No Weak Keys As A Design Goal
The goal of having a 'flat' keyspace (i.e., all keys equally strong) is always a cipher design goal. As in the case of DES, sometimes a small number of weak keys is acceptable, provided that they are all identified or identifiable. An algorithm that has unknown weak keys does not inspire much trust.
The two main countermeasures against inadvertently using a weak key:
- Checking generated keys against a list of known weak keys, or building rejection of weak keys into the key scheduling.
- When the number of weak keys is known to be very small (in comparison to the size of the keyspace), generating a key uniformly at random ensures that the probability of it being weak is a (known) very small number.
A large number of weak keys is a serious flaw in any cipher design, since there will then be a (perhaps too) large chance that a randomly generated one will be a weak one, compromising the security of messages encrypted under it. It will also take longer to check randomly generated keys for weakness in such cases, which will tempt shortcuts in interest of 'efficiency'.
However, weak keys are much more often a problem where the adversary has some control over what keys are used, such as when a block cipher is used in a mode of operation intended to construct a secure cryptographic hash function (e.g. Davies-Meyer).
Read more about this topic: Weak Key
Famous quotes containing the words weak, keys, design and/or goal:
“It was a weak spot in any nation to have a large body of disaffected people within its confusion.”
—Zora Neale Hurston (1891–1960)
“Thou hast the keys of Paradise, oh just, subtle, and mighty opium!”
—Thomas De Quincey (1785–1859)
“I begin with a design for a hearse.
For Christ’s sake not black—
nor white either—and not polished!
Let it be weathered—like a farm wagon—”
—William Carlos Williams (1883–1963)
“Whenever you pray, make sure you do it at school assemblies and football games, like the demonstrative creatures who pray before large television audiences. That is the real goal of the thing. But do not, I urge you, pray all alone in your home where no one can see. That does not get you ratings.”
—Garry Wills (b. 1934)