Assignment #2 – Modified Caesar Cipher

Introduction to Programming


Assignment #2 – Modified Caesar Cipher

Assignment 2 – Modified Caesar Cipher


Your second assignment will consist of a decryption problem.  Some encrypted text will be provided and your job is to write a C program to decrypt it.  As always, the C program should be submitted as a standard C source code file.

Please note that the computer program should comply with the commenting and formatting rules as described in class.  For example, there should be a header for the whole program that gives the author’s name, class name, date, and description.  End braces should be commented, and there are alignment and indenting requirements as discussed.  Please ask if you have any questions.

Program 1:           Modified Caesar Cipher

Basic Caesar Cipher

In cryptography, a Caesar Cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence. (

In a Caesar Cipher, the encryption is performed against a key string, which is often just the 26 letters of the alphabet.  An offset value is defined that consists of a positive or negative integer.  The plaintext (the normal text) is encrypted, one letter at a time, by using that offset value in the following way:  (a) the index position within the key string of the letter to be encrypted is determined, (b) the offset value is applied to that index in order to obtain a new index within the key string, (c) the character at the new index is written as the cipher text.

For a simple example, let’s assume the key string consists of the 26 letters of the alphabet plus a space:

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z \0

Here we have the 26 letters of the English alphabet plus a space with their index positions listed above.  (To simplify, we’ll stick with all caps for the moment.)  We want to encrypt the phrase “I LOVE TO FLY” using an offset of +5.  We get the following:

            Plain text:        I LOVE TO FLY     Cipher text:     NEQT JEYTEKQC

Note that ‘I’ has an index of 8 within the key string.  When we add 5 we get an index of 13, which corresponds to the letter ‘N’, also within the key string.  We then write down that letter as the first character of the encrypted text.  Similarly, the letter ‘Y’ has an index of 24.  When we add 5 we get 29, but since that index doesn’t exist within the array, we “wrap around” and replace the ‘Y’ with a ‘C’.    (Note that the ‘\0’ is not counted.)

To decrypt a message, we apply the offset value in reverse.  If the offsets were added to produce the cipher text, then they should be subtracted to reconstitute the plaintext.

            Cipher text:     NEQT JEYTEKQC    Plain text:        I LOVE TO FLY

Here the plaintext is obtained from the cipher text by subtracting a constant offset of 5.  Note that with this technique, spaces are treated just like any other character.  Note also that we did not access the ASCII table in any way.  All offsets are calculated within the key string only.

Modification of the Caesar Cipher

The basic Caesar Cipher has a characteristic that makes it relatively easy to break:  every character in the plain text is encoded with the same character in the cipher text.  In the above example, every space is encoded with an ‘E’.  Since the space is the most commonly used character, an observant reader could assume that a space is represented by an ‘E’ and determine the offset from that.


A variation on the basic Caesar Cipher uses a variable offset, one that is different for each character encoded.  In this algorithm, the first character of the plain text will be encoded by some firstOffset as above, the next character is encoded by (firstOffset+1), the next by (firstOffset+2), etc.  As you can see, the offsets grow sequentially from the first character of the plain text to the last.  This ensures that instances of a character in the plain text will be represented by different characters in the cipher text.


Here is the above string encoded with a sequential offset that begins with 5.  Notice that the “I” is still encoded by “N”, but all of the other characters are different.


            Plain text:        I LOVE TO FLY     Cipher text:     NFSWDOKEANUAO


Here the “I” is encoded with an offset of 5, yielding a cipher text character “N”.  The first space is encoded with an offset of 6, yielding a cipher text character of “F”.  The “L” is encoded with an offset of 7, yielding a cipher text character of “S”, and so forth.  Each character is encoded by an offset 1 larger than the preceding character until the end of the plain text is reached.


Let’s apply the modified Caesar Cipher described above to a more complicated example.  What follows is the key string you can use to decrypt the cipher text.


Key String:

“hXTFexn39AJ6cmCYgktNuUwBdlKL04R1DSQrH y5q8VpjIZMGivEfasoWOb72zP“


The only characters in the key string are lower case alpha characters, upper case alpha characters, digits from 0 to 9, and a space.  Those characters have been jumbled up to make the problem more difficult.  The double quotes are used to mark the end of the key string; they are not part of it.


The cipher text that you need to decrypt against that key string is:


Ciper text:

“yv uB8pTOAoVh6SrTdlWdq76pJceM3iFu5aSw O27L4pzkTAwkyEl”


Of necessity, all of the characters in the cipher text are contained in the key string.  The sequential offset values used to encrypt the plain text are positive and can begin with any number between 1 and 60.  (You should, therefore, apply negative sequential offsets to decrypt the cipher text.)


Your mission (should you choose to accept it) is to (a) determine the initial offset value, (b) use sequential offsets from that point to decrypt the above cipher text, and (c) do what it says.

Hints on the solution:

One possible way to solve this problem is to generate decryptions for all initial offsets from 1 to

  1. Your output should look like the following, where “Offset” means “Initial Offset”:


Cipher Text:          “yv uB8pTOAoVh6SrTdlWdq76pJceM3iFu5aSw O27L4pzkTAwkyEl”

Offset:   1;  Result:   GQgtrHOGPjRv2tNGnnHFtHvtMG5tqtQvxt2GPtutGiPgQtdHRvPr

Offset:   2;  Result:  HMSYkQrWMzp4i7ktMxxrTkrikZMyk5kSiek7MzkNkMGzYSkBr4izQ

Offset:   3;  Result:  rZDCgSQoZ2V0GbgkZeeQXgQGgIZ gygDGFgbZ2gtgZM2CDgwQ0G2S


Offset:   5;  Result:  SjRcC1DajbqKZWCYjTTDPCDZCpjrCHCRZXCWjbCgCjIbcRCuDKZb1 Offset:   6;  Result:  ……


These are actual data, so you can use these to check your program.  After the full list is generated, visually scan through it to determine the offset and the plain text.


Second, the following functions might be useful to your solution:


  1. A function to determine the length of a C-string. Note that the ‘\0’ is not included in the length calculation.

a. int strlen(char *); (<string.h>)

  1. A function to locate a character within a C-string and return its index value.
  2. This function can be used to find a given character within the key string.

Finally, you can initialize a C-string with the key string and cipher text specified above by copy/pasting those strings into your code, but here is an important point:   never copy/paste from a non-text file, like a PDF for example, directly into your code.  Instead, copy/paste the text into Notepad or Notepad++ first, then copy/paste into your code.  This eliminates most of the problems caused by hidden formatting characters and other issues.

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