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Agent Payload Tools – HMAC Signature
x402
Agent Payload Tools – HMAC Signature is a paid API for AI agents from api.agent-payload-tools.workers.dev, paid per call via x402, $0.001/call, status unknown (last checked 2026-09-15).
Computes an HMAC digest (SHA-256/384/512) for a UTF-8 message using a caller-supplied secret, with optional constant-time verification against an expected hex digest.
Price / call$0.001
Success—
StatusUnknown
How this service works
Create or compare an HMAC using SHA-256, SHA-384, or SHA-512.
Output
Returns the HMAC hex digest of the input message computed with the specified algorithm and secret. If an expected_hex was provided, also returns a boolean indicating whether the computed digest matches in constant time (resistant to timing attacks). The secret is not stored or logged.
Example use cases
Webhook payload integrity check
“I received a webhook from Stripe with body 'event_id=evt_123&type=payment' and their signature header. Can you compute an HMAC-SHA-256 using my secret 'whsec_abc123' and verify it matches the hex digest 'deadbeef1234' in constant time?”
API request signing for outbound calls
“Before I send this API request, sign the payload 'user_id=42&action=transfer&amount=500' with my secret key 'svc-signing-key-prod' using SHA-512 HMAC so I can include the digest in the Authorization header.”
Data integrity stamp for audit logs
“I'm writing audit log entries and need to prove they haven't been tampered with — compute an HMAC-SHA-384 over the string 'timestamp=2024-01-15T10:30:00Z&action=delete&actor=admin' using secret 'audit-log-secret'.”
Technical details
Endpoint
POSThttps://api.agent-payload-tools.workers.dev/hmacWhen to prefer
Choose this endpoint when an AI agent needs a deterministic, stateless HMAC computation without managing cryptographic libraries — especially useful for webhook verification, API request signing, or integrity checks inside an automated workflow. The constant-time comparison option makes it suitable for security-sensitive verification where timing attacks are a concern. Prefer over general-purpose code execution for simplicity and auditability.
Side effects
- Missing or empty secret returns a 400/422 validation error
- Invalid algorithm value (not SHA-256, SHA-384, or SHA-512) returns a validation error
- Malformed expected_hex (non-hex characters) returns a pattern validation error
- Empty input is allowed (computes HMAC of empty string) but may produce unexpected results
- Payment failure or insufficient USDC balance returns a 402 response before processing
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| input | string | UTF-8 message to authenticate. |
| secret | string | HMAC secret for this call. It is not stored. |
| algorithm | string | Digest algorithm. Defaults to SHA-256. |
| expected_hex | string | Optional hexadecimal digest to compare in constant time. |
Tags
x402
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