caesar cipher Algorithm
The Caesar Cipher Algorithm is one of the most basic and earliest known encryption techniques in the history of cryptography. Named after Julius Caesar, who is believed to have used it to communicate with his military generals, this algorithm is a substitution cipher that involves replacing each letter in the plaintext by a letter some fixed number of positions down the alphabet. For instance, with a shift of 3, the letter 'A' would be replaced by 'D', 'B' would become 'E', and so on. The shift value acts as a key for the encryption and decryption process.
Despite its simplicity, the Caesar Cipher was once considered effective in maintaining the secrecy of messages, primarily due to the low literacy rate and lack of knowledge about cryptography at that time. However, in modern times, this algorithm is considered extremely weak and can be easily broken using frequency analysis or even by brute force, trying all possible shift values. Today, the Caesar Cipher serves as a fundamental building block and an educational tool for understanding the basic principles of encryption and cryptography.
from string import ascii_letters
def encrypt(input_string: str, key: int, alphabet=None) -> str:
"""
encrypt
=======
Encodes a given string with the caesar cipher and returns the encoded
message
Parameters:
-----------
* input_string: the plain-text that needs to be encoded
* key: the number of letters to shift the message by
Optional:
* alphabet (None): the alphabet used to encode the cipher, if not
specified, the standard english alphabet with upper and lowercase
letters is used
Returns:
* A string containing the encoded cipher-text
More on the caesar cipher
=========================
The caesar cipher is named after Julius Caesar who used it when sending
secret military messages to his troops. This is a simple substitution cipher
where very character in the plain-text is shifted by a certain number known
as the "key" or "shift".
Example:
Say we have the following message:
"Hello, captain"
And our alphabet is made up of lower and uppercase letters:
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
And our shift is "2"
We can then encode the message, one letter at a time. "H" would become "J",
since "J" is two letters away, and so on. If the shift is ever two large, or
our letter is at the end of the alphabet, we just start at the beginning
("Z" would shift to "a" then "b" and so on).
Our final message would be "Jgnnq, ecrvckp"
Further reading
===============
* https://en.m.wikipedia.org/wiki/Caesar_cipher
Doctests
========
>>> encrypt('The quick brown fox jumps over the lazy dog', 8)
'bpm yCqks jzwEv nwF rCuxA wDmz Bpm tiHG lwo'
>>> encrypt('A very large key', 8000)
's nWjq dSjYW cWq'
>>> encrypt('a lowercase alphabet', 5, 'abcdefghijklmnopqrstuvwxyz')
'f qtbjwhfxj fqumfgjy'
"""
# Set default alphabet to lower and upper case english chars
alpha = alphabet or ascii_letters
# The final result string
result = ""
for character in input_string:
if character not in alpha:
# Append without encryption if character is not in the alphabet
result += character
else:
# Get the index of the new key and make sure it isn't too large
new_key = (alpha.index(character) + key) % len(alpha)
# Append the encoded character to the alphabet
result += alpha[new_key]
return result
def decrypt(input_string: str, key: int, alphabet=None) -> str:
"""
decrypt
=======
Decodes a given string of cipher-text and returns the decoded plain-text
Parameters:
-----------
* input_string: the cipher-text that needs to be decoded
* key: the number of letters to shift the message backwards by to decode
Optional:
* alphabet (None): the alphabet used to decode the cipher, if not
specified, the standard english alphabet with upper and lowercase
letters is used
Returns:
* A string containing the decoded plain-text
More on the caesar cipher
=========================
The caesar cipher is named after Julius Caesar who used it when sending
secret military messages to his troops. This is a simple substitution cipher
where very character in the plain-text is shifted by a certain number known
as the "key" or "shift". Please keep in mind, here we will be focused on
decryption.
Example:
Say we have the following cipher-text:
"Jgnnq, ecrvckp"
And our alphabet is made up of lower and uppercase letters:
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
And our shift is "2"
To decode the message, we would do the same thing as encoding, but in
reverse. The first letter, "J" would become "H" (remember: we are decoding)
because "H" is two letters in reverse (to the left) of "J". We would
continue doing this. A letter like "a" would shift back to the end of
the alphabet, and would become "Z" or "Y" and so on.
Our final message would be "Hello, captain"
Further reading
===============
* https://en.m.wikipedia.org/wiki/Caesar_cipher
Doctests
========
>>> decrypt('bpm yCqks jzwEv nwF rCuxA wDmz Bpm tiHG lwo', 8)
'The quick brown fox jumps over the lazy dog'
>>> decrypt('s nWjq dSjYW cWq', 8000)
'A very large key'
>>> decrypt('f qtbjwhfxj fqumfgjy', 5, 'abcdefghijklmnopqrstuvwxyz')
'a lowercase alphabet'
"""
# Turn on decode mode by making the key negative
key *= -1
return encrypt(input_string, key, alphabet)
def brute_force(input_string: str, alphabet=None) -> dict:
"""
brute_force
===========
Returns all the possible combinations of keys and the decoded strings in the
form of a dictionary
Parameters:
-----------
* input_string: the cipher-text that needs to be used during brute-force
Optional:
* alphabet: (None): the alphabet used to decode the cipher, if not
specified, the standard english alphabet with upper and lowercase
letters is used
More about brute force
======================
Brute force is when a person intercepts a message or password, not knowing
the key and tries every single combination. This is easy with the caesar
cipher since there are only all the letters in the alphabet. The more
complex the cipher, the larger amount of time it will take to do brute force
Ex:
Say we have a 5 letter alphabet (abcde), for simplicity and we intercepted the
following message:
"dbc"
we could then just write out every combination:
ecd... and so on, until we reach a combination that makes sense:
"cab"
Further reading
===============
* https://en.wikipedia.org/wiki/Brute_force
Doctests
========
>>> brute_force("jFyuMy xIH'N vLONy zILwy Gy!")[20]
"Please don't brute force me!"
>>> brute_force(1)
Traceback (most recent call last):
TypeError: 'int' object is not iterable
"""
# Set default alphabet to lower and upper case english chars
alpha = alphabet or ascii_letters
# The key during testing (will increase)
key = 1
# The encoded result
result = ""
# To store data on all the combinations
brute_force_data = {}
# Cycle through each combination
while key <= len(alpha):
# Decrypt the message
result = decrypt(input_string, key, alpha)
# Update the data
brute_force_data[key] = result
# Reset result and increase the key
result = ""
key += 1
return brute_force_data
def main():
while True:
print(f'\n{"-" * 10}\n Menu\n{"-" * 10}')
print(*["1.Encrpyt", "2.Decrypt", "3.BruteForce", "4.Quit"], sep="\n")
# get user input
choice = input("\nWhat would you like to do?: ").strip() or "4"
# run functions based on what the user chose
if choice not in ("1", "2", "3", "4"):
print("Invalid choice, please enter a valid choice")
elif choice == "1":
input_string = input("Please enter the string to be encrypted: ")
key = int(input("Please enter off-set: ").strip())
print(encrypt(input_string, key))
elif choice == "2":
input_string = input("Please enter the string to be decrypted: ")
key = int(input("Please enter off-set: ").strip())
print(decrypt(input_string, key))
elif choice == "3":
input_string = input("Please enter the string to be decrypted: ")
brute_force_data = brute_force(input_string)
for key, value in brute_force_data.items():
print(f"Key: {key} | Message: {value}")
elif choice == "4":
print("Goodbye.")
break
if __name__ == "__main__":
main()
