Valor medio
ponderado de la cantidad de información transmitida por un mensaje, no conocido
a priori, de una fuente (conjunto) de ellos.
En otras
palabras, la entropía de una fuente mide la incertidumbre que, a priori, tiene
un observador acerca de la aparición de un mensaje (no conocido previamente) de
dicha fuente.
Se mide en
bitios y, raramente, en Hartleys (en honor del matemético estadounidense R.V.
Hartley, uno de los primeros estudiosos de la teoría de la información).
[Ribagorda:1997]
Medida de la
impredictibilidad. Se dice que una contraseña tiene mayor entropía cuanto más
impredecible sea. Y viceverse, una
contraseña fácilmente predecible tiene poca entropía.
A measure of the amount of
uncertainty that an Attacker faces to determine the value of a secret. Entropy
is usually stated in bits. [NIST-SP800-63:2013]
A measure of the difficulty
that an Attacker has to guess the average password used in a system. In this
document, entropy is stated in bits. When a password has n-bits of guessing
entropy then an Attacker has as much difficulty guessing the average password
as in guessing an n-bit random quantity. The Attacker is assumed to know the
actual password frequency distribution. [NIST-SP800-63:2013]
A measure of the difficulty
that an Attacker has to guess the most commonly chosen password used in a
system. In this document, entropy is stated in bits. When a password has n-bits
of min-entropy then an Attacker requires as many trials to find a user with
that password as is needed to guess an n-bit random quantity. The Attacker is
assumed to know the most commonly used password(s). [NIST-SP800:63:2013]