man caesar (Jeux) - caesar ,
NAME
caesar ,rot13 - decrypt caesar ciphers
SYNOPSIS
[rotation]
DESCRIPTION
The utility attempts to decrypt caesar ciphers using English letter frequency statistics. reads from the standard input and writes to the standard output.
The optional numerical argument rotation may be used to specify a specific rotation value.
The frequency (from most common to least) of English letters is as follows:
ETAONRISHDLFCMUGPYWBVKXJQZ
Their frequencies as a percentage are as follows:
E(1), T(10.5), A(8.1), O(7.9), N(7.1), R(6.8), I(6.3), S(6.1), H(5.2), D(3.8), L(3.4), F(2.9), C(2.7), M(2.5), U(2.4), G(2), P(1.9), Y(1.9), W(1.5), B(1.4), V(.9), K(.4), X(.15), J(.13), Q(.11), Z(.07).
Rotated postings to USENET and some of the databases used by the fortune() program are rotated by 13 characters.