Message Authentication in Multi-Agent Systems

In multi-agent systems, agents exchange messages to coordinate tasks. Without authentication, any component in the system can forge messages, impersonate other agents, or tamper with message content in transit. Message authentication ensures that the stated sender is the actual sender and that the content has not been modified.

HMAC-Based Authentication

The simplest approach uses HMAC (Hash-based Message Authentication Code). Each agent pair shares a secret key. The sending agent computes an HMAC over the message payload and attaches it as a header. The receiving agent recomputes the HMAC and rejects the message if the values do not match.

This approach works well for small agent networks but has scaling challenges. With N agents, you need N*(N-1)/2 shared keys. Key rotation becomes operationally expensive.

Signed Envelopes

A more scalable approach uses asymmetric cryptography. Each agent has a key pair. Messages are signed with the sender's private key. Any receiving agent can verify the signature using the sender's public key, which is distributed through a trusted registry.

Authensor's action envelope format supports signed envelopes natively. The principal field identifies the signing agent, and the signature covers the entire envelope including the action, parameters, and timestamp.

Replay Protection

Authentication alone does not prevent replay attacks. An attacker who captures a valid signed message can resend it later. Include a timestamp and a nonce in every message. Receiving agents should reject messages with timestamps outside an acceptable window and track seen nonces to detect duplicates.

Integration with Policy

Authensor's policy engine can require message authentication as a precondition for action evaluation. If the envelope signature is invalid or missing, the engine rejects the action before policy rules are even consulted. This fail-closed approach ensures that unauthenticated messages never reach the decision layer.

Authenticate every message, verify every signature, and reject anything that does not pass validation. In multi-agent systems, trust is never assumed.