Atbash Cipher:
how sheshak has been captured, and the praise of the whole earth been seized! how babylon has become an object of horror among the nations! the sea has come up over babylon; she has been engulfed with its tumultuous waves. from jeremiah chapter fifty-one [Jer. 51:41 is one occurrence of Atbash in the Hebrew scriptures, see also Jer. 25:26, 51:11]
The Atbash cipher is an ancient Hebrew cipher used in the scriptures at least three times (see references above), formed by replacing the first letter of the alphabet with the last one, the second letter with the penultimate one, etc. Using Atbash on the Hebrew alphabet, Sheshak becomes Babel, and Leb-kamai becomes Chaldea, both references to the Babylonians who were oppressing the Jewish people at the time when Jeremiah was written. An easy way to write the key (in English, in this case) is to write:
ABCDEFGHIJKLM
ZYXWVUTSRQPON
That is, write one half of the alphabet forwards and then the other backwards. To decipher a 'W', for example, just look above it, and you get a 'd'. Likewise, a 'D' becomes a 'w'. Encrypting using the cipher is just as easy.