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 strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It’s a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity.”
—Jimmy Carter (James Earl Carter, Jr.)
“And yet we constantly reclaim some part of that primal spontaneity through the youngest among us, not only through their sorrow and anger but simply through everyday discoveries, life unwrapped. To see a child touch the piano keys for the first time, to watch a small body slice through the surface of the water in a clean dive, is to experience the shock, not of the new, but of the familiar revisited as though it were strange and wonderful.”
—Anna Quindlen (b. 1952)
“Joe ... you remember I said you wouldn’t be cheated?... Nobody is really. Eventually all things work out. There’s a design in everything.”
—Sidney Buchman (1902–1975)
“[17th-century] Puritans were the first modern parents. Like many of us, they looked on their treatment of children as a test of their own self-control. Their goal was not to simply to ensure the child’s duty to the family, but to help him or her make personal, individual commitments. They were the first authors to state that children must obey God rather than parents, in case of a clear conflict.”
—C. John Sommerville (20th century)