📝Padding (Encryption)

Because block ciphers work on fixed-size blocks, the input for many modes of operation must be padded to the block size. However, care must be taken to ensure that original length can be recovered.

If input is C-style string (does not contain 0 bytes), padding is trivial — just add as many 0 bytes as needed.

Another scheme: append a single 1 bit and as many 0 bits as necessary to pad the block. If the input length is on the boundary of the block, add a whole block of padding.

Another schemes like ciphertext stealing and residual block termination do not cause any extra ciphertext.

Append a byte with value 128 (0x80) and as many zero bytes as necessary.

Pad the last block with n bytes all with value n.

CFB, OFB, and CTR modes do not require any padding, because they can be used as stream ciphers.