2.AdjectiveOf a relation R on a set S, such that xRy if and only if yRx for all members x and y of S (that is, if the relation holds between any element and a second, it also holds between the second and the first).
""Is a sibling of" is a symmetric relation."
3.AdjectiveUsing the same key (or keys that are trivially related) for both encryption and decryption.