What is a substitution cipher method?

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:

Ø  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.

Ø 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.

Ø 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

Ø  Polygraphic substitution: 

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

Ø Mechanical substitution ciphers: 

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

Ø The one-time pad: 

This one is a special substitution cipher which has been proven that it is unbreakable mathematically.

How are strings and arrays defined in C?

A string can be defined as a group of characters, usually letters of the alphabet. When C is either to compare a string with another string, output it, copy it to another string, or whatever, the functions are intended to do what they are called to do until a null, which is a zero, is detected. Such strings are often called an ASCII-Z string. C treats a string as an array termination with a NULL character. C does not have a separate data type for strings and so strings exist as the arrays of characters. An extra byte must be left to store the NULL “\0”. Strings are declared as the arrays of characters. For example:

char name[25];

Strings can be fully or partially initialized depending on the requirements of the user. For example:
char fill[10] = { ‘a’, ‘ ‘, ‘r’,’o’,’p’,’l’,’a’,’n’,’e’,’\0’ };

There exist certain predefined functions for modifying strings. The library cstring.h houses all these functions. Strcpy() is used to copy one string into another string. Strlen() is used to determine the length of the string. Strcmp() is function for comparing two strings i.e., which one is bigger in terms of length. Strcat() is used to concatenate two strings. The second string is appended to the end of the first string. Strrev() is used to reverse a string alphabetically.

An array is a C derived data type and can be defined as a collection of variables of the same data type that are referenced by a common name. All arrays consist of contiguous memory locations where the lowest address corresponds to the first element and highest address corresponds to the last element. Arrays are useful when quite many elements of the same data type are needed to be stored and processed. The numbers in [ ] are called indices or subscripts. A string itself is an array of characters. Arrays are of two types namely one- dimensional arrays and multi dimensional arrays. Multi dimensional array comprises of elements which themselves are arrays. The general form of an array declaration is as follows:

Type array-name [size];

Here type states the data type of the array elements. The array name declares the name of the array, and finally the size defines how many elements will be there in the array. The data type of the array is called the base type.

A multidimensional array consists of M*N elements where M is the number of rows and N is the number of columns. It can be declared as follows:

Type array-name [rows][columns];

Where single dimensional arras can be read using a single loop, it takes nested loops to read and access the elements of a multi dimensional array. One loop is used to process the rows and the other one is used to process the columns. The arrays can be initialized during the run time as follows:

Type array-name [size 1]……….[size N]= {value list};

The value list consists of the array elements. C allows you to skip the size of arrays in an initialization statement. These types of arrays are called unsized arrays. For example:
Int abc[ ] = {1,2,3,5,7};

The best way to assign values to an array is using a "for" or "while" loop because it’s not affordable to write a separate initialization for every element. Arrays can be passed s parameters to the functions to maintain a structured design. Always keep in mind that in C subscripts start from 0. A string from an array of strings can be accessed by using pointers. But keep in mind that pointers hold only address of the strings. From there it is able to obtain the whole chunk of the contiguous memory.