python中的加法密码系统

Question

So I am fairly new in python and I try to work on some programs for practice. Today I started working on an additive cipher system about which I learned in a book. I wrote the code(it's pretty shitty, to be honest🤣) and I tried some inputs. It did well when I entered words with 2 characters but when I tried to encrypt the word "hello" (with shift = 3) for some reason it returns "khuuu" instead of "khoor". Anyone that has any idea please reply. Thank you!!

plain = input("Enter your plaintext in lowercase: ")

key = int(input("Enter your cipher key: "))
alphabet = "abcdefghijklmnopqrstuvwxyz"

for x in range(0, len(plain)):
 if plain[x] == "a":
    plain = plain.replace(plain[x] , alphabet[0 + int(key)])
 elif plain[x] == "b":
    plain = plain.replace(plain[x] , alphabet[1 + int(key)])
 elif plain[x] == "c":
    plain = plain.replace(plain[x] , alphabet[2 + int(key)])
 elif plain[x] == "d":
    plain = plain.replace(plain[x] , alphabet[3 + int(key)])
 elif plain[x] == "e":
    plain = plain.replace(plain[x] , alphabet[4 + int(key)])
 elif plain[x] == "f":
    plain = plain.replace(plain[x] , alphabet[5 + int(key)])
 elif plain[x] == "g":
    plain = plain.replace(plain[x] , alphabet[6 + int(key)])
 elif plain[x] == "h":
    plain = plain.replace(plain[x] , alphabet[7 + int(key)])
 elif plain[x] == "i":
    plain = plain.replace(plain[x] , alphabet[8 + int(key)])
 elif plain[x] == "j":
    plain = plain.replace(plain[x] , alphabet[9 + int(key)])
 elif plain[x] == "k":
    plain = plain.replace(plain[x] , alphabet[10 + int(key)])
 elif plain[x] == "l":
    plain = plain.replace(plain[x] , alphabet[11 + int(key)])
 elif plain[x] == "m":
    plain = plain.replace(plain[x] , alphabet[12 + int(key)])
 elif plain[x] == "n":
    plain = plain.replace(plain[x] , alphabet[13 + int(key)])
 elif plain[x] == "o":
    plain = plain.replace(plain[x] , alphabet[14 + int(key)])
 elif plain[x] == "p":
    plain = plain.replace(plain[x] , alphabet[15 + int(key)])
 elif plain[x] == "q":
    plain = plain.replace(plain[x] , alphabet[16 + int(key)])
 elif plain[x] == "r":
    plain = plain.replace(plain[x] , alphabet[17 + int(key)])
 elif plain[x] == "s":
    plain = plain.replace(plain[x] , alphabet[18 + int(key)])
 elif plain[x] == "t":
    plain = plain.replace(plain[x] , alphabet[19 + int(key)])
 elif plain[x] == "u":
    plain = plain.replace(plain[x] , alphabet[20 + int(key)])
 elif plain[x] == "v":
    plain = plain.replace(plain[x] , alphabet[21 + int(key)])
 elif plain[x] == "w":
    plain = plain.replace(plain[x] , alphabet[22 + int(key)])
 elif plain[x] == "x":
    plain = plain.replace(plain[x] , alphabet[23 + int(key)])
 elif plain[x] == "y":
    plain = plain.replace(plain[x] , alphabet[24 + int(key)])
 elif plain[x] == "z":
    plain = plain.replace(plain[x] , alphabet[25 + int(key)])


print (plain)

Answer 1

replace replaces ALL instances of the first argument with the second argument. If we go through this in steps, we can see what's really happening. This is a fantastic opportunity for you to learn how to use your debugger to step through the code. This example is easy, but future projects might be too difficult to trace down a problem like this. Knowing how to use a debugger is a very valuable skill.

The v indicates where your for loop is at at that step

1. Original: plain = hello 
                             v
2. Replace h with k: plain = kello
                              v
3. Replace e with h: plain = khllo
                               v
4. Replace l with o: plain = khooo <-- Because you replaced ALL l's with o's.
                                v
5. Replace o with r: plain = khrrr <-- Again, because you replaced ALL o's with r's.
                                 v
6. Replace r with u: plain = khuuu <--- Final result

So how do you fix it? Since strings are immutable, you can't change them in place... but you can change lists! And as a bonus, you can iterate over lists just like you do with strings.

plain = list(input("Enter your plaintext in lowercase: "))
print(plain) # <--- It's a list

key = int(input("Enter your cipher key: "))
alphabet = "abcdefghijklmnopqrstuvwxyz"

for x in range(len(plain)):
  # Find the index of the letter in alphabet
  ix = alphabet.index(plain[x].lower())
  plain[x] = alphabet[(ix + key)%26]

# Turn plain back into a string
plain = "".join(plain)
print(plain)

This algorithm is kind of expensive, since you have to find the index of each letter at each iteration but I wanted to keep it in the style as you had.

If you want to improve it, take a look at Jean's comment about using ord and chr so you can skip the index lookup operation.