Sheshach

 This article incorporates text from a publication now in the public domain: Easton, Matthew George (1897). "^{article name needed}". Easton's Bible Dictionary (New and revised ed.). T. Nelson and Sons. 

Sheshach - (Jer. 25:26), supposed to be equivalent to Babel (Babylon), according to a secret mode of writing among the Jews of unknown antiquity, which consisted in substituting the last letter of the Hebrew alphabet for the first, the next to last one for the second, and so on. Thus the letters sh, sh, ch become b, b, l, i.e., Babel (see: Atbash cipher).This code is called the Atbash Cipher and was used to decrypt. The city does not appear on any map nor in any other documents, save the book of Jeremiah. The Atbash Cipher was applied and out of Sheshach came the word Babel. This is supposed to be confirmed by a reference to Jer. 51:41, where Sheshach and Babylon are in parallel clauses. There seems to be no reason to doubt that Babylon is here intended by this name. (See Streane's Jeremiah, l.c.)

A similar instance of this Hebrew cryptographic method occurs in Jer. 51:1, where Leb Kamai is code for Chaldea.