Re: 網路安全概論

[stillflying (阿彬)]

key distribution

Hierarchical Key Control
sessiong key更換的頻率越高 越不容易被別人破解
distribution session key的缺點:會delay
1)fixed period 每隔一段時間就更新一次
        connection-oriented protocol
2)the specific number of transation 一定數目的傳輸後就更改
        connectionless protocol

Decentralized key distribution
1. request||n1
2. Pmk[Ks||request||IDb||n1||f(n2)]
3. Ks[n2]

drawback
1.tag只有8bits 太少 flexible and functionallity
2.tag is not transmmited in clear form


加上control vector(由KDC產生)
encryption:
control vector經過hash function
要和session同長 和session做XOR和session加密

decryption:
一樣!!

advantage:
1.control vectorhh用hash 可以控制key的長度
2.control vector in the clear form

Transparent key control scheme
KDC和FEP共享master key
KDC generate session key for FEP

decentralized key control
不需求trust or protect KDC
n(n-1)/2 master key 當有n個end systems時

different session key
data-encrypting key:general information
PIN-encrypting key :PINs
File-encrypting key:encrypting files store in a publicly accessible location

separate key type
master key :cryptographic hardware physically
session:    available for application programs

DES
8 bits for key tag(56 bits for key)
1 bit for master or session key
1 bit for encryption
1 bit for decryption

drawback:
limited flexibility and functionality
tag is nottransmitted in clear form

control vector
用hash取control vector 沒有length的限制
control vector is available in clear form

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 220.135.222.145
推 nwo:看起來很高級的樣子
> -------------------------------------------------------------------------- <

作者: stillflying (阿彬) 看板: HSNU_975
標題: Re: 網路安全概論
時間: Sat Jan  8 23:28:12 2005

random and unpredictability
random:
uniform distribution    平均分配
independence

unpredictability:
sequence of numbers that appear to be random are generated by some algorithm

Random number
physical noise generator:random and precision
published random numbers:predictable
psedorandom numbers     :deterministic not statistically random

psedorandom number:
Xn+1=(aX(n)+c)mod m
m=2^31

three criteria for random number generate
full-period     :全部的值都出現過
appear random   :generated sequence 是random的
efficiently     :implement with 32-bit arithmetic

cyclic encryption:
master key is deduced by several session keys based on earlier keys

ANSI X9.17
one of the strongest psedorandom number generators
Input1:64-bit of data and time
Input2:64-bit of seed value
Keys  :K1 and K2 56-bit DES key used for 3 Trible-DES encryption modules
Output:64-bit psedorandom number and 64-bit seed value

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 220.135.222.145
推 Leonids:我電腦壞了啦阿gay就我~~
推 stillflying:重灌就OK了啦...所有問題都一樣!!
> -------------------------------------------------------------------------- <

作者: stillflying (阿彬) 看板: HSNU_975
標題: Re: 網路安全概論
時間: Sun Jan  9 02:57:51 2005

distribution of public keys
1.public announcement:
        convenience
        weakness:任何人都可以仿造public annyouncement
2.publicly available directory:
        {name,public key}

        registration 必須要本人(in person)
        或是經過安全認證(secure authentication communication)
3.public key authority:
        Tighter control

fig 10.3
step1   :request||Time1                 (a->public-key authority)
step2   :Ekr[Kub||request||Time1]       (public-key authority->a)
step3   :Eku[IDa||N1]                   (a->b)
step4   :request||time2                 (b->public key authoruty)
step5   :Ekr[Kua||request||time2]       (public-key authority->b)
step6   :Ekua[n1||n2]                   (b->a)-|
step7   :Ekub[n2]                       (a->b)-|----desirable, not required

4.public-key certificate
        exchange key without contacting public-key authority

fig 10.4
a和b在之前就分別給Certificate authority他們之間的public key
certificate authority 會分別generate Ca=Ekr[Time1,IDa,Kua]
                                   和Cb=Ekr[time2,IDb,Kub]
在互相交換
(time即為有效時間 故不用電子簽章)

distribution public-key simple
(互傳public-key和ID only)
Ks會被竊取
public key和ID也會被假造

secret key distribution with confidentiality and authentication
step1. Ekub[N1||ID]     (a->b)
step2. Ekua[N1||N2]     (a<-b)  確定為b
step3. Ekub[N2]         (a->b)  確定為a
step4. Ekub[Ekra[Ks]]   (a->b)

A Hybird Scheme
IBM mainuframes:
        KDC shares a secret master key
        secret session key is encrypted by master key
        public key is used to distribution master key
performance
        session key用PKE太慢
        Public keys 只被用來update master key
Back compatiablity
        with an existing KDC sheme

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 220.135.222.145

> -------------------------------------------------------------------------- <

作者: stillflying (阿彬) 看板: HSNU_975
標題: Re: 網路安全概論
時間: Sun Jan  9 19:16:47 2005

authentication code function
authenticator:用來證明所傳message的值

types of function that prodice a authenticator
1.hash function
2.MAC(message authentication code)
3.message encryption

checksum
error detecting code
frame check sequence (FCS)

internal error control:authentication of messages
external error control:會被forged(做假)

internal error control:
F(m)+m ==> E[F(m)+m] ==> Ek[m||F(m)] ==> D[m||F(m)] ==> 對m做F(m) 比對F(m)
F(m)為checksum

external error control:
先做encryption 再做checksum 但當 Ek[m]+F(Ek[m]) 傳送時可以被造假
對方可以直接從Ek[m]得到F(Ek[m']) 且等於F(Ek[m])

public-key encryption
authentication
signature
confidentiality

authentication                          使用hash 或checksum
receiver確定message沒有被更改過
receiver確定message是從sender送出的
確定receiver是被指定的接收且attacker不能更改內容

MAC and Conventional Encryption
1.for broadcast system,cheaper and reliable
2.authentication is carried out on selective basis
3.可以檢查是否需要decrypt 不用每次都decrypty
4.authenticate messages
5.將authentication和confidentiality分開 使architecture flexible
6.prolong protection time

note: MAC 沒有digital signature的功能 因為是sender和receiver使用相同的key

Hash function
1.可以符合各種size的data block
2.easy to compute

requirement for a hash function
one-way property:
不論給任一個m 不可能去找到一個x 使F(x)=m

weak collision resistance (prevent forgery)
不論任何一個x 不可能找到y =\= x 又 F(y)=F(x)

strong collision resistance
不可能找到一pair (x,y)使H(x)=H(y)

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 220.135.222.145