In
cryptography, a
transposition cipher is a method of encryption by which the positions held by units of
plaintext (which are commonly characters or groups of characters) are shifted according to a regular system, so that the
ciphertext constitutes a
permutation of the plaintext. That is, the order of the units is changed (the plaintext is reordered). Mathematically a
bijective function is used on the characters' positions to encrypt and an
inverse function to decrypt.