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q: digital signature and hash function

jigsaw

KK

CJSDN高级会员

发贴: 3666
积分: 93

于 2005-03-19 02:39

谁能科普一下吗？
我对电子签名的机制感到不解。。。
那，一个电子签名是这样产生的(from user A to B)：
A: original message = *msg*

*msg* ---> encrypt with B's public key ---> *encrypted msg*
----> hash(MD5 or SHA-1) ----> *compressed hash*
-----> encrypt with A's private key ----> *signature*

ok, now A send both *encrypted msg* AND *signature* to B.

B should check the signature (is it from A?) first:
1.*signature* ---> decrypt with A's public key ----> *decrypted sig*
2.*encrypted msg* ---> hash(should be the same as wot A used) ---> *compressed hash*
if the result *decrypted sig* and *compressed hash* are identical,
the B can make sure the message is from A.
after that, B can decrypt *encrypted msg* with B's private key.

--end

now the q:
i dont see why the hash process should be impossible to find collision.
if someone else, say, C, want to forge a message from A, C must know
the private key of A, othersie when B is trying to decrypt the *signature*
with A's public key, he will never get the correct result, is it?
the only thing the hash method do is to compress a long message to a string of given size, is it? why it(hash) must take the shoulder of encrypt since RSA
has already done a good job?
this q must be a silly one, but it has confused me for a long time...

thx

No one knows except both of us.
909090909090909090909090909090909090909090b8533ce76c8d6c241868968a0408c338b4ffbf
ISO/IEC 9899:1999

作者

Re:q: digital signature and hash function [Re:jigsaw]

menzy

版主

发贴: 754
积分: 113

于 2005-03-21 12:31

加密时不必要了，但是签名的主要作用是保证文件的不可否认性，即保证A不能否认他曾经发出过这个文件，这是电子商务合同订立的技术基础。
实际上还有更复杂的，比如说，
A-客户 B - 厂商 C- 银行
如果A购买B的货物，那么A必须向C发送支付请求，让C对B支付，同时把支付信息发送给B，这样B知道C一定会向B进行支付，从而可以可靠的对A发货。
那么，要求同一合同文本只在A/B之间传递，C不知道A购买的货物，而B不应当知道A在C的帐号，如何处理？
实际应用时很怪异的。

作者	Re:q: digital signature and hash function [Re:jigsaw]
jigsaw KK CJSDN高级会员发贴: 3666 积分: 93	于 2005-03-22 10:41 明白了。。。也就是说用到了RSA的时候 hash就不那么重要了谢谢~ No one knows except both of us. 909090909090909090909090909090909090909090b8533ce76c8d6c241868968a0408c338b4ffbf ISO/IEC 9899:1999

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