As usual, create two directories for today's class. Create a
directory called lecture24 under activities, and
a directory called lab24 under labs.
One possible way to create a word puzzle is to take the characters of the word, and re-arrange them in some random fashion. This is known as a transposition cipher, and is typically used to create word jumbles that you have likely seen either in a newspaper (or more likely on a newspapers website) or at the movies, during the pre-roll they show before the previews. Today, you will write a function that can perform such a transposition on an input word.
Create a file called transpose.py in your
lecture24 directory. In this file, you will write a function
called transpose_characters, which takes a single
parameter: a string of lowercase characters. Your function should
also return a string. This
returned string should contain the same characters from the input
string, but in some random order. Recall that you can use the string
slicing syntax (some_string[start : exclusive_end]) as a
mechanism to remove a specific charater from a string, as demonstrated
in class on Friday.
For example, if the input to transpose_characters is
"williams", your function might return "lialimws" or "smailliw". It
should never return "llllllll", or even "wiliamsz", since they do not
contain the exact same characters (some characters are missing, or
there are extra characters).
Cryptography is the science of hiding secrets. A typical cryptographic protocol takes some plaintext message P and some key k, and produces a ciphertext C that is unreadable by any entity that does not know the key k
One very easy (and surprisingly incredibly old) cryptographic protocol is know as rot13. In the early days of the Internet, rot13 was used to hide solutions to puzzles posted on message boards. rot13 is very simplistic, and is considered the Internet's version of printing the answers upside down on the same page.
The key in rot13 is always 13. To encrypt, you chose the letter 13 spaces further in the alphabet of the entered character. For example, the encryption of the character 'a' is the character 'n'. The end of the alphabet wraps around to the beginning of the alphabet. Thus, the character 'z' is actually encoded as 'm'. Recall that you can use the modulus (A.K.A. remainder, the % operator) to cause this cyclic process to occur. The nice thing about rot13 is that decryption is the same process, so you do not need to write a separate decrypt function.
Write a program rot13.py which reads a sentence from
the user, and prints the rot13 encryption of the sentence the user
typed in. Your program should only encode the lower-case alphabetic
characters of the typed sentence. So, for example, "hello world!"
would become "uryyb jbeyq!".
While there are no required functions for this assignment, you should still use functions where appropriate.
Make sure you test your program well. How many test cases do you need? Include your test cases in your program file. Comment them appropriately.
When you have finished, create a tar file of your lab24
directory. To create a tar file, execute the following commands:
cd ~/cs120/labs tar czvf lab24.tgz lab24/
To submit your activity, go to cseval.roanoke.edu. You should
see an available assignment called Lab Assignment 24. Only
one of your pair should submit your activity. Make sure both partners
are listed in the header of your files.
Do not forget to email your partner today's files!