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Question

1 简介
DES是Data Encryption Standard（数据加密标准）的缩写。 数据加密算法的数据加密标准（Data Encryption Standard，DES）是规范的描述，它出自 IBM 的研究工作，并在 1997 年被美国政府正式采纳。它很可能是使用最广泛的秘钥系统，特别是在保护金融数据的安全中，最初开发的 DES 是嵌入硬 件中的。通常，自动取款机（Automated Teller Machine，ATM）都使用 DES。
IBM 曾对 DES 拥有几年的专利权，但是在 1983 年已到期，并且处于公有范围中，允许在特定条件下可以免除专利使用费而使用。二十年来，它一直活跃在国际保密通信的舞台上，扮演了十分重要的角色。
DES是一个分组加密算法，他以64位为分组对数据加密。同时DES也是一个对称算法：加密和解密用的是同一个算法。它的密匙长度是56位（因为每个第8位都用作奇偶校验），密匙可以是任意的56位的数，而且可以任意时候改变。其中有极少量的数被认为是弱密匙，但是很容易避开他们。所以保密性依赖于密钥。
2 算法框架：
DES对64(bit)位的明文分组M进行操作，M经过一个初始置换IP置换成m0，将m0明文分成左半部分和右半部分m0=(L0,R0)，各32位长。然后进行16轮完全相同的运算，这些运算被称为函数f，在运算过程中数据与密匙结合。经过16轮后，左，右半部分合在一起经过一个末置换，这样就完成了。
在每一轮中，密匙位移位，然后再从密匙的56位中选出48位。通过一个扩展置换将数据的右半部分扩展成48位，并通过一个异或操作替代成新的32位数据，在将其置换换一次。这四步运算构成了函数f。然后，通过另一个异或运算，函数f的输出与左半部分结合，其结果成为新的右半部分，原来的右半部分成为新的左半部分。将该操作重复16次，就实现了。
3 DES解密
在经过所有的代替、置换、异或盒循环之后，你也许认为解密算法与加密算法完全不同。恰恰相反，经过精心选择的各种操作，获得了一个非常有用的性质：加密和解密使用相同的算法。
DES加密和解密唯一的不同是密匙的次序相反。如果各轮加密密匙分别是K1,K2,K3….K16那么解密密匙就是K16,K15,K14…K1。
4 DES的几种工作方式
第一种电子密本方式（ECB）
将明文分成n个64比特分组，如果明文长度不是64比特的倍数，则在明文末尾填充适当数目的规定符号。对明文组用给定的密钥分别进行加密，行密文C=(C0,C1,……,Cn-1)其中Ci=DES(K,xi),i=0,1,…..,n-1。
第二种密文分组链接方式（CBC）
在CBC方式下，每个明文组xi在加密前与先一组密文按位模二加后，再送到DES加密，CBC方式克服了ECB方式报内组重的缺点，但由于明文组加密前与一组密文有关，因此前一组密文的错误会传播到下一组。
第三种密文反馈方式（CFB），可用于序列密码
明文X＝(x0,x1,……,xn-1)，其中xi由t个比特组成0 第四种输出反馈方式（OFB），可用于序列密码.
与CFB唯一不同的是OFB是直接取DES输出的t个比特，而不是取密文的t个比特，其余都与CFB相同。但它取的是DES的输出，所以它克服了CFB的密文错误传播的缺点 .

