We present a multi-input functional encryption scheme (MIFE) for the inner product functionality based on the k-Linear assumption in prime-order bilinear groups. Our construction works for any polynomial number of encryption slots and achieves security against unbounded collusion, while relying on standard polynomial hardness assumptions. This is the first MIFE scheme for a non-trivial functionality based on standard cryptographic assumptions, as well as the first to achieve polynomial security loss for a super-constant number of slots under falsifiable assumptions. Prior works required stronger non-standard assumptions such as indistinguishability obfuscation or multi-linear maps.