  The secure summation problem is considered, where $K$ users, each holds an
input, wish to compute the sum of their inputs at a server securely, i.e.,
without revealing any information beyond the sum even if the server may collude
with any set of up to $T$ users. First, we prove a folklore result for secure
summation - to compute $1$ bit of the sum securely, each user needs to send at
least $1$ bit to the server, each user needs to hold a key of at least $1$ bit,
and all users need to hold collectively some key variables of at least $K-1$
bits. Next, we focus on the symmetric groupwise key setting, where every group
of $G$ users share an independent key. We show that for symmetric groupwise
keys with group size $G$, when $G > K-T$, the secure summation problem is not
feasible; when $G \leq K-T$, to compute $1$ bit of the sum securely, each user
needs to send at least $1$ bit to the server and the size of each groupwise key
is at least $(K-T-1)/\binom{K-T}{G}$ bits. Finally, we relax the symmetry
assumption on the groupwise keys and the colluding user sets; we allow any
arbitrary group of users to share an independent key and any arbitrary group of
users to collude with the server. For such a general groupwise key and
colluding user setting, we show that secure summation is feasible if and only
if the hypergraph, where each node is a user and each edge is a group of users
sharing the same key, is connected after removing the nodes corresponding to
any colluding set of users and their incident edges.
