There are two classic methods for cryptography
namely transposition cipher method and the substitution cipher method.

**In this article we shall discuss about the latter one i.e., the substitution cipher method.**
- This method of encoding involves replacement of the units or letters of
the plain text with some other units or letters.

**- The encoded text is then called as the cipher text.**

- The replacement of the units is made based up on
some regular system.

- These units might be individual letters, pairs or triplets
of letters and so on.

- On the receiver’s side, an inverse substitution is
required for deciphering the text.

- We can make a comparison between the
transposition ciphers and the substitution ciphers.

- In the former ciphers, the
plain text units are rearranged unlike in substitution cipher where units are
replaced.

- The order of rearrangement in the transposition ciphers is somewhat
more complex than what is followed by the substitution ciphers and the units
are not changed.

- On the other side, the sequence of the units remains same in
the substitution cipher but they are themselves altered.

__There are various types of substitution cipher as mentioned below:__

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__Simple substitution ciphers:__
- This involves substitution of the single letters and thus
has been termed as the simple substitution.

- The alphabet can be written out in
some order so as to represent the substitution.

- This alphabet is referred to as
the substitution alphabet.

- This alphabet might be revered or shifted or
scrambled in some complex manner.

- In such cases, it is termed as the deranged
alphabet or the mixed alphabet.

- The creation of the mixed alphabets involves
writing out a keyword while removing the repeating letters and then rewriting
the leftovers in the same sequence.

- For avoiding the transmission errors, the
cipher text is written in block form and the spaces and the punctuation are
omitted.

- This also helps in creating disguises for the boundaries between the
words.

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__Homophonic substitution:__
- This method is followed for increasing the difficulty for the
frequency analysis attacks.

- The frequencies of the letters of the plain text
are disguised by homophony.

- Here the letters of the plain text are mapped to
many symbols of the cipher text.

- Normally the plain text symbols with highest
frequencies are mapped with more equivalents when compared to their low
frequency counterparts.

- This leads to the flattening of the frequency
distribution which in turn raises the difficulty of frequency analysis.

- For the
invention of larger alphabets a number of solutions are employed.

**-**

**The simplest of these solutions is using a numeric substitution alphabet.**

- Another method
uses the variations of the existing alphabet i.e., writing it upside down, or
in upper case and lower case etc.

- Nomenclature is also a variant of the
homophonic substitution.

- The other two types of homophonic ciphers namely
straddling checker board and book cipher.

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__Polyalphabetic substitution:__
- It involves the use of the multiple cipher alphabets.

- For the
facilitation of the encryption process, these alphabets are written out in a
big table which is referred to as the tableau.

- The particular poly alphabetic
cipher is defined by the method with which the tableau is filled and the
alphabet is chosen.

**- Some types of the polyalphabetic ciphers are:**

1. Beaufort cipher

2. Gronsfeld cipher

3. Running key cipher

4. Autokey cipher

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__Polygraphic substitution:__
Here the letters of the plain text are substituted in terms of
large groups instead of individual letter substitution.

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__Mechanical substitution ciphers:__
Some examples of this type of substitution ciphers are
enigma, rotor cipher machines etc.

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__The one-time pad:__
This one is a special substitution cipher which has been proven that
it is unbreakable mathematically.

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