arXiv:2610.40928v1 Announce Type: cross Abstract: The omission of source IP authentication remains a fundamental vulnerability in the Internet. Existing approaches rely on routing plausibility, topology assumptions, per-flow state, or wide deployment. These requirements have kept strong source authentication out of the data plane. As simplicity is the key to deployability, we present LiHA, a lightweight source authentication mechanism built around operations that routers already perform efficiently. LiHA binds secret keys to source prefixes using longest prefix match and authenticates each packet with a single message authentication code (MAC). The data plane requires no handshakes, per-flow state, or per-packet control plane access. A sender performs one prefix lookup and one fixed-size hash. The verifier repeats the same operations and forwards the packet only if the MAC matches. LiHA is designed for incremental deployment. It primarily protects against off-path attackers that spoof source prefixes they do not control. An on-path attacker can replay valid authenticated packets until the key rotates. LiHA does not eliminate every denial-of-service threat. Instead, it confines accepted traffic to authenticated source prefix space within participating networks. We argue that simpler source authentication with clear limits is more likely to be deployed. We implemented LiHA in three environments: as a set of user-space libraries, as eBPF programs, and in an FPGA. We found that the user-space implementation had very high overheads. The eBPF implementation got reasonable single thread performance even with a software based cryptographic hash. We found that only the FPGA could obtain 100 Gbps single thread performance, but that was only when bypassing the kernel using a DPDK user-level networking stack.