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:
“A weak mind is the only defect out of our power to mend.”
—François, Duc De La Rochefoucauld (1613–1680)
“Thou hast the keys of Paradise, O just, subtle, and mighty opium!”
—Thomas De Quincey (1785–1859)
“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)
“An older child, one who possesses a conscience, will be troubled with self-reproaches and feelings of shame for his naughtiness, even if he is not discovered. But our two-year-olds and our three-year- olds experience guilt feelings only when they feel or anticipate disapproval from the outside. In doing this, they have taken the first steps toward the goal of conscience, but there is a long way ahead before the policeman outside becomes the policeman inside.”
—Selma H. Fraiberg (20th century)