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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.