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)
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