Answer 1

One brief introduction
The DES is the abbreviation of the Standard(the data encrypt standard) of the Data Encryption. Data's encrypting the data of calculate way to encrypt standard(the Standard of the Data Encryption, DES) is a norm of description, it come from IBM research work, and at 1997 drive the United States government formal adopt.It very may is usage most extensive of Mi key system, especially in the protection the finance the safety of the data, first development of the DES imbed a hard piece.Usually, cash dispenser(Automated Teller Machine, ATM) all usage DES.
The IBM once owns the patent right for several years to the DES, but has already expire in 1983, and be placed in owned by public scope in, allow under the particular condition can absolution patent usage fee but usage.20 in the last years, it be always active to keep secret correspondence in the nations of satge up, impersonation very importance of role.
The DES is that a cent set encrypt calculate way, he take 64 as cent set logarithms according to encrypt.The DES is also a symmetry calculate way in the meantime:Encrypt to reach agreement airtight what to use be the same of calculate way.It of the airtight spoon length be 56(used to because of each the 8th strange accidentally the school check), the airtight spoon can be arbitrarily 56 number, and can arbitrarily time change.Have among them very few quantity of number is think is weak and airtight spoon, but very easy avert from them.So the confidentiality dependence is at the airtight key.
2 calculate way frame:
DES rightness the clear statement of 64(bit) cent set the M carry on operation, M process a beginning start displacement IP displacement m 0, m 0 clear statements' being divided into left half part and right half be parts of m 0=(L0, R0), 32 for each long.Then carry on 16 complete homology of operation, these operation is called function f, the data and the airtight spoon combine in the operation the process.Through 16, left, the right half part match together through an end displacement, so completion.
In each 1, the airtight spoon move, then win election 48 from 56 of airtight spoon.Expand displacement through 1 data of the right half part expand into 48, and pass a difference or operation act for new of 32 data be once changing its displacement.This four step operation constitute function f.Then, pass an another difference or operation, function f of output with left half part combine, it result become new of right half part, originally of right half part become new of left half part.Will should operation repeated 16 times, realization.
3 The DES decrypt
Has been allly replaced, displacement, difference or box circulation after, you perhaps think decrypt calculate way with encrypt calculate way totally different.Exactly the opposite, process with meticulous care various operation of choice, acquired a property of great use:Encrypt calculate way of reach agreement the airtight usage homology.
DES's encrypting a dissimilarity of reach agreement the airtight unique is that the order of sequence of airtight spoon is contrary.If each encrypt an airtight spoon is a K1, K2 respectively, the K be 3 … .K16's so decrypting an airtight spoon is soa K16, K15, K 14 … K1.
4 DES a few kinds work way
A kind of electronics airtight origin way(ECB)
Be divided into clear statement n 64 ratio especially cent set, if clear statement length is not 64 ratio especially of multiple, fill appropriate number at the clear statement end tail of provision sign.Be respectively carry on encrypt to clear statement set the airtight key which use to give to settle, line secret content C=(C0, C1, ……, Cn-1)among them Ci=DES(K, xi), i=0,1, ….., N-1.
Two kinds of secret content cents set chain connect a way(CBC)
Under the CBC way, each clear statement set xi at before encrypt with first a set of secret content by mold two add after, again send to DES to encrypt, the CBC way overcame an ECB way report the heavy weakness is inside the set, but because of the clear statement set encrypt front have relation with a set of secret content, therefore a set of ex- secret content of mistake would dissemination arrive next.
Three kinds of secret content feedback way(CFB) can used for a sequence password
Clear statement X=(x 0, x 1, ……, xn-1), among them xi from t ratio constitute especially 04 kinds output a feedback way(OFB), can used for a sequence password.
Different from CFB unique of is an OFB is direct take the DES output's t ratio especially, not take the t of secret content ratio especially, rest all with CFB homology.But what it take be a DES exportation, so it overcome the weakness that the CFB secret content mistake spread.

Answer 2

1 Introduction

DES is the Data Encryption Standard (Data Encryption Standard) abbreviation. Data Encryption Data Encryption Standard algorithm (Data Encryption Standard, DES) is to standardize the description, it comes from IBM's research work, and in 1997 formally adopted by the U.S. government. It is probably the most widely used secret key system, especially in protecting the security of financial data, the initial development of the DES is embedded in hardware. Typically, ATM (Automated Teller Machine, ATM) are the use of DES.

DES had several years IBM has a patent, but in 1983 has expired, and is in the public area, allowing under certain conditions can be royalty-free use. The last two decades, it has been active in the international arena and secure communication have played an important role to play.

DES is a block cipher algorithm, his 64-bit data encryption for the packet. At the same time, DES is a symmetric algorithm: encryption and decryption using the same algorithm. Its key length is 56 (since each section are used as 8-bit parity), keys can be arbitrary number of 56, but the time can change. Very little of which is considered the number of weak keys, it is easy to avoid them. Therefore, dependent on key confidentiality.

