In cryptography, a permutation box (or P-box) is a method of bit-shuffling used to permute or transpose bits across S-boxes inputs, retaining diffusion while transposing.
In block ciphers, the S-boxes and P-Boxes are used to make the relation between the plaintext and the ciphertext difficult to understand (see Shannon's property of confusion). P-boxes are typically classified as compression, expansion, and straight, according as the number of output bits is less than, greater than, or equal to the number of input bits. Only straight P-boxes are invertible.
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“However low and poor the taking Snuff argues a Man to be in his own Stock of Thought, or Means to employ his Brains and his Fingers, yet there is a poorer Creature in the World than He, and this is a Borrower of Snuff; a Fellow that keeps no Box of his own, but is always asking others for a Pinch.”
—Richard Steele (1672–1729)