[SOLVED] Cmput 175 – lab 2: caesar cipher
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Learn how to include docstrings for each function you write. Learn about another
encryption method, and more about the Unicode table.Useful functions: chr(), ord()
* Note: you don’t need to use these functions in your solution, but you may still be asked how
they work during your demo.
Look up details about how to use chr() and ord() in the Python 3 online documentation:
https://docs.python.org/3/index.html.
Look up the Unicode table: https://unicode-table.com/en/
Look up examples of Python docstrings: https://www.geeksforgeeks.org/python-docstrings/The Caesar cipher is a simple encryption method that works by substituting each letter in a word
(or message) with a letter that is a specific number of letters ahead of it in the alphabet. That
specific number is called the cipher key, and both the encrypter and decrypter must know the
value of the cipher key in order to successfully exchange secret (encoded) messages.Example: If the cipher key is 5, then the letter ‘a’ in a clear message will be encoded as the letter
‘f’ in the encrypted message. Using the same cipher key, the letter ‘v’ in a clear message will be
encoded as the letter ‘a’ (because we wrap around to the beginning of the alphabet when we
reach the end).In order to decrypt a message encoded with a Caesar cipher, simply perform the encryption
process in reverse. Example: If the cipher key is 5, then the letter ‘a’ in the encoded message
becomes the letter ‘v’ in the decoded message.
For more details about the Caesar cipher, please watch the optional online video “Encryption &
Public Keys Explained”: https://www.youtube.com/watch?v=mEFko9nf5UUIn this lab, you will create a new Python program (lab2.py). In that program, you will ask the
user for the name of a text file. You should check that this file ends with a “.txt” extension. If it
does not, print an error message on the screen and continue to ask the user for the name of a text
file until a valid one is provided. This is the only validation you need to do for the file name –
you can assume that the .txt file exists in the same directory as your Python file. This should all
be done in a function called getInputFile(), which returns the valid name of the text file as a
string.When provided with a valid text file name, you can assume that the file will contain exactly two
lines. The first line will consist of a positive integer: this is the cipher key. The second line will
consist of a series of encrypted words, where each word is encoded using the Caesar cipher
described above. You are tasked with decrypting the message, and printing the clear message
(all in lower case letters) to the screen. For example, if you have a cipher key of 1, the encrypted
message IfmmP XpsMe becomes hello world. This should all be done in a function called
decrypt(filename).Things to keep in mind:
1. Both lines in the text file may or may not have leading and/or trailing whitespace that you
should strip away before processing the data.
2. There are an unknown number of encrypted words in the text file, and these words are
separated by inconsistent whitespace (e.g. tabs, single space, multiple spaces). Your
decrypted message should separate each word by a single space. You can include a
single space after the last word in your decrypted message.3. You can assume that the encrypted words will only contain letters (i.e. no numbers,
punctuation, or symbols), but these letters could be uppercase or lowercase. Your
decrypted message should only contain lowercase letters.
4. The cipher key can be greater than 26.5. Letters should wrap around, so that if you have a cipher key of 1, the encrypted message
qjaab becomes pizza.Be sure to include a docstring at the top of each of your function definitions. In Python,
docstrings are located on the line directly after def function_name(param1, …): and is
indented one level. The docstring may be one line or many lines, but must be enclosed by triple
quotes (“””). Each docstring should describe the purpose of the function, the function
parameters, and what the function returns. Test your docstrings by calling
help(getInputFile) and help(decrypt) in your main function.Partial Sample Run 1 (does not show output from help calls):
Enter the input filename: secretMessage1
Invalid filename extension. Please re-enter the input filename: secretMessage1.jpg
Invalid filename extension. Please re-enter the input filename: secretMessage1.txtThe decrypted message is:
congratulations
Partial Sample Run 2 (does not show output from help calls):
Enter the input filename: secretMessage2.txt
The decrypted message is:
i came i saw i conqueredTesting:
Download the files called secretMessage1.txt and secretMessage2.txt from eClass, and save it
in the same directory as your lab2.py. When you test your code using these files, your output
should match what is shown in the above sample runs. However, the sample runs only test some
of the functionality of your getInputFile and decrypt functions. Once you are satisfied that your
program meets these minimum requirements, modify these files or create your own text files to
test all 5 points under “things to keep in mind.” Try to be efficient in your testing (i.e. don’t test
the same thing multiple times unless you’re testing it in a different way), and consider any
“edge” cases. DO NOT submit your .txt test files on eClass, but be prepared to show them to
your TA during your demo.Challenge Problem (optional – not for marks, but may help with future exercises):
Once you’ve created (and tested) your getInputFile and decrypt functions, try writing an
encrypt() function. This function will ask the user for the name of a text file – this file may or
may not exist yet, but it must have a “.txt” extension. If the file already exists, the contents of it
will be overwritten during the encryption; otherwise, a new file will be created. The user should
be asked to enter a message that s/he would like to be encrypted – this message may consist of
multiple words, but you can assume that it only consists of letters and spaces (i.e. no numbers,
punctuation, or special characters). The user will then be asked to provide a cipher key.Your
program should validate that the user has entered an integer number for this key. (Hint: lookup
string methods isalpha(), isalnum(), isdecimal(), isdigit(), and isnumeric() – can you use any of
those to help?) If it is not a whole number, an error message should be printed, and the user
should be re-prompted to enter a valid integer cipher key. Finally, encrypt() should encode the
entered message using the Caesar cipher method and the entered cipher key. The cipher key
should be printed to line 1 of the text file, and the encoded message should be printed in all
CAPITAL letters to line 2 of the text file.You may want to create additional functions to support your encrypt function. You can also
rename/modify functions that you created in the non-optional decryption part of this lab, if
appropriate – just be sure to test any changes you make to ensure that your decryption still
works.