Transposition ciphers don't transform the plaintext letters but rearrange (or permute) them. There are two basic methods:
Examples: 1. An aperiodic example is provided by the spiral The plaintext is written along the path, and the ciphertext is read off by rows.
ISOPSN TPANAA This is an aperiodic transposition. ---> IEHTSR ---> ISOPS NTPAN AAIEH TSROR ISITN IODIC ORISIT NIODIC
2. For a periodic transposition we take the period l = 5 and let the permutation defined by the keyword APPLE the alphabetic order of whose letters is (14532).
This is a periodic transposition. ---> THISI SAPER IODIC TRANS POSIT IONXX TSIIH SERPA IICDO TNSAR PITSO IXXNO
+ | Transposition breaks patterns in the plaintext. |
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+ | In general there is no unique cryptanalytic solution because each string has a lot of anagrams. |
– | Letter frequencies, MFL score, LW score, and coincidence index φ are invariant. |
– | This observation easily reveals the encryption method. |
For a compehensive treatment of transposition ciphers and their cryptanalysis consult the books by F. L. Bauer, Gaines, Sinkov, and Nichols (Vol. II), see the reference list.
The earliest known hostorical appearances of transposition ciphers are the Scytale used by the Spartans and the work by AL-KINDI about 850.