是的，你所要求的一切都是可能的。 在没有帮助的情况下“解密”MD5密码是不可能的，但是可以将MD5密码重新加密到另一种算法中，只是不能一蹴而就。

您要做的就是让用户能够使用旧的MD5密码登录到新系统。在他们登录时，他们已经给了你的登录程序一个未加密的密码版本，你证明与你拥有的MD5哈希匹配。然后，您可以将这个非散列密码转换为新的散列算法。

显然，这是一个扩展的过程，因为您必须等待用户告诉您密码是什么，但它确实有效。

(注意:七年后，哦，希望有人会发现它有用)

MD5哈希算法是不可逆的，所以MD5解码在不可能，但一些网站有批量设置密码匹配，所以你可以尝试在线解码MD5哈希。

在网上试试:

MD5解密

md5online

md5decrypter

MD5有它的弱点(见维基百科)，所以有一些项目，试图预先计算哈希值。维基百科也暗示了其中一些项目。我所知道(并且尊敬)的一个是ophrack。您不能告诉用户他们自己的密码，但您可以告诉他们一个有效的密码。但我想:还是给他们寄个新密码吧，以防他们忘了。

不，不可能反转诸如MD5这样的哈希函数:给定输出哈希值，除非已知关于输入消息的足够信息，否则不可能找到输入消息。

解密不是为哈希函数定义的函数;加密和解密是CBC模式下AES等密码的功能;哈希函数不加密也不解密。哈希函数用于摘要输入消息。顾名思义，没有反向算法可以设计。

MD5 has been designed as a cryptographically secure, one-way hash function. It is now easy to generate collisions for MD5 - even if a large part of the input message is pre-determined. So MD5 is officially broken and MD5 should not be considered a cryptographically secure hash anymore. It is however still impossible to find an input message that leads to a hash value: find X when only H(X) is known (and X doesn't have a pre-computed structure with at least one 128 byte block of precomputed data). There are no known pre-image attacks against MD5.

It is generally also possible to guess passwords using brute force or (augmented) dictionary attacks, to compare databases or to try and find password hashes in so called rainbow tables. If a match is found then it is computationally certain that the input has been found. Hash functions are also secure against collision attacks: finding X' so that H(X') = H(X) given H(X). So if an X is found it is computationally certain that it was indeed the input message. Otherwise you would have performed a collision attack after all. Rainbow tables can be used to speed up the attacks and there are specialized internet resources out there that will help you find a password given a specific hash.

It is of course possible to re-use the hash value H(X) to verify passwords that were generated on other systems. The only thing that the receiving system has to do is to store the result of a deterministic function F that takes H(X) as input. When X is given to the system then H(X) and therefore F can be recalculated and the results can be compared. In other words, it is not required to decrypt the hash value to just verify that a password is correct, and you can still store the hash as a different value.

重要的是使用密码哈希或PBKDF(基于密码的密钥派生函数)来代替MD5。这样的函数指定如何将盐和散列一起使用。这样就不会为相同的密码(来自其他用户或其他数据库)生成相同的散列。由于这个原因，密码哈希也不允许使用彩虹表，只要盐足够大并且正确随机。

Password hashes also contain a work factor (sometimes configured using an iteration count) that can significantly slow down attacks that try to find the password given the salt and hash value. This is important as the database with salts and hash values could be stolen. Finally, the password hash may also be memory-hard so that a significant amount of memory is required to calculate the hash. This makes it impossible to use special hardware (GPU's, ASIC's, FPGA's etc.) to allow an attacker to speed up the search. Other inputs or configuration options such as a pepper or the amount of parallelization may also be available to a password hash.

然而，它仍然允许任何人验证给定H(X)的密码，即使H(X)是密码哈希。密码哈希仍然是确定的，所以如果有人知道所有的输入和哈希算法本身，那么X可以用来计算H(X)，并且-再说一次-结果可以进行比较。

常用的密码散列有bcrypt、scrypt和PBKDF2。还有各种形式的Argon2，它是最近密码哈希竞赛的赢家。在CrackStation上有一篇很好的关于密码安全的博文。

可以使对手无法执行哈希计算来验证密码是否正确。为此，可以使用胡椒作为密码散列的输入。或者，哈希值当然可以使用AES等密码和CBC或GCM等操作模式进行加密。然而，这需要独立存储秘密/密钥，并且比密码哈希有更高的访问要求。

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。

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。