Symmetric-Key Cryptography - Codes: The Guide to Secrecy from Ancient to Modern Times - page 133

3.3 Modes ofOperation

In architecture as in all other operative arts, the end must direct the opera-
tion. Theendistobuildwell. Wellbuildinghaththreeconditions. Commodity,
firmness, and delight.

Henry Wotton (1568-1639), English poet and diplomat
— from Elements of architecture (1624, page 1)

We need to examine how block ciphers, such as DES, may be applied to a
variety of situations, called modes of operation . Symmetric-key block ciphers
have five modes of operation recommended by NIST, and defined in FIPS 81,
December 2, 1980, as well as in ANSI X 3 . 106 1983, with the number of modes

expanded from four to five in Special Publication 800

−

38 A , December 2001.
These modes (initially intended for DES) are meant to address every conceivable
application for cryptology to which block ciphers can be applied.
Before describing the formal details of each mode, we present a brief verbal
introduction.

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Block Cipher Modes — Overview
1. Electronic Codebook (ECB) : Each 64-bit block of plaintext is enci-
phered with the same key, albeit independently. This mode is typically
used to send small amounts of data such as a symmetric key.

2. Cipher Block Chaining (CBC) : The input is the addition, modulo 2,
of the previous 64 bits of ciphertext with the succeeding 64 bits of plain-
text. Normally, this mode is used as a general-purpose block-transport
mechanism, but also may be employed for authentication purposes.

3. Cipher Feedback Mode (CFB) : This mode employs a chaining mecha-
nism similar to CBC. It uses prior ciphertext as input and outputs pseu-
dorandom strings that are added, modulo 2, with plaintext to produce the
next quantity of ciphertext. This mode is employed as a stream-cipher-
oriented means for general-purpose messaging since it processes n

∈ N bits

at a time.

4. Output Feedback (OFB) : This is comparable to CFB mode with the
exception that its input is the prior block cipher's output. This mode is
usually employed for stream-cipher-oriented communications, especially
those requiring message authentication, such as a MAC (see Chapter 7).

5. Counter Mode (CTR) : The ciphertext is formed via a modulo 2 addition
of a plaintext block with an enciphered counter, which is updated for each
succeeding block. This mode is remarkably easy to use, and is typically
utilized for high-speed transmission. In fact, this is the least-known of the
modes, but is rapidly gaining ground with working cryptographers in the
field as an excellent means of using block ciphers in a variety of situations.