Which of these statements are correct

A)$)$ The Kerckhoffs's principle states that cryptosystems should be secure even when only the decryption key is kept secret.

B)$)$ In cryptography using finite sets is not very common.

C)$)$ A group is a set of elements G together with an operation which combines two elements of G,$,$ having following properties: the group operation is closed and associative, there is a neutral element in the group and there is an inverse element in the group ($($a group is abelian or commutative if for all elements of the group the operation combining two elements does not depend on the order of elements).$).$

D)$)$ A stream cipher using a truly random key binary sequence is the unbreakable One−$-$Time Pad cipher ($($under a condition that each key stream bit is used only once, i.$.$e.$.$ the key binary sequence is as long as the message binary sequence and is used only once).$).$