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).
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Famous quotes containing the words weak, keys, design and/or goal:
“The weak and misbegotten shall perish: first principle of our brotherly love. And they shall be given every assistance.”
—Friedrich Nietzsche (1844–1900)
“McCoy: That shark’s been following us ever since the surgeon died, waiting for the burial. Couldn’t I have a musket to shoot it, sir?
Fletcher Christian: Take the deck, McCoy. I’ll get the keys to the arms chest.
McCoy: Get two muskets, sir. I’d like to shoot that shark on board.”
—Talbot Jennings (1896–1985)
“Nowadays the host does not admit you to his hearth, but has got the mason to build one for yourself somewhere in his alley, and hospitality is the art of keeping you at the greatest distance. There is as much secrecy about the cooking as if he had a design to poison you.”
—Henry David Thoreau (1817–1862)
“To enumerate the different trades by which the women in New York are endeavoring—not to live—that for many of them is as utterly unattainable a goal as the end of the rainbow—but simply to postpone as long as possible their appearance at the morgue or the cemetery—to attempt to do this would be useless.”
—Katharine Pearson Woods (1853–1923)