代写Python作业，实现简单的加密算法。
Introduction
For this assignment you will write one program that employs multiple
functions.
Reminder
If you talk to someone else (including the professor) about the assignment,
you need to cite this collaboration. If you use any external resources besides
class notes and the book, you also need to cite it.
Please limit yourself to techniques that you have learned and used in class.
Preliminaries
Log in to gemini and enter these commands to start the project:
cs110
start cipher
cd ~/110/cipher
This will copy a “skeleton program” to your account. Use the first and third
commands above to start each work session. Do not run “start cipher” more than
once; if you do, it will replace any programs you have written with new blank
programs.
Filename: ~/110/cipher/keyword.py
You are an Orc cryptanalyst working for the wizard Saruman from an undisclosed
location near Emyn Arnen.
Allies have intercepted a message that originated in Lothlrien; it appears to
have been on its way to Helm’s
Deep when the messenger was captured.1
The captured text will be in the form
FLVYM NFZKQ MVVZL YSIUM DBOOV
That the code is broken into five-letter groups inspires you: this may very
well be a keyword cipher.
To decrypt a keyword cipher, it is important to know how to encrypt a message
using a keyword: the message (called the plaintext) is written, as is
customary, in five-letter groups, with a keyword written repeatedly above.
For example,
RINGR INGRI NGRIN GRING RINGR
ELVES HAVEA TTACK ERIAD ORXXX
shows the keyword RING written above the plaintext ELVES HAVE ATTACKED
ERIADOR. Notice that: (a) if the length of the plaintext (with spaces removed)
is not a multiple of five, then the author fills in the remainder with the
letter X; and (b) if needed, only part of the keyword is used to complete the
top line.
Once this task is completed, the encrypted message is found by adding the
values (with A as 0, B as 1, and so on up to Z as 25) of the pairs of letters
in columns, using modular arithmetic if needed, to find the result.
For example, in the first column we have R (letter 17 in the alphabet) added
to E (letter 4 in the alphabet) to give us letter 21 in the alphabet: V. Thus,
the letter E in the plaintext is encrypted as V thanks to the letter R in the
keyword.
In the fifth column, we add R (17) to S (18) to get 35, which is beyond the
end of the alphabet. We calculate 35 % 26 to get 9, and thus use the letter J.
This process is continued to fully encrypt the plaintext. The result is called
the ciphertext.
A keyword cipher is extremely safe as long as the adversary does not know the
keyword, but if the adversary has some idea of what the keyword might be, they
can try all of the possibilities and, with luck, recover the plaintext.
Your program will simulate the work of the many Orcs in your command, who will
of course not use computers, but will write with foul ink on paper pressed
from the trees of Fangorn Forest. The skeleton program has a main() function,
already provided. Your job is to provide the implementations for three other
functions:
- remove_spaces, called by main,
- decrypt, called by main, and
- subtract, called by decrypt.
The user will input the captured_text (the ciphertext) and a keyword, and some
clues to search for. Your program should decrypt the captured text and check
it for these clues that indicate a useful decryption. In addition, the
report() and main() functions are already written for you.
Here is a sample interaction with the program. Bold/underlined values indicate
user input.
Enter the captured text: FLVYM NFZKQ MVVZL YSIUM DBOOV
Enter a keyword: MERRY
Enter the clues separated by one space: HOBBITS MORDOR RING
The decrypted message is THEHOBBITSAREINMORDORXXXX
Clue HOBBITS discovered in THEHOBBITSAREINMORDORXXXX
Clue MORDOR discovered in THEHOBBITSAREINMORDORXXXX
Here is another sample interaction with the program.
Enter the captured text: LFZOI MTRCT HSOBX ZLPUF
Enter a keyword: GOLLUM
Enter the clues separated by one space: MORDOR FRODO RING
The decrypted message is FRODOANDRINGINMORDOR
Clue MORDOR discovered in FRODOANDRINGINMORDOR
Clue FRODO discovered in FRODOANDRINGINMORDOR
Clue RING discovered in FRODOANDRINGINMORDOR
If the decrypted message does not contain any of the clues, your program
should just print the decrypted message.
Enter the captured text: GHYUP AHEER PWASE NMVWR HHBZM
Enter a keyword: EMRYN
Enter the clues separated by one space: MORDOR FRODO RING
The decrypted message is CVHWCWVNGELKJURJAEYEDVKBZ
What to hand in
Submit your work using the course submission procedure. In this case it is
cd ~/110/cipher
submit cipher
This will submit the file keyword.py.
Evaluation criteria
Your work will be evaluated according to criteria in two general categories,
style and correctness. Each category counts for half of your grade. For this
assignment, the criteria are:
- Style
* The header (large docstring at the top) contains all the appropriate information: your name, the name of the file, etc; and that the program description is accurate and complete,
* The logic (approach to solving the problem) is clear and reasonably simple,
* Variable names are descriptive, that is, they accurately describe the values they name,
* Commenting is accurate and complete, and
* The horizontal and vertical spacing effectively enhances the structure of the program.
* The program is efficient and avoids redundant computations. If the same calculation is used twice, it is better to do it once and store the result in a variable.
* No variables are unnecessary. A rule-of-thumb is that a variable is necessary only when it names an important part of the logic, or when it is used in the logic more than once after it is assigned. - Correctness
- The submission system will evaluate how well your program passes the automatic tests.