2 Algorithm Framework:

DES of 64 (bit)-bit sub-M to operate explicitly, M, after an initial permutation IP replaced by m0, will be divided into m0 explicit part of the left part and right m0 = (L0, R0), the 32-bit long. 16 exactly the same and then proceed to computing, the computing function is called f, in the computing process of combining data and key. After 16, the left and right parts together after a late replacement, this is complete.

In each round, the key shift bit, and then from the key 56 of the 48 selected. Through an expansion of the right replacement part will extend the data into 48, and through an XOR operation to replace a new 32-bit data for one of its replacement. This constitutes a four-step computing function f. Then, through another XOR operation, the output function f and the left part of combination, the results become part of the new right, the right part of the original to become part of the new left. Repeat the operation 16 times, on the reality.

3 DES decryption

After all the alternative, replacement, or box the cycle varies, you may think that encryption algorithm and decryption algorithm is entirely different. The contrary, carefully selected a variety of operations, was a very useful property: using the same encryption and decryption algorithms.

DES encryption and decryption keys only difference is the opposite order. If the round of encryption keys are K1, K2, K3 .... K16 decryption key is then K16, K15, K14 ... K1.

4 DES Several working methods

The first is the way the electronic density of the (ECB)

N will be explicitly divided into a 64-bit packet, if the explicit length is not a multiple of 64 bits, then at the end of the express provisions of an appropriate number of filler symbols. On the explicit group were given the keys for encryption, the ciphertext C = (C0, C1, ... ..., Cn-1) in which Ci = DES (K, xi), i = 0,1, ... .., n-1.

The second sub-link ciphertext (CBC)

In CBC mode, each xi expressly group first before encryption ciphertext by a group of two plus-bit mode, and then to the DES encryption, CBC ways to overcome the way the ECB reported that weaknesses within the group of heavy, but the explicit group before encryption ciphertext with a group, so a group of former ciphertext error will spread to the next group.

The third ciphertext feedback (CFB), the password can be used to sequence

Express X = (x0, x1, ... ..., xn-1), of which xi composed by the t-bit 0 of the fourth output feedback (OFB), the password can be used in sequence.

CFB and OFB only difference is that DES is a direct output of the t-bit, rather than take the t ciphertext bits, and the rest are the same as CFB. But it is taking the output of DES, it has overcome the CFB ciphertext of the shortcomings of error propagation.

Answer 3

DES is the Data Encryption Standard abbreviation. Data Encryption Data Encryption Standard algorithm is to standardize the description, it comes from IBM's research work, and in 1997 formally adopted by the U.S. government. It is probably the most widely used secret key system, especially in protecting the security of financial data, the initial development of the DES is embedded in hardware. Typically, ATM are the use of DES.
DES had several years IBM has a patent, but in 1983 has expired, and is in the public area, allowing under certain conditions can be royalty-free use. The last two decades, it has been active in the international arena and secure communication have played an important role to play.
DES is a block cipher algorithm, his 64-bit data encryption for the packet. At the same time, DES is a symmetric algorithm: encryption and decryption using the same algorithm. Its key length is 56 , keys can be arbitrary number of 56, but the time can change. Very little of which is considered the number of weak keys, it is easy to avoid them. Therefore, dependent on key confidentiality.
2 Algorithm Framework:
DES of 64 (bit)-bit sub-M to operate explicitly, M, after an initial permutation IP replaced by m0, will be divided into m0 explicit part of the left part and right m0，the 32-bit long. 16 exactly the same and then proceed to computing, the computing function is called f, in the computing process of combining data and key. After 16, the left and right parts together after a late replacement, this is complete.
In each round, the key shift bit, and then from the key 56 of the 48 selected. Through an expansion of the right replacement part will extend the data into 48, and through an XOR operation to replace a new 32-bit data for one of its replacement. This constitutes a four-step computing function f. Then, through another XOR operation, the output function f and the left part of combination, the results become part of the new right, the right part of the original to become part of the new left. Repeat the operation 16 times, on the reality.
3 DES decryption
After all the alternative, replacement, or box the cycle varies, you may think that encryption algorithm and decryption algorithm is entirely different. The contrary, carefully selected a variety of operations, was a very useful property: using the same encryption and decryption algorithms.
DES encryption and decryption keys only difference is the opposite order. If the round of encryption keys are K1, K2, K3 .... K16 decryption key is then K16, K15, K14 ... K1.
4 DES Several working methods
The first is the way the electronic density of the
N will be explicitly divided into a 64-bit packet, if the explicit length is not a multiple of 64 bits, then at the end of the express provisions of an appropriate number of filler symbols. On the explicit group were given the keys for encryption, the ciphertext C in which Ci = DES，i = 0,1, ... .., n-1.
The second sub-link ciphertext
In CBC mode, each xi expressly group first before encryption ciphertext by a group of two plus-bit mode, and then to the DES encryption, CBC ways to overcome the way the ECB reported that weaknesses within the group of heavy, but the explicit group before encryption ciphertext with a group, so a group of former ciphertext error will spread to the next group.
The third ciphertext feedback , the password can be used to sequence
Express X , of which xi composed by the t-bit 0 of the fourth output feedback , the password can be used in sequence.
CFB and OFB only difference is that DES is a direct output of the t-bit, rather than take the t ciphertext bits, and the rest are the same as CFB. But it is taking the output of DES, it has overcome the ciphertext CFB error propagation of the shortcomings of

