For this assignment, you will be creating a simple cryptography library for the Caesar Cipher and Vigenere Cipher as well as a program to test these ciphers.

Your program should be divided into the following files:

ciphers.c: Will contain the code for the encryption and decryption functions for each cipher. ciphers.h: A header file containing function prototypes for each function in ciphers.c. cipher main.c: Contains your main function and code to test your cipher functions.

Your cryptography library should contain the following functions:

char * caesar encrypt(char *plaintext, int key)

This function takes a null terminated string, plaintext, and integer key as arguments. The result is a new dynamically allocated null terminated string containing the ciphertext (the result of applying the Caesar cipher on the plaintext). Your function should not alter the plaintext string in any way.

The Caesar cipher works by shifting each letter in the plaintext by the amount given in the key. If this amount would shift the letter past the last letter in the alphabet it is wrapped around to the beginning of the alphabet. For example, A shifted by 5 is F, T shifted by 20 is N, H shifted by 300 is V and D shifted by -4 is Z.

Your function should convert all letters to uppercase before encrypting and ignore any punctuation, numbers or other nonletter characters (they should be left in the ciphertext unencrypted).

The string your function returns must be null terminated, the same length as the plaintext string and not take up any more space than required. The string should be allocated on the heap and not the stack.

Hint 1: You should use the remainder operator (%) to handle the wrapping of the letters. Keep in mind that this operator may work differently then you expect for negative numbers.

Hint 2: Functions like malloc or calloc might be of use here. Hint 3: You are allowed to use character and string functions like toupper and strcpy so long as

you meet the other requirements of the function.

char * caesar decrypt(char *ciphertext, int key)

This function takes a null terminated string, ciphertext, and integer key as arguments. The result is a new dynamically allocated null terminated string containing the plaintext (the result of decrypting the Caesar cipher using the key). Your function should not alter the ciphertext string in any way.

You can decrypt the Caesar cipher by simply shifting the letters in the other direction by the amount given in the key argument.

Your function should ignore any punctuation, numbers or other nonletter characters (they should be left in the plaintext as they are in the ciphertext).

The string your function returns must be null terminated, the same length as the plaintext string and not take up any more space than required. The string should be allocated on the heap and not the stack.

Hint: You are allowed to call your other functions from this function.

char * vigen encrypt(char *plaintext, char *key)

This function takes a null terminated string, plaintext and a second null terminated string key as arguments. The result is a new, dynamically allocated null terminated string containing the ciphertext (the result of apply the Vigenere cipher on the plaintext). Your function should not alter the plaintext or key strings in any way.

The Vigenere cipher works in a similar manner as the Caesar cipher, however, instead of shifting each letter by a fixed amount, the letter is shifted by the letter in the same place in the key. If the key is shorter than the plaintext, pad the key by repeating it.

Example:

Plaintext: Key: Padded Key: Ciphertext:

ATTACKATDAWN LEMON LEMONLEMONLE LXFOPVEFRNHR 

The first letter of the plaintext is A and is shifted by L, as L is the 11th letter of the alphabet (if we start counting at 0) A is shifted by 11, resulting in L. The second letter of the plaintext is T and is shifted by E (the 4th letter of the alphabet) and the result is X. The third letter T is shifted by 12 (M is the 12th letter of the alphabet) and wrapped around to F.

Your function should convert all letters in the plaintext and key to uppercase before encrypting. Ignore any nonletter characters in the plaintext (they should be left in the ciphertext unencrypted). When a nonletter character is encountered in the plaintext, this uses a letter in the key even if the character is not encrypted. Assume the key contains only letters.

The string your function returns must be null terminated, the same length as the plaintext string and not take up any more space than required. The string should be allocated on the heap and not the stack.

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char * vigen decrypt(char *ciphertext, char *key)

This function takes a null terminated string, ciphertext and a second null terminated string key as arguments. The result is a new dynamically allocated null terminated string containing the plaintext (the result of decrypting the ciphertext using the Vigenere cipher and key). Your function should not alter the ciphertext or key strings in any way.

You can decrypt the Vigenere cipher by simply shifting the letters in the other direction by the value of the letter in the same place in the key argument.

Your function should ignore any punctuation, numbers or other nonletter characters (they should be left in the plaintext as they are in the ciphertext). Assume the key only contains letters.

The string your function returns must be null terminated, the same length as the plaintext string and not take up any more space than required. The string should be allocated on the heap and not the stack.

void freq analysis(char *ciphertext, double letters[26])

This function takes a null terminated string, ciphertext and an array of doubles of size 26. There is no return value, rather your function will be setting the values of the letters arrays. Your function should not alter the ciphertext string in any way.

The purposes of this function is to perform a frequency analysis of the cipher text. That is, count the number of occurrences of each letter. The result of this analysis should be stored in the letters array such that letters[0] is the percentage of A or as in the ciphertext and letters[25] is the percentage of Z or zs in the ciphertext. The percentage should be a double value between 0 and 100 (inclusive).

You can not assume that the letters array is initialized to zero. Assume that it may start with any values in the array. Ignore any punctuation, numbers or other nonletter characters. These should not be counted in the total number of characters or in the letters array. Uppercase and lowercase letters should be treated as the same letter.

