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27 May 2012

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What is MD5 Hash and How to Use It ?


Today In this post, I am going to explain one of my favourite and interesting cryptographic algorithm called MD5 (Message-Digest algorithm 5). This algorithm is mainly used to perform file integrity checks under most circumstances. The MD5 Message-Digest Algorithm is a widely used cryptographic hash function that produces a 128-bit (16-byte) hash value. Specified in RFC 1321, MD5 has been employed in a wide variety of security applications, and is also commonly used to check data integrity. MD5 was designed by Ron Rivest in 1991 to replace an earlier hash function, MD4. An MD5 hash is typically expressed as a 32-digit hexadecimal number etc.

 

Security

The security of the MD5 hash function is severely compromised. A collision attack exists that can find collisions within seconds on a computer with a 2.6 GHz Pentium 4 processor (complexity of 224.1). Further, there is also a chosen-prefix collision attack that can produce a collision for two chosen arbitrarily different inputs within hours, using off-the-shelf computing hardware (complexity 239). The ability to find collisions has been greatly aided by the use of off-the-shelf GPUs. On an NVIDIA GeForce 8400GS graphics processor, 16–18 million hashes per second can be computed. An NVIDIA GeForce 8800 Ultra can calculate more than 200 million hashes per second.

These hash and collision attacks have been demonstrated in the public in various situations, including colliding document files and digital certificates.

Other vulnerabilities:

A number of projects have published MD5 rainbow tables online, that can be used to reverse many MD5 hashes into strings that collide with the original input, usually for the purposes of password cracking.

The use of MD5 in some websites' URLs means that search engines such as Google can also sometimes function as a limited tool for reverse lookup of MD5 hashes. Both these techniques are rendered ineffective by the use of a sufficiently long salt.


A MD5 hash is nothing but a 32 digit hexadecimal number which can be something as follows

A Sample MD5 Hash

e4d909c290d0fb1ca068ffaddf22cbd0

This hash is unique for every file irrespective of it’s size and type. That means two .exe files with the same size will not have the same MD5 hash even though they are of same type and size. So MD5 hash can be used to uniquely identify a file.

How to use MD5 Hash to check the Integrity of Files?

Suppose you have a file called backup.tar on your server. Before you download, you need to generate MD5 hash for this file on your server. To do so use the following command.

For UNIX:

md5sum backup.tar

When you hit ENTER you’ll see something as follows

e4d909c290d0fb1ca068ffaddf22cbd0

This is the MD5 hash for the file backup.tar. After you download this file onto your PC, you can cross check it’s integrity by again re-generating MD5 hash for the downloaded file. If both the hash matches then it means that the file is perfect. Otherwise it means that the file is corrupt. To generate the MD5 hash for the downloaded file on your Windows PC use the following freeware tool

MD5 Summer (Click on the link to download)

Cheers and have fun! Please pass your valuable comments below regarding this topic…



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