Java IO.pdf - Nguyen Dang Binh

Java I/O
downloads the page, and compares it to a previously retrieved copy for changes. However, if
you need to do this for hundreds or thousands of web pages, the space to store the pages
becomes prohibitive. Email clients have similar needs. Many broken mail clients and mailing
list managers send multiple copies of the same message. A mail client should recognize when
multiple copies of the same message are being passed through the system and delete them. On
an ISP level, it might be possible to use this as a spam filter by comparing messages sent to
different customers.
All these tasks need a way to compare files at different times without storing the files
themselves. You can write a special kind of method called a hash function that reads an
indefinite number of sequential bytes and assigns a number to that sequence of bytes. This
number is called a hash code or digest. The size of the number depends on the hash function.
It is not necessarily the same size as any Java primitive data type like int or long. For
instance, digests calculated with the SHA algorithm are 20-byte numbers. You can store the
digest of the files, then compare the digests. The digests are generally much smaller than the
files themselves.
Hash functions are also used in digital signatures. To indicate that you actually authored a
document, you first calculate the hash function for the message, then encrypt the hash code
with your private key. To check your signature, the recipient of the message decrypts the hash
code with your public key and compares it to the hash function you calculated. If they match,
then only someone who knew your private key could have signed the message. Although you
could simply encrypt the entire message with your private key rather than a hash code, public
key algorithms are rather slow, and encrypting a 20-byte hash code is much faster than
encrypting even a short email message. In Java, digital signatures are implemented through
the java.security.Signature class. We won't talk much about that class in this book, but it
is dependent on the MessageDigest classes we will discuss.
10.1.1 Requirements for Hash Functions
Hash codes are calculated by hash functions, and there are better and worse hash functions.
Good hash functions (also called strong hash functions) make it extremely unlikely that two
different documents will share a hash value. Furthermore, hash functions used for
cryptography must also be one-way hash functions—that is, given a hash code, you should
not be able to create a document with that hash code. A strong one-way hash function must
meet several related criteria. Among these criteria are the following:
• Hash functions are deterministic. The same document always has the same hash code.
The hash code does not depend on the time it's calculated, a random number, or
anything other than the sequence of bytes in the document. Without this requirement,
the same document could have different hash codes at different times, thus indicating
that documents had changed, when in fact they hadn't.
• Hash codes should be uniformly distributed throughout the available range. Given any
sample of the documents you wish to track, all hash codes are equally likely. For
instance, given a 64-bit hash code, which might be interpreted as a long integer, it
would be an error if even numbers were substantially more likely than odd numbers.
• Hash codes should be extremely difficult to reverse engineer. Given a hash code, there
should be no means easier than brute force to produce a document that matches that
hash code. For instance, if I know the hash code is 9,423,456,789, I shouldn't be able
to then create a file that happens to have that exact hash code.
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