在这里可以看到所有其他的答案，关于它如何以及为什么不可逆，以及为什么你不想这样做。

为了完整起见，这里有彩虹表，你可以在上面查找可能的匹配。不能保证彩虹表中的答案是用户选择的原始密码，这样会使用户非常困惑。

同样，这也不适用于咸散列。许多安全专家都建议用盐腌制。

MD5是一种哈希算法，不能还原哈希值。

您应该添加“更改密码功能”，用户提供另一个密码，计算散列并将其存储为新密码。

MD5被认为是坏的，不是因为您可以从散列中获得原始内容，而是因为通过工作，您可以生成两个散列到相同散列的消息。

不能取消MD5哈希。

不。MD5不是加密(尽管它可能被用作一些加密算法的一部分)，它是一个单向哈希函数。作为转换的一部分，大部分原始数据实际上“丢失”了。

Think about this: An MD5 is always 128 bits long. That means that there are 2128 possible MD5 hashes. That is a reasonably large number, and yet it is most definitely finite. And yet, there are an infinite number of possible inputs to a given hash function (and most of them contain more than 128 bits, or a measly 16 bytes). So there are actually an infinite number of possibilities for data that would hash to the same value. The thing that makes hashes interesting is that it is incredibly difficult to find two pieces of data that hash to the same value, and the chances of it happening by accident are almost 0.

A simple example for a (very insecure) hash function (and this illustrates the general idea of it being one-way) would be to take all of the bits of a piece of data, and treat it as a large number. Next, perform integer division using some large (probably prime) number n and take the remainder (see: Modulus). You will be left with some number between 0 and n. If you were to perform the same calculation again (any time, on any computer, anywhere), using the exact same string, it will come up with the same value. And yet, there is no way to find out what the original value was, since there are an infinite number of numbers that have that exact remainder, when divided by n.

That said, MD5 has been found to have some weaknesses, such that with some complex mathematics, it may be possible to find a collision without trying out 2128 possible input strings. And the fact that most passwords are short, and people often use common values (like "password" or "secret") means that in some cases, you can make a reasonably good guess at someone's password by Googling for the hash or using a Rainbow table. That is one reason why you should always "salt" hashed passwords, so that two identical values, when hashed, will not hash to the same value.

一旦一段数据通过哈希函数运行，就没有回头路了。

解密(直接从散列值中获取纯文本，以一种算法方式)，没有。

然而，有一些方法使用所谓的彩虹表。如果你的密码是没有盐的散列，这是非常可行的。

不直接。由于鸽子洞原理，有(可能)不止一个值散列到任何给定的MD5输出。因此，你不能肯定地扭转它。此外，MD5是为了使查找任何这样的反向哈希变得困难(然而，已经有产生冲突的攻击-也就是说，产生两个哈希到相同结果的值，但您无法控制最终的MD5值是什么)。

但是，如果将搜索空间限制为长度小于N的普通密码，则可能不再具有不可逆性属性(因为MD5输出的数量远远大于感兴趣域中的字符串数量)。然后，您可以使用彩虹表或类似的反向哈希。