Specifically , Michael Luby and Charles Rackoff analyzed the Feistel block cipher construction , and proved that if the round function is a cryptographically secure pseudorandom function , with Ki used as the seed , then 3 rounds is sufficient to make the block cipher a pseudorandom permutation , while 4 rounds is sufficient to make it a `` strong '' pseudorandom permutation ( which means that it remains pseudorandom even to an adversary who gets oracle access to its inverse permutation ) . Specifically , Michael Luby and Charles Rackoff analyzed the Feistel cipher construction , and proved that if the round function is a cryptographically secure pseudorandom function , with Ki used as the seed , then 3 rounds is sufficient to make the block cipher a pseudorandom permutation , while 4 rounds is sufficient to make it a `` strong '' pseudorandom permutation ( which means that it remains pseudorandom even to an adversary who gets oracle access to its inverse permutation ) .