Jiasi Weng, Jian Weng, Minrong Chen et al.
The rapid adoption of artificial intelligence (AI)-driven and agentic workflows is transforming traditional government and enterprise systems into language-based, tool-using and increasingly autonomous infrastructures. Conventional key management services authenticate who may invoke a cryptographic primitive, but remain agnostic to which workflow steps are authorized at runtime: an authenticated agent can still be hijacked by direct or indirect prompt injection into executing malicious actions that satisfy identity checks yet violate user intent. We present \emph{Neural Cryptographic Services} (NCS), an active security governance plane grounded in a neural-symbolic design and interposed between LLM agents and privileged tools. Under NCS, an untrusted neural planner compiles natural-language instructions into structured plan drafts but holds no execution authority. Execution is instead gated by a deterministic symbolic controller operating over an offline-signed, hash-chained instruction stream. NCS verifies the signature, incrementally validates the hash chain, releases exactly one instruction payload at a time, and enforces strict binding between the agent's proposed tool parameters and the verified payload. Mismatched or out-of-order tool calls are rejected fail-closed, while previously verified states are retained for post-hoc auditing. We evaluate NCS on AgentDojo and a custom argument-hijacking benchmark. NCS drives attack success rates to near zero while preserving acceptable utility on benign workflows. NCS thus reframes agent security from asking whether a model's intent complied to asking whether a proposed dispatch matches a cryptographically authorized step.