29_03-чтение


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Цыкарев - 3

The A ES is likely to be the commercial-grade symmetric algorithm of choice for years, if not **decades**. __Let us look at it more clo s ely__.

The A ES Contest
In J anuary 1997, NIST called for cryptographers to develop a new encryption system. __As with the c a ll__ for candidates from which DES was selected, NIST made several important restrictions. The algorithms had to be In August 1998, fifteen algorithms were ch o sen from among those submitted; in August 1999,
 *  unclassified
 *  publicly disclos e d
 *  available roya lt y-free for use worldwide
 *  symmetric block cipher algorithms, for blocks of 128 bits
 *  usable with key sizes of 128, 192, and 256 bits

Нефёдов 4 -

2.6. The AES Encryption Algorithm
The A ES is likely to be the commercial-grade symmetric algorithm of choice for years, if not **decades**. Let us look at it more clo s ely.

The AES Contest
In January 1997, NIST called for cryptographers to develop a new encryption system. As with the call for **candidat e s** from which DES was selected, NIST made several important restrictions. The algorithms had to be In August 1998, fifteen algorithms were ch o sen from among those submitted; in August 1999, Макарихина 4
 *  unclassified
 *  publicly disclosed
 *  available royalty-free for use worldwide
 *  symmetric block cipher algorithms, for blocks of 128 bits
 *  usable with k ey siz es of 128, 192, and 256 bits

Overview of Rijndael
Each cycle consists of four steps.
 * Rijndael ** is a fast algorithm that can be implemented eas i ly on simple processors. **Although** it has a strong ma th ematical foundation, it primarily uses substitution; transposition; and the shift, exclusive OR, and addition operations. Like DES, AES uses repeat cycles. There are 10, 12, or 14 cycles for keys of 128, 192, and 256 bits, respectively. In Rijndael, the **cycles** **are called "** **rounds.** **"**
 *  Byte substitution: This step uses a substitution box structure similar to the DES, substituting each byte of a 128-bit block according to a substitution table. This is a straight diffusion operation.
 *  Shift row: A transposition step. For 128- and 192-bit block sizes, row n is shifted left circular ( n - 1) bytes; for 256-bit blocks, row 2 is shifted 1 byte and rows 3 and 4 are shifted 3 and 4 bytes, respectively. This is a straight confusion operation. ||

Смирнов 3 -

Each cycle consists of four steps.
 * Rijndael ** is a f a st algorithm that can be imple ment ed easily on simple **processors**. **Al th ough** it has a strong ma th ematical foundation, it primarily us e s substitution; transposition; and the shift, **exclusive** OR, and addition operations. Like DES, AES uses repeat cycles. There are 10, 12, or 14 cycles for keys of 1 28, 1 92, and 256 bits, resp e ctively. In Rijndael, the **cycles** **are c a lled "** **rounds.** **"**

Shift row: A transposition step. For 128- and 192-bit block sizes, row n is shifted left circular ( n - 1) bytes; for 256-bit blocks, row 2 is shifted 1 byte and rows 3 and 4 are shifted 3 and 4 bytes, respectively. This is a straight confusion operation.
 *  Byte substitution: This step uses a substitution box struc tur e similar to the DES, substituting each byte of a 1 28-bit block according to a substitution table. This is a straight diffusion operation.

Strength of the Algorithm
Казаков 3 Ошибок в произношении мало, но осталась проблема с прошлого раза: полное отсутствие слитности. The Rijndael algorithm is quite new, so there are few reports of extensive experience with its use. However, between its submission as a **candid a te** for AES in 1997 and its selection in 2001, it underwent extensive cryptanalysis by both government and independ ent **cryptographers**. Its Dutch inventors have no relationship to the NSA or any other part of the U.S. government, so there is no __suspicion__ that the government somehow weakened the algorithm or added a trapd oo r. Although the steps of a cycle are simple to describe and seem to be r a ther random transformations of bits, in fact (as described in some detail in  Chapter 12 <span style="font-family: "Times New Roman"; font-size: 12.0pt; mso-ansi-language: EN-US; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: RU;">), these transformations have a sound mathematical origin. With nearly 30 years of use, suspicions of weakness (intentional or not) and backdoors have pretty much been quashed. Not only have analysts failed to find any significant flaws, but in fact research has shown that seemingly insignificant changes weaken the strength of the algorithmthat is, the algorithm is the best it can be. The second question, about how long DES would last, went unanswered for a long time but then was answered very quickly by two experiments in which DES was cracked in days. Thus, after 20 years, the power of individual specialized processors and of massive parallel searches finally overtook the fixed DES key size.

