使用2个不同的哈希函数一个好的方法来检查文件完整性？ [英] Is using 2 different hash functions a good way to check for file integrity?

问题描述

我有一个网站，用户可以上传他们的文件;这些都存储在服务器上，并且它们的元数据记录在数据库中。我正在实现一些简单的完整性检查，即这个文件的内容现在是字节为字节相同，当它被上传吗？

I have a website where users can upload their files; these are stored on the server and their metadata recorded in a database. I'm implementing some simple integrity checks, i.e. "is the content of this file now byte-for-byte identical as when it was uploaded?"

一个例子：for内容 userfile.jpg ，MD5哈希是 39f9031a154dc7ba105eb4f76f1a0fd4 ，SHA-1哈希是 878d8d667721e356bf6646bd2ec21fff50cdd4a9 。 如果此文件的内容更改，但前后都有相同的MD5哈希，那么SHA-1哈希值是否也会保持不变？（使用哈希，有时可能会发生哈希冲突 - ）

An example: for content of userfile.jpg, MD5 hash is 39f9031a154dc7ba105eb4f76f1a0fd4 and SHA-1 hash is 878d8d667721e356bf6646bd2ec21fff50cdd4a9. If this file's content changes, but has the same MD5 hash before and after, is it probable that the SHA-1 hash will also stay the same? (With hashing, sometimes you can get a hash collision - could this happen with two different hashing algorithms at once?)

或者是为一个文件无节点计算两个不同的哈希值（我应该尝试其他验证完整性的机制）？

Or is computing two different hashes for a file pointless (and I should try some other mechanism for verifying integrity)?

编辑：我不是真的担心意外损坏，但我应该防止用户更改文件不注意（生日攻击和朋友）。

I'm not really worried about accidental corruption, but I'm supposed to prevent users changing the file unnoticed (birthday attack and friends).

我可能会用一个哈希，SHA-512 - 检查不会发生，经常是一个性能瓶颈，反正正如布鲁斯Schneier说，有足够的快速，不安全的系统已经有了。 - @ MichaelGG在注释

I'll probably go with one hash, SHA-512 - the checks don't happen that often to be a performance bottleneck and anyway, "As Bruce Schneier says, there's enough fast, insecure systems out there already. –@MichaelGG in the comments".

推荐答案

MD5可能对你正在做的是安全的，但没有理由继续使用哈希与已知缺陷。事实上，没有理由你不应该使用SHA256或SHA512，除非你有一些已知的主要性能瓶颈。

MD5 is probably safe for what you're doing, but there's no reason to continue to use a hash with known flaws. In fact, there's no reason you shouldn't be usign SHA256 or SHA512, unless you have some known major performance bottleneck.

编辑：为了澄清，没有理由使用两种算法;只是使用一个适合你需要的。如果你担心人们对你进行MD5冲突（如，这是一个安全威胁吗？），那么使用不那么弱的算法，如SHA256。

To clarify, there's no reason to use two algorithms; just use one that fits what you need. If you're worried about people doing MD5 collisions on you (as in, is this a security threat?), then use an algorithm that isn't as weak, such as SHA256.

编辑2：为了解决一个显然仍然常见的误解：在散列上找到随机碰撞不是1/2 ^ n的概率。它接近1/2 ^（n / 2）。因此，128位散列可能与2 ^ 64次尝试冲突。有关详情，请参见生日攻击。

Edit 2: To address an apparently still common misunderstanding: Finding a random collision on a hash is not a 1/2^n probability. It's closer to 1/2^(n/2). So a 128-bit hash can probably be collided with 2^64 attempts. See birthday attack for details.

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