Atbash Cipher: Encode and Decode Online

in "Ciphers"

The Atbash cipher is a monoalphabetic substitution cipher that was used for the Hebrew alphabet. It is one of the earliest known substitution ciphers to have been used, and is very simple.

The cipher simply reverses the plaintext alphabet to create the ciphertext alphabet.

That is, the first letter of the alphabet is encrypted to the last letter of the alphabet, the second letter to the penultimate letter and so forth.

Basically, Atbash cipher encodes letter A to Z, letter B to Y and so on. In the original Hebrew this means that “alef” is encrypted to “tav”, “bet” to “shin” and so on.

The simplicity of the Atbash cipher is also its biggest weakness, as it does not use a key.

Hence every piece of plaintext encrypted using the Atbash cipher uses the same ciphertext alphabet, and so can be easily broken, since the encryption algorithm is known to all.

For the Hebrew alphabet (unicode representation), we get the following conversion table.

Plaintext Alphabet

Ciphertext Alphabet

‘Alef

Tav

Bet

Shin

Gimel

Resh

Dalet

Qof

He

Tsadi

Vav

Pe

Zayin

‘Ayin

Het

Samekh

Tet

Nun

Yod

Mem

Kaf

Lamed

Lamed

Kaf

Mem

Yod

Nun

Tet

Samekh

Het

‘Ayin

Zayin

Pe

Vav

Tsadi

He

Qof

Dalet

Resh

Gimel

Shin

Bet

Tav

‘Alef

 

For the Roman alphabet of 26 letters, we have the ciphertext alphabet as given in the table below.

Plaintext Alphabet

Ciphertext Alphabet

A

Z

B

Y

C

X

D

W

E

V

F

U

G

T

H

S

I

R

J

Q

K

P

L

O

M

N

N

M

O

L

P

K

Q

J

R

I

S

H

T

G

U

F

V

E

W

D

X

C

Y

B

Z

A

 

Example

How to encrypt using the Atbash cipher

Atbash encryption uses a substitution alphabet and its reciprocal, a combination of the normal alphabet and its reverse alphabet (mirrored).

For example, using the English alphabet (ABCDEFGHIJKLMNOPQRSTUVWXYZ) and its reverse (ZYXWVUTSRQPONMLKJIHGFEDCBA), let’s encrypt the message “Attack at noon”.

Plaintext Letter

Ciphertext Letter

A

Z

T

G

T

G

A

Z

C

X

K

P

A

Z

T

G

N

M

O

L

O

L

N

M

 

Thus, “Attack at noon” is encrypted as “Zggzxp zg mllm”.

How to decrypt Atbash cipher

Decryption is identical to encryption because of the reverse alphabet (due to the symmetry of the backwards alphabet).

Use the above Atbash cipher decoder and encoder to encrypt/decrypt simple messages.

How to recognize an Atbash ciphertext

An Atbash ciphertext has a coincidence index similar to an unencrypted text.

If the encryption used the classical Latin alphabet, letters V, G, R, L, and M appears the most frequently.

Otherwise the presence of Hebrew characters or a reference to the Dead Sea can be a clue.

The notions of mirror, reflection, axis, direction or sense are also clues.

Why is this cipher called “Atbash”?

In the Hebraic alphabet, aleph (first letter) was replaced by tav (last letter) and beth (second letter) with shin (second last) and so on.

Initials make A, T, B, SH.

Other Implementations

The Atbash cipher can be implemented as an Affine cipher by setting both “a” and “b” to 25.

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