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。

It is not yet possible to put in a hash of a password into an algorithm and get the password back in plain text because hashing is a one way thing. But what people have done is to generate hashes and store it in a big table so that when you enter a particular hash, it checks the table for the password that matches the hash and returns that password to you. An example of a site that does that is http://www.md5online.org/ . Modern password storage system counters this by using a salting algorithm such that when you enter the same password into a password box during registration different hashes are generated.

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，你不能解密/反向md5，因为它是一个单向哈希函数，直到你不能在md5中发现广泛的漏洞。 另一种方法是有一些网站有大量的密码集数据库，所以你可以尝试在线解码你的MD5或SHA1哈希字符串。 我尝试了http://www.mycodemyway.com/encrypt-and-decrypt/md5这样的网站，它对我来说工作得很好，但这完全取决于你的哈希，如果哈希存储在数据库中，那么你可以得到实际的字符串。

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。