What is the name of a one way transformation of a string of characters into a usually shorter fixed-length value or key that represents the original string? Such a transformation cannot be reversed?
Correct Answer: A
A cryptographic hash function is a transformation that takes an input (or 'message')
and returns a fixed-size string, which is called the hash value (sometimes termed a message
digest, a digital fingerprint, a digest or a checksum).
The ideal hash function has three main properties - it is extremely easy to calculate a hash for any
given data, it is extremely difficult or almost impossible in a practical sense to calculate a text that
has a given hash, and it is extremely unlikely that two different messages, however close, will
have the same hash.
Functions with these properties are used as hash functions for a variety of purposes, both within
and outside cryptography. Practical applications include message integrity checks, digital
signatures, authentication, and various information security applications. A hash can also act as a
concise representation of the message or document from which it was computed, and allows easy
indexing of duplicate or unique data files.
In various standards and applications, the two most commonly used hash functions are MD5 and
SHA-1. In 2005, security flaws were identified in both of these, namely that a possible
mathematical weakness might exist, indicating that a stronger hash function would be desirable. In
2007 the National Institute of Standards and Technology announced a contest to design a hash
function which will be given the name SHA-3 and be the subject of a FIPS standard.
A hash function takes a string of any length as input and produces a fixed length string which acts
as a kind of "signature" for the data provided. In this way, a person knowing the hash is unable to
work out the original message, but someone knowing the original message can prove the hash is
created from that message, and none other. A cryptographic hash function should behave as
much as possible like a random function while still being deterministic and efficiently computable.
A cryptographic hash function is considered "insecure" from a cryptographic point of view, if either
of the following is computationally feasible:
finding a (previously unseen) message that matches a given digest
finding "collisions", wherein two different messages have the same message digest.
An attacker who can do either of these things might, for example, use them to substitute an
authorized message with an unauthorized one.
Ideally, it should not even be feasible to find two messages whose digests are substantially
similar; nor would one want an attacker to be able to learn anything useful about a message given
only its digest. Of course the attacker learns at least one piece of information, the digest itself,
which for instance gives the attacker the ability to recognise the same message should it occur
again.
REFERENCES:
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten
Domains of Computer Security, 2001, John Wiley & Sons, Pages 40-41.
also see:
http://en.wikipedia.org/wiki/Cryptographic_hash_function
