Any message written over a fixed set of symbols can be represented as a binary string (a sequence of 0's and 1's) Binary digits 0 and 1 are called bits To reduce computation overhead, encryption algorithms should only use operations that are easy to implement For a binary string X: The length of X, denoted by |X|, is the number of bits in X If |X| = l, X is an l-bit binary string Let a be a binary bit and k a non-negative integer. Denote by ak a binary string consisting of k copies of a Denote the concatenation of X and Y by XY or.
