MD5 : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
Go to : punctuation 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z (all)

MD5

a message-digest algorithm developed by RSA Laboratories used for creating unforgeable digital signatures. MD5 produces an 128-bit (16 byte) message digest. Most existing software applications that handle certificates only support MD5. It is also used in signing Netscape jar files. Its advantage is rapid computation. Its disadvantage is cryptographic weakness. Longer digests such as SHA-1, SHA-256, and SHA-512 are used for Sun jar signing. You compute an MD5 digest using a MessageDigest object created like this:

Note that MD5 involves no private or public key. The algorithm for computing it is completely public. It represents a summary of an entire file or message. Only that summary needs to be digitally signed/encrypted, not the entire file.

A digest looks like an byte[16] of apparently random bytes, though they are completely repeatable. You need to armour it if you want to make it printable for transporting in an email, for example. The MD5 digests that you see in the manifest of a Netscape digitally signed jar file are base64 encoded.

MD5 is a trap-door, one-way function. You can easily compute the MD5 digest of a document, but you can’t go backwards and compose a document with a given digest. MD5 is not an encryption method.

Learning More

Sun’s Javadoc on the MessageDigest/MD5 class : available:

on the web at java.Sun.com
in the JDK 1.6.0_07 or in JDK 1.5.0_16 on your local J: drive.

armouring
base64
certificate
digest
digital signatures
encryption
JCE
manifest
SHA-1

	Please email your feedback for publication, errors, omissions, broken/redirected link reports and suggestions to improve this page to Roedy Green :
	Canadian Mind Products
	mindprod.com IP:[65.110.21.43]
	Your face IP:[38.103.63.16]	The information on this page is for non-military use only.
	You are visitor number 31,415.	Military use includes use by defence contractors.
	You can get a fresh copy of this page from:	or possibly from your local J: drive (Java virtual drive/Mindprod website mirror)
	http://mindprod.com/jgloss/md5.html	J:\mindprod\jgloss\md5.html