AI agents call hmac_sha256 to retrieve information from TinyFn without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
| Parameter | Type | Required | Description |
|---|---|---|---|
key | string | Yes | Secret key |
message | string | Yes | Message to authenticate |
Parameters from the server's own tool schema.
HMAC-SHA256 is a pure cryptographic function that takes input (a message and a secret key) and produces a deterministic hash output. It performs no side effects, creates no persistent changes, executes no external operations, and does not retrieve sensitive data from storage. It is a read-like operation in that it computes and returns a derived value.
From the tool's definition Tool name: 'hmac_sha256'. Description: 'Generate HMAC-SHA256.' HMAC generation is a deterministic cryptographic hashing operation that produces output without modifying any state or data in the system.
Attacks that exploit this kind of access
Generate HMAC-SHA256. It is categorised as a Read tool in the TinyFn MCP Server, which means it retrieves data without modifying state.
hmac_sha256 accepts 2 parameters: key, message. Required: key, message. The full parameter table on this page comes from the server's own tool schema.
Register the TinyFn MCP server in PolicyLayer and add a rule for hmac_sha256: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches TinyFn. Nothing to install.
hmac_sha256 is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the hmac_sha256 rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for hmac_sha256. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
hmac_sha256 is provided by the TinyFn MCP server (https://api.tinyfn.io/mcp/all/). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
hmac_sha256 is one line of TinyFn's registry record.
The record carries the whole server: verified identity, auth posture, risk grade, every tool classified, recommended policy — re-checked continuously.
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