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{{Short description|Book for ciphers}}
{{for|the concept in machine learning|Vector quantization}}
[[File:State Department code book 1899, open to title page, at NMC.agr.jpg|thumb|U.S. State Department code book issued in 1899, an example of a one-part code, at the [[National Cryptologic Museum]]]]
[[File:State Department code book 1899, code page 187.agr.jpg|thumb|Page 187 of the State Department 1899 code book, a one-part code with a choice of code word or numeric ciphertext. Numeric codes are prefixed by the page number.]]

A '''codebook''' is a type of document used for gathering and storing [[cryptography code]]s. Originally, codebooks were often literally [[books|<span lang="en-us" dir="ltr">books</span>]], but today "codebook" is a byword for the complete record of a series of codes, regardless of physical format.

==Cryptography==
In [[cryptography]], a codebook is a document used for implementing a [[code (cryptography)|code]]. A codebook contains a [[lookup table]] for coding and decoding; each word or phrase has one or more strings which replace it. To decipher messages written in code, corresponding copies of the codebook must be available at either end. The distribution and [[physical security]] of codebooks presents a special difficulty in the use of codes compared to the secret information used in [[cipher]]s, the [[cryptographic key|key]], which is typically much shorter.

The United States [[National Security Agency]] documents sometimes use ''codebook'' to refer to [[block cipher]]s; compare their use of ''combiner-type algorithm'' to refer to [[stream cipher]]s.

Codebooks come in two forms, one-part or two-part: 
* In one-part codes, the plaintext words and phrases and the corresponding code words are in the same alphabetical order. They are organized similar to a standard dictionary. Such codes are half the size of two-part codes but are more vulnerable since an attacker who recovers some code word meanings can often infer the meaning of nearby code words. One-part codes may be used simply to shorten messages for transmission or have their security enhanced with [[superencryption]] methods, such as adding a secret number to numeric code words.<ref name="pommerening" />
* In two-part codes, one part is for converting plaintext to ciphertext, the other for the opposite purpose. They are usually organized similarly to a language translation dictionary, with plaintext words (in the first part) and ciphertext words (in the second part) presented like dictionary headwords. 

The earliest known use of a codebook system was by Gabriele de Lavinde in 1379 working for the [[Antipope Clement VII]].<ref name="pommerening" >
Klaus Pommerening.
[https://www.staff.uni-mainz.de/pommeren/Cryptology/Classic/1_Monoalph/Codebook.html "Codebooks"].
</ref>

Two-part codebooks go back as least as far as 
[[Antoine Rossignol]] in the 1800s.<ref name="pommerening" />

From the 15th century until the middle of the 19th century,
nomenclators (named after [[nomenclator (nomenclature)|nomenclator]]) were the most used cryptographic method.<ref>
Chris Christensen.
[https://www.nku.edu/~christensen/section%2023%20codes.pdf "Codes and Nomenclators"].
p. 4
</ref>

Codebooks with superencryption were the most used cryptographic method of World War I.<ref name="pommerening" />

The [[JN-25]] code used in World War II used a codebook of 30,000 code groups superencrypted with 30,000 random additives.

The book used in a [[book cipher]] or the book used in a [[running key cipher]] can be any book shared by sender and receiver and is different from a cryptographic codebook.

==Social sciences==
In social sciences, a codebook is a document containing a list of the codes used in a set of data to refer to variables and their values, for example locations, occupations, or clinical diagnoses.

==Data compression==
Codebooks were also used in 19th- and 20th-century [[Commercial code (communications)|commercial codes]] for the non-cryptographic purpose of data compression.

Codebooks are used in relation to [[precoding]] and [[beamforming]] in mobile networks such as [[5G]] and [[LTE (telecommunication)|LTE]]. The usage is standardized by [[3GPP]], for example in the document [https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3197 TS 38.331], NR; Radio Resource Control (RRC); Protocol specification.

==See also==
*[[Block cipher modes of operation]]
*[[The Code Book]]

==References==
{{Reflist}}

[[Category:Cryptography]]
[[Category:Social research]]