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{ }

Definition(s):

  In this Recommendation, the curly braces { } are used in the following three situations: (1) {x} is used to indicate that the inclusion of x is optional; for example, the notation “Input: w, {x}, y, and z” implies that the inclusion of x as an input is optional. (2) If both X and Y are binary strings, the notation of binary string “Y{||X}” implies that the concatenation of string X is optional. (3) {x1, x2, …, xk} indicates a set with elements x1, x2, …, xk.
Source(s):
NIST SP 800-56A Rev. 2 [Superseded]