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。

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。

不可能，至少在合理的时间内是不可能的。

通常的处理方式是密码“重置”。也就是说，你给他们一个新的(随机的)密码，并在电子邮件中发送给他们。

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。

无法恢复md5密码。(任何语言)

但是你可以:

给用户一个新的。

找个彩虹表，也许能找回旧的。

理论上，对哈希值进行解密是不可能的，但您可以使用一些肮脏的技术来获取原始的纯文本。

Bruteforcing: All computer security algorithm suffer bruteforcing. Based on this idea today's GPU employ the idea of parallel programming using which it can get back the plain text by massively bruteforcing it using any graphics processor. This tool hashcat does this job. Last time I checked the cuda version of it, I was able to bruteforce a 7 letter long character within six minutes. Internet search: Just copy and paste the hash on Google and see If you can find the corresponding plaintext there. This is not a solution when you are pentesting something but it is definitely worth a try. Some websites maintain the hash for almost all the words in the dictionary.