Дешифрование методом Виженера. Не могу обнаружить ошибку

import string
 
text_plain = """Alan Mathison Turing was a British mathematician."""
text_plain = text_plain.upper()
key=input("Enter the key_word -> ")
key=key.upper()
key=list(key)
print(text_plain)
print(key)
 
 
rep=len(text_plain)//len(key)
residue=len(text_plain)%len(key)
print("len_text_plain=",len(text_plain)," len_key=",len(key)," rep=",rep," residue=",residue)
key=key*rep+key[:residue]
print("\nlen_key=",len(key))
 
text_cipher=""
 
for i,letter in enumerate(text_plain):
    if letter in string.ascii_uppercase:
        code_cipher=ord(letter)+(ord(key[i])-65)
        if code_cipher>90:
            code_cipher-=26        
    else:
        code_cipher=ord(letter)
    letter_cipher=chr(code_cipher)
    #print(i," - code_cipher=",code_cipher," letter_cipher - ",letter_cipher)
    text_cipher+=letter_cipher
 
print("\ntext_cipher=",text_cipher)
 
text_decipher=""
 
for i,letter in enumerate(text_plain):
    if letter in string.ascii_uppercase:
        code_cipher=ord(letter)-(ord(key[i])-65)
        if code_cipher<65:
            code_cipher+=26        
    else:
        code_cipher=ord(letter)
    letter_cipher=chr(code_cipher)
    #print(i," - code_cipher=",code_cipher," letter_cipher - ",letter_cipher)
    text_decipher+=letter_cipher
 
print("\ntext_decipher=",text_decipher)

print('Впишите текст на зашифровку, пожалуйста')
#Get plaintext and key by user
plain = input().upper()
print('Впишите ключ для зашифровки, пожалуйста')
key = input().upper()
#Здесь ключ умножается на необходимое число так, что len(key) >= len(plain). Без обрезки на конце, если лишку.
key *= len(plain) // len(key) + 1 if not len(plain) == len(key) else 1
#Сама зашифровка происходит здесь.
ciphered = ''.join([chr((ord(chplain) + ord(key[indx])) % 26 + ord('A')) for indx, chplain in enumerate(plain)])
print(ciphered)

print('Getting plain text from ciphered by key')
plain = ''.join([chr((ord(chplain) - ord(key[indx])) % 26 + ord('A')) for indx, chplain in enumerate(ciphered)])
print(plain)