> For the complete documentation index, see [llms.txt](https://dailysharing.supermouse.cn/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dailysharing.supermouse.cn/2020/december/14th.md).

# 14th

The **MD5 message-digest algorithm** is a widely used [hash function](https://en.wikipedia.org/wiki/Hash_function) producing a 128-[bit](https://en.wikipedia.org/wiki/Bit) hash value. Although MD5 was initially designed to be used as a [cryptographic hash function](https://en.wikipedia.org/wiki/Cryptographic_hash_function), it has been found to suffer from extensive vulnerabilities. It can still be used as a [checksum](https://en.wikipedia.org/wiki/Checksum) to verify [data integrity](https://en.wikipedia.org/wiki/Data_integrity), but only against unintentional corruption. It remains suitable for other non-cryptographic purposes, for example for determining the partition for a particular key in a partitioned database.[\[3\]](https://en.wikipedia.org/wiki/MD5#cite_note-3)

MD5 was designed by [Ronald Rivest](https://en.wikipedia.org/wiki/Ronald_Rivest) in 1991 to replace an earlier hash function [MD4](https://en.wikipedia.org/wiki/MD4),[\[4\]](https://en.wikipedia.org/wiki/MD5#cite_note-Ron_Barak-4) and was specified in 1992 as [RFC 1321](https://tools.ietf.org/html/rfc1321).

One basic requirement of any cryptographic hash function is that it should be [computationally infeasible](https://en.wikipedia.org/wiki/Computational_complexity_theory#Intractability) to find two distinct messages that hash to the same value. MD5 fails this requirement catastrophically; such [collisions](https://en.wikipedia.org/wiki/Collision_resistance) can be found in seconds on an ordinary home computer.

The weaknesses of MD5 have been exploited in the field, most infamously by the [Flame malware](https://en.wikipedia.org/wiki/Flame_malware) in 2012. The [CMU Software Engineering Institute](https://en.wikipedia.org/wiki/CMU_Software_Engineering_Institute) considers MD5 essentially "cryptographically broken and unsuitable for further use".[\[5\]](https://en.wikipedia.org/wiki/MD5#cite_note-5)

As of 2019, MD5 continues to be widely used, in spite of its well-documented weaknesses and deprecation by security experts.[\[6\]](https://en.wikipedia.org/wiki/MD5#cite_note-Cimpanu2019-6)

参考资料：<https://en.wikipedia.org/wiki/MD5#cite_note-3>

MD5 消息摘要算法是一种应用广泛的散列函数，可以生成一个 128 个比特位的散列值。MD5 是 Ronald Rivest 在 1991 年设计的，用以取代 MD4，1992 年 MD5 收录于 RFC 1321。虽然 MD5 的设计初衷是用于密码散列，但是他已经被发现存在漏洞——两条不同的消息可能会生成相同的散列值，即散列冲突，在一个普通的家用电脑上，只需几秒钟便可以找到这种冲突，这对一个密码散列函数来说无疑是致命的。2012 年臭名昭著的 Flame 病毒便是利用了 MD5 的漏洞。

MD5 可以作为校验和，来验证数据的完整性，但是，MD5 只能针对数据意外损坏的情况（换言之，人为篡改是无法有效检验的）。一些非加密场景也能用到 MD5，比如数据库分区时确定特定关键字所处的分区。