##### Wiktionary

**substitution cipher**

n. (context cryptography English) a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.

##### Wikipedia

**Substitution cipher**

In cryptography, a **substitution cipher** is a method of encoding by which units of plaintext are replaced with ciphertext, according to a fixed system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution.

Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.

There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a **simple substitution cipher**; a cipher that operates on larger groups of letters is termed **polygraphic**. A **monoalphabetic cipher** uses fixed substitution over the entire message, whereas a **polyalphabetic cipher** uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.

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#### Usage examples of "substitution cipher".

It could take any English- language message and transform it by a simple __substitution cipher__ into something that looked like gibberish.

I recalled another message, too, which we had intercepted, a well-disguised but simple __substitution cipher__.

Egyptian hieroglyphics are, in significant part, a simple __substitution cipher__.

The signals, incidentally, are not a __substitution cipher__, for the languages of the red savages, not being written languages, in any conventional sense, do not have a standardized alphabet or syllabary.