It's lambda encoded as a unary vector, i.e. a vector (1, ..., 1) of lambda ones. It means KeyGen is parametrized by a natural number lambda, like KeyGen(lambda)

It's lambda encoded as a unary vector, i.e. a vector (1, ..., 1) of lambda ones. It means KeyGen is parametrized by a natural number lambda, like KeyGen(lambda). The notion is from computer science or complexity theory I think. The reason it's written like this is because the complexity of functions is described as a function of the length of the inputs, rather than the input itself.