Letters or groups of letters are replaced by other ones.
Monoalphabetic substitution:
Each letter is replaced by another letter that is always the same.
Polyalphabetic substitution:
Each letter is replaced—depending on its position in the text—by
another letter.
(Most important method of classic cryptography in the 20th century
up to the sixties)
Monographic substitution:
Letters are replaced by symbols one at a time.
Polygraphic substitution:
In each step one or more letters are replaced by several symbols.
Homophonic substitution:
For some plaintext letters or groups there are several choices
of ciphertext symbols.
This can also be described as probabilistic encryption.
The ciphertext is randomly chosen.
Transposition:
The letters of the plaintext are permuted.
Codebook:
Letter groups of various lengths (for example entire words) are replaced
by other ones according to a list.
An example is compression—here usually the list is generated
dynamically.
Since the Renaissance this was in use under the denomination Nomenclator.
It was the most used encryption method even in the 20th Century, especially
by diplomats.
Source coding (superencrypted code):
The plaintext is transformed with a codebook, and the resulting "intermediate
text" is encrypted by some kind of substitution.
Book cipher:
Plaintext words or letters are looked up in a certain book. As ciphertext
one takes the position of the word or letter in the book, for example
page number, line number, number of the word (or number of the letter).
Block cipher:
In each step a fixed number of letters is substituted at once.
Stream cipher:
In each step a single letter is substituted, each time in another way,
depending on its position in the plaintext.
Product cipher:
A sequence of several transpositions and block substitutions is applied
one after the other. Also called cipher cascade.
![[JoGu]](../../JGU.gif)