Answer 4

Introduction
Data Encryption Standard (DES is the abbreviation of Data Encryption Standard). Data Encryption algorithm Data Encryption Standard (DES) Data Encryption Standard, it is Standard, from IBM research work, and in 1997 by the U.S. government formally adopted. It is probably the most widely used secret-key systems, especially in the financial data security protection, was originally developed DES is embedded hard thing. Usually, Automated Teller machines (atms) Teller Machine, o use DES.
IBM has to have several patents, DES in 1983, has expired and in public scope permitted in particular conditions can avoid patent royalty and use. 2 years, it has been active in international communication stage, plays an important role.
DES is a group encryption algorithm, with his 64-bit data encryption for grouping. While DES is a symmetric algorithms of encryption and decryption: with the same algorithm. It's the key length is 56 (for each article 8 bits are used for parity), key can be arbitrary 56 number, but can change any time. One of a number of key is believed to be the weak, but easy to avoid them. So the secrecy depends on the keys.
2 algorithm framework:
DES 64 bit of expressly grouping operation, M j, after an initial displacement IP will become replacement m0 expressly into the left part and m0 L0 = (m0 right part, the 32-bit R0), long. Then on wheels identical operation, the operation is called function f, in operation process data and key combination. After 16 rounds left and right after part together after a replacement, so late.
In each round, the key position shift, and then from the key of 56 elected 48. Through a replacement will expand the right data into a part, and through a vision or operation and alternative into new 32 data, in its displacement in time. This four-step operation constitutes a function f. Then, through another or operation, the output and the left () function f part, the result became part of new right and left hemisphere has become a new part. This operation will repeat 16 times, can achieve.
3 DES decryption
After all, replacement, or replaced, you may circulation box that decryption algorithm and encryption algorithm is totally different. Contrary to all kinds of choices, meticulously, received a very useful properties: encryption and decryption use the same method.
DES encryption and decryption only different is the key of the order. If each chakra MiMi add keys were K1, K2 and erp. K16. So MiMi solution is K16 keys, K15, K14... K1.
Four kinds of working style. DES
The first kind of electron dense this way (ECB)
Will a 64 bit expressly into n, if not 64 bit length ratio, the end of a reasonable amount of filling expressly prescribed symbols. With a given set of plaintext encrypted respectively, key plaintext C = C0 (C1,..., Cn, including 1 -), Ci = DES (K), I = 0,1 xi,... .. , n - 1.
The second group links (plaintext CBC)
In CBC mode, every plaintext encrypted before and during group xi before a group of ciphertext bitwise mode, after two plus DES encrypted, CBC ways to overcome the ECB way in groups of weakness, but because expressly set with a group of plaintext encrypted before, so ago a group of errors will spread to plaintext under a group.
The third kind of plaintext way (CFB), feedback can be used to sequence the password
X0 (X = expressly xn, clamps its x1,..., 1 -), including a bit from t xi composed 0 fourth output feedback (OFB), can be used to sequence the password.
The only difference is with CFB OFB is directly take a bit of t DES output, rather than take a bit of plaintext; the rest are identical with CFB. But it is the output of DES, it overcomes the ciphertext error propagation of CFB faults本回答被提问者采纳