What is the MD5 hash?
The MD5 hash also known as checksum for a file is a 128-bit value,
something like a fingerprint of the file. There is a very small possibility of
getting two identical hashes of two different files. This feature can be useful
both for comparing the files and their integrity control.
Let us imagine a situation that will help to understand how the MD5 hash
works.
Alice and Bob have two similar huge files. How do we know that they are
different without sending them to each other? We simply have to calculate the
MD5 hashes of these files and compare them.
MD5 Hash Properties
The MD5 hash consists of a small amount of binary data, typically no
more than 128 bits. All hash values share the following properties:
Hash length
The length of the hash value is determined by the type of the used
algorithm, and its length does not depend on the size of the file. The most
common hash value lengths are either 128 or 160 bits.
Non-discoverability
Every pair of nonidentical files will translate into a completely
different hash value, even if the two files differ only by a single bit. Using
today's technology, it is not possible to discover a pair of files that
translate to the same hash value.
Repeatability
Each time a particular file is hashed using the same algorithm, the exact
same hash value will be produced.
Irreversibility
All hashing algorithms are one-way. Given a checksum value, it is
infeasible to discover the password. In fact, none of the properties of the
original message can be determined given the checksum value alone.
The algorithm was
invented by:
Professor Ronald L. Rivest (born 1947, Schenectady, New York) is a
cryptographer, and is the Viterbi Professor of Computer Science at MIT's
Department of Electrical Engineering and Computer Science. He is most celebrated
for his work on public-key encryption with Len Adleman and Adi
Shamir, specifically the RSA algorithm, for which they won the
2002 ACM Turing Award.
Simple
Code for Android:
public static final String
md5Digest(final String text)
{
try
{
//
Create MD5 Hash
MessageDigest
digest = java.security.MessageDigest.getInstance("MD5");
digest.update(text.getBytes());
byte messageDigest[] = digest.digest();
//
Create Hex String
StringBuffer
hexString = new StringBuffer();
int messageDigestLenght = messageDigest.length;
for (int i = 0;
i < messageDigestLenght; i++)
{
String
hashedData = Integer.toHexString(0xFF & messageDigest[i]);
while (hashedData.length() < 2)
hashedData
= "0" + hashedData;
hexString.append(hashedData);
}
return hexString.toString();
}
catch (NoSuchAlgorithmException e)
{
e.printStackTrace();
}
return ""; // if text is null then return nothing
}
