blocks|key|1717051|text|它的可能性有多大并不重要；它是可能的。这可能发生在您散列的前两件事情上(非常不可能，但也是有可能的)，所以您需要从一开始就支持冲突。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1717052|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Doesn't really matter how likely it is; it is possible. It could happen on the first two things you hash (very unlikely, but possible), so you'll need to support collisions from the beginning.

blocks|key|1717110|text|碰撞的粗略经验法则是值范围的平方根。您的MD5签名可能有128位长，因此您可能会看到超过2%5E64个图像的冲突。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1717111|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

A rough rule of thumb for collisions is the square-root of the range of values. Your MD5 sig is presumably 128 bits long, so you're going to be likely to see collisions above and beyond 2^64 images.

blocks|key|1717178|text|S3可以有子目录。只需在键名中添加"/“，您就可以访问这些文件，就像它们位于不同的目录中一样。我使用它在S3中根据用户ID将用户文件存储在不同的文件夹中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1717179|例如："mybucket/users/1234/somefile.jpg“。它与文件系统中的目录并不完全相同，但是S3应用编程接口有一些特性使它的工作方式几乎相同。我可以要求它列出所有以"users/1234/“开头的文件，它会显示该”目录“中的所有文件。|1717180|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

S3 can have subdirectories. Just put a "/" in the key name, and you can access the files as if they were in separate directories. I use this to store user files in separate folders based on their user ID in S3. 

For example: "mybucket/users/1234/somefile.jpg". It's not exactly the same as a directory in a file system, but the S3 API has some features that let it work almost the same. I can ask it to list all files that begin with "users/1234/" and it will show me all the files in that "directory".

blocks|key|1717252|text|等等，是不是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1717253|md5(filename)+%2B+timestamp|code-block|syntax|javascript|1717254|或者：|1717255|md5(filename+%2B+timestamp)|1717256|如果是前者，你基本上就是GUID了，我不担心这一点。如果是后者，那么请参阅Karg关于最终将如何遇到冲突的帖子。|1717257|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

So wait, is it:

<pre><code>md5(filename) + timestamp
</code></pre>

or:

<pre><code>md5(filename + timestamp)
</code></pre>

If the former, you are most of the way to a GUID, and I wouldn't worry about it. 
If the latter, then see Karg's post about how you will run into collisions eventually.

blocks|key|1717338|text|虽然由于冲突而导致的MD5问题广为人知，但随机数据之间的无意冲突是exceedingly+rare。另一方面，如果你对文件名进行散列，那就不是随机数据，我希望很快就会发生冲突。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1717339|entityMap|0|LINK|mutability|MUTABLE|url|http://www.miketaylor.org.uk/tech/law.html^0|X|G|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

While there have been well publicized problems with MD5 due to collisions, UNINTENTIONAL collisions among random data are <a href="http://www.miketaylor.org.uk/tech/law.html" rel="noreferrer">exceedingly rare</a>. On the other hand, if you are hashing on the file name, that's not random data, and I would expect collisions quickly.

blocks|key|1502254|text|尽管随机MD5冲突非常罕见，但如果您的用户可以提供文件(将逐字存储)，那么他们就可以控制冲突的发生。也就是说，他们可以故意创建两个MD5sum相同但数据不同的文件。确保您的应用程序能够以合理的方式处理这种情况，或者使用更强的散列，如SHA-256。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1502255|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Although random MD5 collisions are exceedingly rare, if your users can provide files (that will be stored verbatim) then they can engineer collisions to occur. That is, they can deliberately create two files with the same MD5sum but different data. Make sure your application can handle this case in a sensible way, or perhaps use a stronger hash like SHA-256.

I've got an image library on Amazon S3. For each image, I md5 the source URL on my server plus a timestamp to get a unique filename. Since S3 can't have subdirectories, I need to store all of these images in a single flat folder.

Do I need to worry about collisions in the MD5 hash value that gets produced?

Bonus: How many files could I have before I'd start seeing collisions in the hash value that MD5 produces?

How many random elements before MD5 produces collisions?

文件存储

文件系统

服务器

我在亚马逊S3上有一个图像库。对于每个图像，我md5我的服务器上的源URL加上一个时间戳，以获得唯一的文件名。因为S3不能有子目录，所以我需要将所有这些图像存储在一个平面文件夹中。我需要担心生成的MD5散列值中的冲突吗？额外的好处:在我开始看到MD5产生的哈希值发生冲突之前，我可以有多少个文件？

问在MD5产生冲突之前，有多少随机元素？
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