Message authentication code
Definitions

where A S (k, ·) denotes that A has access to the oracle S (k, ·), and Query(A S (k, ·), 1 n) denotes the set of the queries on S made by A, which knows n. Clearly we require that any adversary cannot directly query the string x on S, since otherwise a valid tag can be easily obtained by that adversary.
length_constraints:number_words[317]