HMAC signature API: SHA-256 webhook signature check

$0.01 per call · failed calls are never charged · no account or subscription: pay per call with a card key or USDC

Compute an HMAC of a message with a key using SHA-256 (default), SHA-1, SHA-384 or SHA-512, in hex and base64. Use to check webhook signatures. The key is used only for this call and is not stored.

When to use it

Run it free (25 free calls) Buy credits by card (from $1) Example result (JSON)

Try it with your own inputs

The message
The key

Uses one of your 25 free calls. Nothing is charged, and a call that fails is not counted.

Inputs

NameTypeDescription
message (required)stringThe message
key (required)stringThe key
algorithmstringsha256 (default), sha1, sha384 or sha512

Request

Over HTTP, with a card key from /credits:

curl -H "Authorization: Bearer adt_…" "https://tools.funwords.eu/hmac?message=hello&key=secret"

Or one of your 25 free calls, no key:

curl "https://tools.funwords.eu/hmac?message=hello&key=secret&trial=1"

Without a key, the same URL answers 402 Payment Required with x402 and MPP terms, so an agent with a wallet pays per call in USDC (Base, Polygon, Arbitrum or Solana over x402; Base or Tempo over MPP) and retries.

Example result

{
  "algorithm": "sha256",
  "hex": "88aab3ede8d3adf94d26ab90d3bafd4a2083070c3bcce9c014ee04a443847c0b"
}

From an AI agent (MCP)

Add the MCP server to Claude, Cursor or any MCP client. This tool is hmac, and the first 25 successful calls are free:

https://tools.funwords.eu/mcp

Questions

Is my key stored?

No. The key is used only for this call and is not stored.

How do I pay?

Each successful call costs $0.01; failed calls are not charged. The first 25 calls are free. Then pay with card credits (from $1; the key goes in an Authorization header or, for clients that can only fetch a URL, as api_key= in the URL) or per call in USDC over x402 or MPP. No account is needed.

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