Cryptography

Cryptography#

https://upload.wikimedia.org/wikipedia/commons/3/3f/Confederate_cipher_disk.png

Fig. 15 A reproduction of the Confederacy’s cipher disk
CC BY-SA 3.0. By RadioFan (talk). Source: Wikimedia Commons#

You will implement two cryptosystems:

Example run#

$ code-prj/crypto/crypto

Usage:
  code-prj/crypto/crypto MODE ALGORITHM TEXT [KEY]

Modes:
  e, encrypt        Encrypt the input text
  d, decrypt        Decrypt the input text

Algorithms:
  c, caesar         Caesar cipher
  v, vigenere       Vigenère cipher

Examples:
  code-prj/crypto/crypto e c HELLO
  code-prj/crypto/crypto decrypt v LQVRRBOFTQJ UGLE

$ code-prj/crypto/crypto e caesar LEVERPOSTEJ
OHYHUSRVWHM

Caesar#

This cipher shifts each character in a plaintext by three letters forward with a wraparound in the end:

plain

A

B

C

…

Y

Z

cipher

D

E

F

…

B

C

For example encrypting LEVERPOSTEJ gives

LEVERPOSTEJ
OHYHUSRVWHM

You have to implement two functions:

char *caesar_encrypt(char *plaintext);
char *caesar_decrypt(char *ciphertext);

These functions work in-place, in other words, you don’t have to create a new string for the encrypted/decrypted text.

Only alphabetic characters (A - Z) must be processed, others must be left as they are. You can assume that all characters will be uppercase.

Vigenère#

Similar to Caesar, however every character in the plaintext can be shifted by a variable amount. The amount to shift is determined by the key of alphabetic characters, where A corresponds to 0, B 1, etc. There is a wrap-around if necessary like in Caesar.

The key is repeated or truncated as necessary. For example if the key is LEMON:

Plaintext:		ATTACKATDAWN
Key:			LEMONLEMONLE
Ciphertext:	 	LXFOPVEFRNHR

  • The first character A encrypted with L gives L, because A + L = 0 + 11 = 11 -> L.

  • The third character of the ciphertext is F because T + M = 19 + 12 = 31 -> 5 -> F

Implement the functions:

char *vigenere_encrypt(char *plaintext, char *key);
char *vigenere_decrypt(char *ciphertext, char *key);

You can assume that plaintext:

  • is uppercase

  • contains no spaces, numbers or punctuation

Testing#

You can use these tables for testing.

Warm-up activities#

Activity 32 (Flowchart command-line interface)

Draw a flowchart for the program. Focus on the command-line interface (CLI).

🤔 Question to ponder

Which C syntax features or statements would you use to implement the flowchart?

Credits#

I copied and modified the assignment materials by Parth Sarin, Michael Cooper and Sam Redmond.

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