This page consists of a list of terms dealing with cryptology used throughout the website. They are displayed in alphabetical order.

## Bigram / Bigraph

A bigram is a combination of two letters. Every language uses some bigrams that are more common than others. For example, in English the bigrams 'TH' and 'SH' are more common than 'XL' and 'SR'. Some ciphers convert one plaintext letter into a bigram to obscure frequency analysis.

## Cipher

The cipher is the cryptographic algorithm used to encode or decode information. The cipher is part of the cryptosystem.

## Ciphertext

The plaintext is the output of an encryption algorithm or the input for a decryption algorithm. The ciphertext is the encrypted information that needs to be revealed using a cryptosystem.

## Code

The code is the method used to encrypt or decrypt information. In a sense it is analogous to the cryptosystem.

## Cryptanalysis

From the Greek 'kryptós' = 'hidden' and 'analýein' = 'to loosen, to untie'. I.e., the process of revealing a hidden message. Cryptanalysis is the process of looking for weaknesses in an algorithm used to hide information or the algorithm itself. The process of decrypting information without (full) prior knowledge of the cryptosystem.

## Cryptography

From the Greek 'kryptós' = 'hidden' and 'gráphein' = 'to write'. I.e., the process of writing hidden information. Cryptography is the actual process of converting a plaintext to ciphertext and ciphertext to plaintext.

## Cryptology

From the Greek 'kryptós' = 'hidden' and 'lógos' = 'word'. I.e., the science dealing with hidden information. Cryptology is the combination of cryptography and cryptanalysis.

## Cryptosystem

The combinaton of procedures, protocols, cryptographic algorithms and instructions used to encode or decode a message with cryptography. The cryptosystem holds the keys, methods, algorithms, agreements between sender and recipient, i.e. all necessary information to properly encode and decode information.

## Decipher / Decode

The deciphering / decoding / decrypting process converts ciphertext into the original plaintext with prior knowledge of the used cryptosystem.

## Decrypt

The decrypting process converts ciphertext into the original plaintext without prior knowledge of the cryptosystem used.

See bigram

## Encipher / Encode / Encrypt

The enciphering / encoding / encrypting process converts plaintext into ciphertext by means of a cryptosystem to prevent unauthorized access.

## Fractionation

The process of fractionation embodies the dividing of plaintext into fractions to increase difficulty to analyse a ciphertext.

## Homophonic Substitution

A homophonic substitution cipher employs one cipher alphabet that is larger than the standard alphabet. The purpose is to replace plaintext letters with more than one possible ciphertext letter. A homophonic cipher may employ an invented alphabet of symbols to use. Do not confuse this with polyalphabetic substitution, which employs more than one cipher alphabet. The Voynich Manuscript is possibly the most famous example of an invented symbolic alphabet used for homophonic substitution. The script is never deciphered though and some claim that it is a fraude.

## Key

A key is a piece of information that controls the algorithm of a cryptosystem. A key can be a keyword or a key phrase, but also some other kind of information necessary for the encryption or decryption process.

## Monoalphabetic Substitution

The process of monoalphabetic substitution replaces one letter from the plaintext with another letter or symbol. The same letter or symbol in the ciphertext always represents the same letter in the plaintext. I.e. if the plaintext 'A' is replaced by an 'S', then all As are replaced by an S and all Ss from the ciphertext decode back to an A.

## Null

A null is a meaningless character with regards to the plaintext to confuse or obscure analysis. Nulls are sometimes also used to make a plaintext or ciphertext message a certain length.

## Plaintext

The plaintext is the input for an encryption algorithm or the output of a decryption algorithm. The plaintext is the unencrypted information that is to be hidden using a cryptosystem.

## Polyalphabetic Substitution

The polyalphabetic substitution consists of using multiple cipher alphabets to encode or decode the same message. This usually results in the same plaintext letters being replaced by different ciphertext letters. This makes cryptanalysis a bit more difficult when compared to monoalphabetic substitution.

## Polybius Square

A table invented by the Greek scholar and historian Polybius to fractionate plaintext letters in order to be represented by a different set of characters. This usually is a 5x5 square filled with 25 characters (I=J or U=V) or a 6x6 square with all letters and ten digits. Some ciphers use more than one Polybius square to encode and decode.

## Polygraphic Substitution

A polygraphic substitution cipher replaces one plaintext letter with a multiple of ciphertext letters or symbols. If one plaintext letter is replaced by two ciphertext letters we say it is a digraphic system. The advantage of a polygraphic substitution is the disturbance of the frequence of letters. This makes cryptanalysis a lot harder.

## Steganography

From the Greek words 'stégein' = 'to cover' and 'gráphein' = 'to write'. I.e. to cover a written message. Steganography is the science of hiding the very existence of information within other information. This in contrast to cryptography, where the information is not hidden but rather scrambled. A well known form of steganography is hiding any type of computer data file within an image graphic file; an unsuspected onlooker would see only an image, but the intended recipient knows there is more information than can be seen at first glance.

## Substitution

Substitution is the process of replacing a part of plaintext with something else. For detailed information see: monoalphabetic substitution, homophonic substitution, polyalphabetic substitution and polygraphic substitution.

## Transposition

Transposition is the process of re-arranging plaintext letters to form a ciphertext. Transposition is often used in combination with a substitution. There are a multitude of transposition techniques, some examples are columnar transposition, rail fence and route cipher.

## Trigram / Trigraph

A trigram is a combination of three letters. Every language uses some trigrams that are more common than others. For example, in English the bigrams 'ING' and 'STR' are more common than 'COF' and 'SIR'. With that knowledge frequency analysis can be performed on trigrams. Some ciphers convert one plaintext letter into a trigram to obscure normal frequency analysis.