1. Symmetrical.
  2. Of 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. Using the same key (or keys that are trivially related) for both encryption and decryption.

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