Cryptography/Breaking transposition ciphers

Earlier, we discussed how ../Permutation cipher/ and ../Transposition ciphers/ work for people who know the secret key. Next, we'll discuss how, in some cases, it is possible for a person who only has the ciphertext -- who doesn't know the secret key -- to recover the plaintext.

The frequency distribution of the letters in any transposition or permutation ciphertext is the same as the frequency distribution for plaintext.

breaking columnar transposition ciphers
The frequency distribution of digrams can be used to help break columnar transposition ciphers.

breaking turning grille ciphers
Turning grilles, also called Fleissner grilles, ...

A guess at some sequence of two or more consecutive holes of the grill in one position of the grill (by a "known word" or an expected common digraph) can be "checked" by seeing if those holes, after the grill is rotated a half-turn, produce reasonable digraph.