Берёзкина 4

We must ask the same questions about AES: Does it have fl aw s, and for how long will it remain sound? We cannot address the question of fl aw s yet, o ther than to say that teams of cryptanalysts pored over the design of Rijndael during the two-year review **period** without finding any problems. The lon g evity question is more difficult, but also more optimistic, to answer for AES than for DES. The AES algorithm as defined can use 128-, 192-, or 256-bit keys. This characteristic means that AES starts with a key more than double the size of a DES key and can extend to double it yet again. (Remember that doubling the key length squares the number of possible keys that need to be tested in attempts to break the encryption.) But because there is an evident underlying structure, it is also possible to use the same general approach on a slightly different underlying problem and accommodate keys of even larger size. (Even a key size of 256 is prodigious, however.) Thus, unlike DES, AES can move to a longer key length any time technology seems to allow an analyst to overtake the current key size.

Ярушин 4 - Moreover, the number of cycles can be extended __in a natural__ way. With DES the algorithm was defined for precisely 16 cycles; to extend that number would require substantial __redefinition__ of the algorithm. The internal structure of AES has no a priori limitation on the number of cycles. If a cryptanalyst ever concluded that 10 or 12 or 14 rounds were too l ow, the only change needed to improve the algorithm would be to change the limit on a rep ea t loop. A mark of confidence is that the U.S. government has approved AES for protecting Secret and Top Secret classified documents. This is the first time the United States has ever approved use of a commercial algorithm derived outside the government (and furthermore outside the United States) to encrypt classified data.

<span style="display: block; height: 1px; left: -40px; overflow: hidden; position: absolute; top: 1300px; width: 1px;"> The A ES is likely to be the commercial-grade symmetric algorithm of choice for years, if not **decades**. __Let us look at it more clo s ely__.

The A ES Contest
In J anuary 1997, NIST called for cryptographers to develop a new encryption system. __As with the c a ll__ for candidates from which DES was selected, NIST made several important restrictions. The algorithms had to be In August 1998, fifteen algorithms were ch o sen from among those submitted; in August 1999,
 * <span style="mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> unclassified
 * <span style="mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> publicly disclos e d
 * <span style="mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> available roya lt y-free for use worldwide
 * <span style="mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> symmetric block cipher algorithms, for blocks of 128 bits
 * <span style="mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> usable with key sizes of 128, 192, and 256 bits

2.6. The AES Encryption Algorithm
The A ES is likely to be the commercial-grade symmetric algorithm of choice for years, if not **decades**. Let us look at it more clo s ely.

The AES Contest
In January 1997, NIST called for cryptographers to develop a new encryption system. As with the call for **candidat e s** from which DES was selected, NIST made several important restrictions. The algorithms had to be <span style="font-family: "Times New Roman"; font-size: 12.0pt; mso-ansi-language: EN-US; mso-bidi-language: AR-SA; mso-fareast-font-family: "Times New Roman"; mso-fareast-language: RU;">In August 1998, fifteen algorithms were ch o sen from among those submitted; in August 1999,
 * <span style="mso-list: l0 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> unclassified
 * <span style="mso-list: l0 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> publicly disclosed
 * <span style="mso-list: l0 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> available royalty-free for use worldwide
 * <span style="mso-list: l0 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> symmetric block cipher algorithms, for blocks of 128 bits
 * <span style="mso-list: l0 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list 36.0pt;"> usable with k ey siz es of 128, 192, and 256 bits