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## Code Snippet

``````def caesar_cipher(message, key, action):
letters = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
'1', '2', '3', '4', '5', '6', '7', '8', '9']

number_in_list = len(letters)
output_message = ""
for next_character in message:
in_list = False
i = 0
while i < number_in_list:
new_position = 0
if next_character == letters[i]:
in_list = True
# What happens next determined by value of action.
if action == 'E':
new_position = i + key
else:
new_position = i - key
if new_position >= number_in_list:
new_position -= number_in_list
output_char = letters[new_position]
i += 1
if in_list:
output_message += output_char
else:
output_message += next_character
return output_message

#Main program
print("CAESAR CIPHER")
print("")
print("OPTIONS")
print("")
flag = True
while flag:
print("")
print("1. Encrypt a message")
print("2. Decrypt a message")
print("3. Exit the program")
message = input("Enter the message you want to encrypt: ")
key = input("Enter the key: ")
action = "E"
print("The encoded message is: ")
print(caesar_cipher(message, int(key), action))