nExhaustive search of
keyspace must be infeasible
nMore about this
later…
nIt must also be
infeasible to find the key given:
nSample ciphertext
and corresponding plaintext (“known-plaintext
attack”)
nThe ability to feed
ciphertext in and see what plaintext comes out
(“chosen-ciphertext attack”)
nor
the other way around (“chosen-plaintext attack”)
nIf someone can find
keys under any of these conditions,
the cipher isn’t considered secure