substr md5碰撞 [英] substr md5 collision

问题描述

我需要一个4字符的散列。目前我正在采用 md5（）哈希的前4个字符。我正在对80个字符或更少的字符串进行哈希处理。这会导致碰撞吗？或者，碰撞的可能性是多少？假设我将散列少于65,536（16 ⁴ ）的不同元素？

解决方案

的确令人惊讶地高。正如您从这个近似碰撞概率图（公式来自维基百科页面），只有几百个元素，您的碰撞概率超过50％。

当然，如果您面临攻击者提供字符串的可能性，那么您可以假设它是100％ - 在任何现代PC上几乎可以瞬间完成扫描以发现16位搜索空间中的冲突。甚至任何现代手机，就此而言。

I need a 4-character hash. At the moment I am taking the first 4 characters of a md5() hash. I am hashing a string which is 80 characters long or less. Will this lead to collision? or, what is the chance of collision, assuming I'll hash less than 65,536 (16⁴) different elements?

解决方案

Surprisingly high indeed. As you can see from this graph of an approximate collision probability (formula from the wikipedia page), with just a few hundred elements your probability of having a collision is over 50%.

Note, of course, if you're facing the possibility of an attacker providing the string, you can probably assume that it's 100% - scanning to find a collision in a 16-bit search space can be done almost instantaneously on any modern PC. Or even any modern cell phone, for that matter.

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