Which of these statements are correct

$$ 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$).$

$$ 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$).$

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

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