An addition or subtraction in a numbered alphabet modulo its size.
Simple substitution Simple substitution: A monoalphabetic substitution that maps individual plaintext units one to one to ciphertext units.
Solution: Decipher's saved statement of plaintext and key or settings for a Challenge; not every plausible projection.
Square: A grid with equal row and column counts; in cipher pages it usually means an alphabet or Polybius-style square.
Substitution Substitution: A transformation that replaces units while normally preserving their order.
Symbol: A visible mark or token treated as one unit under the current analysis rule.

T

Terminal Patterns and pairs: A repeated initial or final sequence of a selected length, usually within word-like units.
Text preparation: The Cipher-Method-specific conversion of Plaintext Material into the units the cipher actually transforms.
Transposition Transposition: A transformation that changes the positions of units without changing their identities.
Transposition unit: The unit whose position is moved, such as a letter, coordinate pair, cell, or other token.
Trigram Overlapping groups: A sequence of three adjacent selected units.

U

Uniform distribution Chi-square against uniform: A distribution assigning the same probability to each unit in the stated inventory.
Unit: The item counted, aligned, grouped, or transformed by an operation: for example a letter, grapheme, word, coordinate pair, or cell.

V

Variant Beaufort Variant Beaufort: The periodic polyalphabetic relation described on Decipher's Variant Beaufort page; not another name for Beaufort.
Vigenère Vigenère: Also Vigenere; A periodic polyalphabetic shift cipher.

W

Word boundary: A division treated as separating word-like units. Ciphertext grouping is not automatically a word boundary.
Word signature Word signatures: Also word pattern; A canonical record of which positions in a word contain the same unit, such as MEET → 1.2.2.3.

Sources