In cryptography, a block cipher operates on blocks of fixed length, often 64 or 128 bits. Because messages may be of any length, and because encrypting the same plaintext under the same key always produces the same output (as described in the ECB section below), several modes of operation have been invented which allow block ciphers to provide confidentiality for messages of arbitrary length.
The earliest modes described in the literature (eg, ECB, CBC, OFB and CFB) provide only confidentiality or message integrity, but do not perform both simultaneously. Other modes have since been designed which ensure both confidentiality and message integrity in one pass, such as IAPM, CCM, EAX, GCM, and OCB modes. Tweakable narrow-block encryption (LRW) mode, and wide-block encryption (CMC and EME) modes, designed to securely encrypt sectors of a disk, are described in the article devoted to disk encryption theory.
Here's a striking example of the degree to which ECB can leave plaintext data patterns in the ciphertext. A pixel-map version of the image on the left was encrypted with ECB mode to create the center image:
|- align="center" |||Original||Encrypted using ECB mode||Encrypted using other modes|
The image on the right is how the image might look encrypted with CBC, CTR or any of the other more secure modes -- indistinguishable from random noise. Note that the random appearance of the image on the right tells us very little about whether the image has been securely encrypted; many kinds of insecure encryption have been developed which would produce output just as 'random-looking'.
ECB mode can also make protocols without integrity protection even more susceptible to replay attacks, since each block gets decrypted in exactly the same way. For example, the Phantasy Star Online: Blue Burst online video game uses Blowfish in ECB mode. Before the key exchange system was cracked leading to even easier methods, cheaters repeated encrypted "monster killed" message packets, each an encrypted Blowfish block, to illegitimately gain experience points quickly.
If the first block has index 1, the mathematical formula for CBC encryption is
while the mathematical formula for CBC decryption is
CBC has been the most commonly used mode of operation. Its main drawbacks are that encryption is sequential (i.e., it cannot be parallelized), and that the message must be padded to a multiple of the cipher block size. One way to handle this last issue is through the method known as ciphertext stealing.
Note that a one-bit change in a plaintext affects all following ciphertext blocks, and a plaintext can be recovered from just two adjacent blocks of ciphertext. As a consequence, decryption can be parallelized, and a one-bit change to the ciphertext causes complete corruption of the corresponding block of plaintext, and inverts the corresponding bit in the following block of plaintext.
PCBC is used in Kerberos v4 and WASTE, most notably, but otherwise is not common. In fact when, on a message encrypted in PCBC mode, two adjacent blocks are exchanged, this does not affect the subsequent blocks and is thus no longer used in Kerberos v5.
The cipher feedback (CFB) mode, a close relative of CBC, makes a block cipher into a self-synchronizing stream cipher. Operation is very similar; in particular, CFB decryption is almost identical to CBC encryption performed in reverse:
Like CBC mode, changes in the plaintext propagate forever in the ciphertext, and encryption cannot be parallelized. Also like CBC, decryption can be parallelized. When decrypting, a one-bit change in the ciphertext affects two plaintext blocks: a one-bit change in the corresponding plaintext block, and complete corruption of the following plaintext block. Later plaintext blocks are decrypted normally.
Because each stage of the CFB mode depends on the encrypted value of the previous ciphertext XORed with the current plaintext value, a form of pipelining is possible, since the only encryption step which requires the plaintext is the final XOR. This is useful for applications that require low latency between the arrival of plaintext and the output of the corresponding ciphertext, such as certain applications of streaming media.
CFB shares two advantages over CBC mode with the stream cipher modes OFB and CTR: the block cipher is only ever used in the encrypting direction, and the message does not need to be padded to a multiple of the cipher block size.
Because of the symmetry of the XOR operation, encryption and decryption are exactly the same:
Each output feedback block cipher operation depends on all previous ones, and so cannot be performed in parallel. However, because the plaintext or ciphertext is only used for the final XOR, the block cipher operations may be performed in advance, allowing the final step to be performed in parallel once the plaintext or ciphertext is available.
Like OFB, counter mode turns a block cipher into a stream cipher. It generates the next keystream block by encrypting successive values of a "counter". The counter can be any simple function which produces a sequence which is guaranteed not to repeat for a long time, although an actual counter is the simplest and most popular. CTR mode has similar characteristics to OFB, but also allows a random access property during decryption. Note that the nonce in this graph is the same thing as the initialization vector (IV) in the other graphs. The IV/nonce and the counter can be concatenated, added, or XORed together to produce the actual unique counter block for encryption. CTR mode is well suited to operation on a multi-processor machine where blocks can be encrypted in parallel.
Before the message integrity problem was widely recognized, it was common to discuss the "error propagation" properties of a mode of operation as a suitability criterion. It might be observed, for example, that a one-block error in the transmitted ciphertext would result in a one-block error in the reconstructed plaintext for ECB mode encryption, while in CBC mode such an error would affect two blocks:
Some felt that such resilience was desirable in the face of random errors (eg, line noise), while others argued that it increased the scope for attackers to modify messages without assurance of detection if checked.
However, when proper integrity protection is used, such an error will result (with high probability) in the entire message being rejected. If resistance to random error is desirable, error-correcting codes should be applied to the ciphertext before transmission.
Some modes of operation have been designed to combine security and authentication. Examples of such modes are: XCBC, IACBC, IAPM, OCB, EAX, CWC, CCM, and GCM. These authenticated encryption modes are classified as single pass modes or double pass modes. Some modes also allow for the authentication of unencrypted associated data, and these are called AEAD (Authenticated-Encryption with Associated-Data) schemes. For example, EAX mode is a double pass AEAD scheme while OCB mode is single pass.
CFB, OFB and CTR modes do not require any special measures to handle messages whose lengths are not multiples of the block size, since they all work by XORing the plaintext with the output of the block cipher. The last partial block of plaintext is XORed with the first few bytes of the last keystream block, producing a final ciphertext block that is the same size as the final partial plaintext block. This characteristic of stream ciphers makes them suitable for applications that require the encrypted ciphertext data to be the same size as the original plaintext data, and for applications that transmit data in streaming form where it is inconvenient to add padding bytes.
Many more modes of operation for block ciphers have been suggested. Some of them have been accepted, fully described (even standardised), and are in use. Others have been found insecure, and should never be used. NIST maintains a list of proposed modes for AES at
Disk encryption often uses special modes. Tweakable narrow-block encryption modes (LRW, XEX, and XTS) and wide-block encryption (CMC and EME) modes are designed to securely encrypt sectors of a disk. (See disk encryption theory)
Block ciphers can also be used in other cryptographic protocols. They are generally used in modes of operation similar to the block modes described here. As with all protocols, to be cryptographically secure, care must be taken to build them correctly.
Cryptographically secure pseudorandom number generators (CSPRNGs) can also be built using block ciphers.
Authenticated encryption also uses block ciphers as components. It means to both encrypt and MAC at the same time. That is to both provide confidentiality and authentication. IAPM, CCM, EAX, GCM and OCB are such authenticated encryption modes.
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