The Atbash cipher is a substitution cipher with a specific key where the letters of the alphabet are reversed. I.e. all 'A's are replaced with 'Z's, all 'B's are replaced with 'Y's, and so on. It was originally used for the Hebrew alphabet, but can be used for any alphabet.
The Atbash cipher offers almost no security, and can be broken very easily. Even if an adversary doesn't know a piece of ciphertext has been enciphered with the Atbash cipher, they can still break it by assuming it is a substitution cipher and determining the key using hill-climbing. The Atbash cipher is also an Affine cipher with a=25 and b = 25, so breaking it as an affine cipher also works.
The Algorithm §
The Atbash cipher is essentially a substitution cipher with a fixed key, if you know the cipher is Atbash, then no additional information is needed to decrypt the message. The substitution key is:
To encipher a message, find the letter you wish to encipher in the top row, then replace it with the letter in the bottom row. In the example below, we encipher the message 'ATTACK AT DAWN'. The first letter we wish to encipher is 'A', which is above 'Z', so the first ciphertext letter is 'Z'. The next letter is 'T', which is above 'G', so that comes next. The whole message is enciphered:
ATTACK AT DAWN ZGGZXP ZG WZDM
To decipher a message, the exact same procedure is followed. Find 'Z' in the top row, which is 'A' in the bottom row. Continue until the whole message is deciphered.
Other Implementations §
To encipher your own messages in python, you can use the pycipher module. To install it, use pip install pycipher. To encipher messages with the Atbash cipher (or another cipher, see here for documentation):
>>>from pycipher import Atbash >>>Atbash().encipher('defend the east wall of the castle') 'wvuvmwgsvvzhgdzoolugsvxzhgov' >>>Atbash().decipher('wvuvmwgsvvzhgdzoolugsvxzhgov') 'defendtheeastwallofthecastle'
The Atbash cipher is trivial to break since there is no key, as soon as you know it is an Atbash cipher you can simply decrypt it. If you didn't know it was an Atbash cipher, you could break it by assuming the ciphertext is a substitution cipher, which can still be easily broken, see here. Alternatively, it can be broken if it is assumed to be an Affine cipher.
References § Wikipedia has a good description of the encryption/decryption process, history and cryptanalysis of this algorithm
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