For this function only use pointer notation and not array subscript notation. You should have no [ or ] characters in the code for this function after the function header.

Your main function Your main function in cipher main.c should ask the user for a line of text to encrypt (that may

contain spaces), the encryption cipher to use, and the key to use. Read in these values via standard input using any method you think is appropriate (getchar, scanf, gets, fgets, etc.).

You should use the functions from your cryptography library and print the following: Display the plaintext before encrypting.

Display the ciphertext after encrypting. Display the plaintext after decrypting. Print a nicely formatted table of letters and their frequency in the ciphertext.

You should do any needed error checking on the user input including checking for invalid characters in the key (for the Vigenere cipher) and return the correct exit status from your program.

For all of the functions in ciphers.c you should output an error and exit the program with a failure exit status if you were unable to allocate memory on the heap.

Ensure that you free the dynamically allocated strings before exiting your program.

Other than error messages, only the main function should output text to the screen.

Hint: When reading in the key for the Vigenere cipher, you need to be careful that you do not include the linebreak ( ) on the end of the string. If you use gets or fgets you will have to manually remove this linebreak before sending the key to the Vigenere functions.

Example Input/Output

Note that your formatting does not have to match these examples exactly. Your frequency table for example could be split between multiple rows or columns, so long as it is easily readable.

Example 1:

Input plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 1 Input key as number: 23

Plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Ciphertext: XYZABCDEFGHIJKLMNOPQRSTUVW

Decrypted plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8

Example 2:

Input plaintext: The Quick Brown FoX jUMPs Over The Lazy Dog!!

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 1 Input key as number: -3

Plaintext: The Quick Brown FoX jUMPs Over The Lazy Dog!!

Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD!!

Decrypted plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG!!

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

2.9 8.6 2.9 2.9 5.7 2.9 2.9 2.9 2.9 2.9 2.9 11.4 2.9 2.9 5.7 2.9 5.7 5.7 2.9 2.9 2.9 2.9 2.9 2.9 2.9 2.9

Example 3:

Input plaintext: The five boxing wizards jump quickly.

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 1 Input key as number: 1337

Plaintext: The five boxing wizards jump quickly.

Ciphertext: ESP QTGP MZITYR HTKLCOD UFXA BFTNVWJ.

Decrypted plaintext: THE FIVE BOXING WIZARDS JUMP QUICKLY.

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

3.2 3.2 3.2 3.2 3.2 6.5 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 6.5 3.2 3.2 3.2 12.9 3.2 3.2 3.2 3.2 3.2 3.2

Example 4:

Input plaintext: ATTACKATDAWN

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 2

Input key as string: LEMON

Plaintext: ATTACKATDAWN

Ciphertext: LXFOPVEFRNHR

Decrypted plaintext: ATTACKATDAWN

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

0.0 0.0 0.0 0.0 8.3 16.7 0.0 8.3 0.0 0.0 0.0 8.3 0.0 8.3 8.3 8.3 0.0 16.7 0.0 0.0 0.0 8.3 0.0 8.3 0.0 0.0

Example 5:

Input plaintext: BlewJs computer quiz favored proxy hacking.

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 2

Input key as string: DucKs

Plaintext: BlewJs computer quiz favored proxy hacking.

Ciphertext: EFGGBM MGPJWDWU SEAC HKNRLGN SLQHQ BCMCLHI.

Decrypted plaintext: BLEWJS COMPUTER QUIZ FAVORED PROXY HACKING.

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

2.7 5.4 8.1 2.7 5.4 2.7 10.8 8.1 2.7 2.7 2.7 8.1 8.1 5.4 0.0 2.7 5.4 2.7 5.4 0.0 2.7 0.0 5.4 0.0 0.0 0.0

Example 6:

Input plaintext: The quick onyx goblin jumps over the lazy dwarf.

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 2

Input key as string: AAAYNAUIIRVHOSVVQRXVRZUONKGFOFPBRHGZCAAAAWIMOWNDUCKS

Plaintext: The quick onyx goblin jumps over the lazy dwarf.

Ciphertext: THE DUCKS JUMP BESIDE DIZZY CATS AND LAZY LIONS.

Decrypted plaintext: THE QUICK ONYX GOBLIN JUMPS OVER THE LAZY DWARF.

Frequency analysis: ABCDEFGHIJKLMNOPQRSTUVWXYZ

7.7 2.6 5.1 10.3 7.7 0.0 0.0 2.6 7.7 2.6 2.6 5.1 2.6 5.1 2.6 2.6 0.0 0.0 10.3 5.1 5.1 0.0 0.0 0.0 5.1 7.7

Example 7:

Input plaintext: This is a test!

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 2

Input key as string: This is a bad key Error: bad key, invalid char!

Example 8:

Input plaintext: Hello World!

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 1

Input key as number: Also a bad key Error: bad key!

Example 9:

Input plaintext: Numb3rs are al0wed in plaintext

Available Ciphers: 1) Caesar 2) Vigenere

Select Cipher: 8 Error: bad selection!