Roger A. Grimes wrote:
Length is always more important than complexity because password
keyspace is expressed as Y^X, where Y is the number of possible
characters and X is the password length. Thus, any similar increase in X
has significantly more impact than to Y.
Roger,
That relies upon the assumption of all attackers performing attacks that
attempt all possible characters all the time. In most attempts to break
passwords, the attacker will remove the uncommonly used characters from
being attempted. Since users try and follow the bare minimum
requirements, not adding complexity requirements can have a detrimental
effect. Consider the following hypothetical situation:
An internal employee has sniffed hashes from a network (we will assume
there are no shortcuts/weaknesses in the algorithm). The internal
company policy only requires 8 character length passwords and nothing
more. Which will be broken first by the attacker who is only trying to
crack a hash with lowercase letters [a-z]?
A hash generated from a 10 character password that was created with
only lowercase [a-z].
or
A hash generated from a 8 character password that was created with
lowercase [a-z], uppercase [A-Z], numerical [0-9].
The likely combinations to guess are not only derived from the length of
the password but also from the minimum requirements instituted by the
password policy. Having password complexity requirements forces
attackers into using more possible combinations. I will not argue that
length or complexity is more important than the other because situations
can arise that expose the weakness of either. Both are required (and
complement each other) when instituting a sound password policy.
--Eoin
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