继续改进前次作业的 Vigenre Cipher 算法。
Introduction
For this assignment you will need to create a basic cryptography program based
the Vigenre cipher.
To Note
- You can assume that the length of the text to be encrypted/decrypted is always greater than zero.
- If you find a character that’s not a letter, i.e. a number, a space, or a punctuation character, do not change it, leave as is, and copy it over.
- Although the examples given in these instructions are a good place to start. Your program must work with any encrypted/decrypted messages given in the inputs.
The Vigenre cipher decodes text based on a second string that is called key.
The key must be the same key used to encrypt the text if you want to decrypt
the encrypted text to discover the original text. The alphabetical position of
each of the characters in the key determines how many positions the character
in the original text must be shifted. The letter “A” in the key means a shift
of zero, “B” produces a shift of 1, “C” a shift of 2 and so on. A key can
contain more than one character, in those cases you need to ‘extend’ the key
by repeating it as many times as necessary until it reaches the same length of
the whole text. Your program must ask the user for the Decoding Key. To learn
more about the cipher visit:
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher .
For example, suppose that the text to be encrypted is “pittsburgh pa” and the
key is “yes”. The first step is to concatenate “yes” over and over until it’s
same length as “pittsburgh pa”, consider white spaces as a special character.
We did this task in lab 1.
Part 2
- Write a function decrypt_vigenere(), that doesn’t take any arguments. The function should ask the user for the text to be decrypted (use the input() function) and for an decoding key.
IMPORTANT: To make the process of decrypting easier, convert both text and key
to lower case using the .lower() method. (You will learn more about lower when
we cover chapter 8).
After that, your function should call adjusted_key(), passing text and key so
an adjusted key can be created for the encryption process.
The next step is to use text and the adjusted key to create the decrypted
string. Now that you know how to encrypt text, decrypting should be very
simple. Use a loop to visit every character in the text and to concatenate the
resulting decrypted letter to a variable that holds the decrypted text.
Remember that only letters should be decrypted.
Numbers, punctuation, and other special characters should not be touched and
concatenated to result as is. Finally, print the decrypted text to the screen - Reading and writing to files
Modify encrypt_vigenere() to prompt the user to enter the name of a file in
.txt format containing the text to be encrypted. Use the text in that file as
the source to be encrypted text instead of obtaining it from the keyboard
input. Also, prompt the user for a name for an output file. The function now
should write the encrypted message to that file instead of printing it to the
screen.
Modify decrypt_vigenere() to prompt the user to enter the name of a file in
.txt format containing the text to be decrypted. Use the text in that file as
the source to be decrypted text instead of obtaining it from the keyboard
input.
Also, prompt the user for a name for an output file. The function now should
write the decrypted message to that final instead of printing it to the
screen.
When testing your program, you original text should match the decrypted text
when passing in the same key for encoding and decoding. View the example
below:
Encoding
textfile1.txt contains 'pittsburgh pa' [input file for encryption]
encodedtextfile1.txt contains 'nmlrwtsvyf ts' [output file for encryption]
Decoding
encodedtextfile1.txt contains 'nmlrwtsvyf ts' [input file for decryption]
decodedtextfile1.txt contains 'pittsburgh pa' [new output file for decryption]
Notes
How to determine if a character is a letter:
Recall from the lectures that every character in the ASCII table has a numeric
code.
‘a’ = 97, ‘b’ = 98, …, ‘z’ = 122
‘A’= 65, ‘B’ = 66, …, ‘Z’ = 90
You can obtain the numeric code of a character by using the function ord()
ord(‘a’) = 97, ord(‘t’)= 116
So, if ord(character) is a number between 97 and 122 it is a letter in lower-
case. If ord(character) is a number between 65 and 90 it is a letter in upper-
case.
To convert a numeric code into the corresponding character, we can use the
chr() function.
chr(72) = ‘H’, chr(114) = ‘r’, chr(65) = ‘A’
Applying a shift for decoding
Consider a shift of 3 and the character ‘c’
ord(‘c’) = 99
(original character from file) 99 + 3 = 102
(code for character + shift) chr(102) = ‘f’
(final character encrypted)
What if shift + character code exceeds 122?
To keep the result encoded using only letters, if shift is greater than the
numeric code for z (122) we must begin again at a. i.e.
For ‘y’ and shift of 10 -> ord(‘y’) = 121
a->b->c->d->e->f->g->h->i ord(‘I’) = 105
How to do that?
for lower-case letters we must keep the numeric codes within [97, 122]. That
can be done as follows:
- obtain code using ord()
- subtract 97 from it
- subtract shift to code
- add 26 to that
- obtain the remainder of the division of the previous number by 26 (there are 26 letters in the alphabet)
- add 97 to that
- covert code back to string using chr().
- obtain code using ord()
- subtract 65 from it
- subtract shift to code
- add 26 to that
- obtain the remainder of the division of the previous number by 26 (there are 26 letters in the alphabet)
- add 65 to that
- covert code back to string using chr().
