1. Atbash Cipher

2. Atbash
Atbash (c. 500 BC)
Atbash (also transliterated Atbaš) is a simple substitution cipher originally for the Hebrew alphabet, but possible with any known alphabet.
History
Atbash is a simple substitution cipher for the Hebrew alphabet. It consists of substituting aleph (the first letter) for tav (the last), beth (the second) for shin (the one before last), and so on, reversing the alphabet.
Note that modern Hebrew is written right-to-left.
In the Book of Jeremiah, לב קמי Lev Kamai (51:1) is Atbash for כשדים Kasdim (Chaldeans), and ששך Sheshakh (25:26; 51:41) is Atbash for בבל Bavel (Babylon).

3. Atbash
It is considered a 'weak' or 'simple' cipher because it only has one possible key, and it is a simple monoalphabetic substitution cipher.
However, this may not have been an issue at the time when the cipher was first devised.

4. Atbash Substitution Ciphers
Atbash cipher substitutes the corresponding letter from the reverse alphabet
Caesar cipher substitutes the letter three later in the alphabet (wrapping back around to the beginning)
ABC  ZYX ABC  DEF
HELLO  SVOOL HELLO  KHOOR
substitution ciphers are easy to understand and use

5. Example
NZGS RH UFM
Decrypt using Atbash
MATH IS FUN

6. Example GSV VZTOV SZH OZMWVW The eagle has landed A B C D E F G H I JK L M Z Y X W V U T S R Q P O N
GSV VZTOV SZH OZMWVW
The eagle has landed