cryptography Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or ''-logy, -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of Adversary (cryptography), ...

, residual block termination is a variation of

cipher block chaining In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transform ...

mode (CBC) that does not require any

padding Padding is thin cushioned material sometimes added to clothes. Padding may also be referred to as batting or wadding when used as a layer in lining quilts or as a packaging or stuffing material. When padding is used in clothes, it is often done in ...

. It does this by effectively changing to

cipher feedback In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transform ...

mode for one

block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...

. The cost is the increased complexity.

Encryption procedure

If the

plaintext In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of comp ...

length ''N'' is not a multiple of the block size ''L'': *Encrypt the ⌊''N''/''L''⌋ full blocks of plaintext using the

mode Mode ( meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * MO''D''E (magazine), a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is the setting fo ...

; *Encrypt the last full encrypted block again; *

XOR Exclusive or, exclusive disjunction, exclusive alternation, logical non-equivalence, or logical inequality is a logical operator whose negation is the logical biconditional. With two inputs, XOR is true if and only if the inputs differ (one ...

the remaining bits of the plaintext with leftmost bits of the re-encrypted block.

Decryption procedure

Decrypt In cryptography, encryption (more specifically, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information, known as plai ...

the ⌊''N''/''L''⌋ full encrypted blocks using the Cipher Block Chaining mode; *

Encrypt In cryptography, encryption (more specifically, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information, known as plai ...

the last full encrypted block; *

the remaining bits of the ciphertext with leftmost bits of the re-encrypted block.

Short message

For messages shorter than one block, residual block termination can use an encrypted

initialization vector In cryptography, an initialization vector (IV) or starting variable is an input to a cryptographic primitive being used to provide the initial state. The IV is typically required to be random or pseudorandom, but sometimes an IV only needs to be un ...

instead of the previously encrypted block.

References

Cryptographic algorithms {{cryptography-stub