# HMAC-SHA3-256 Base64url Signing Service

> HMAC-SHA3-256 Base64url Signing Service is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.006/call, status unknown (last checked 2026-10-02).

Computes an HMAC-SHA3-256 message authentication code for a given text using a caller-supplied key, returning the result encoded as base64url.

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/hmac-sha3-256-base64url?utm_source=zero.xyz
- Price: $0.006/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-02
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hmac-sha3-256-base64url-signing-service-10013e3b
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_Wo3jMgoe3MRAv9LaynJks

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability hmac-sha3-256-base64url-signing-service-10013e3b -d '<json body>'
```

Example prompt: Compute the HMAC-SHA3-256 of the text 'Hello, World!' using the secret key 'mysecretkey123' and give me the result as a base64url string.

## When to prefer this

Choose this endpoint when you specifically need HMAC-SHA3-256 (not HMAC-SHA256 or HMAC-SHA1) with base64url output encoding, for use cases like URL-safe tokens, webhook signing with modern hash functions, or when a downstream system explicitly requires SHA3-256. Use the free sandbox endpoint POST /v1/sandbox/hmac-sha3-256-base64url for testing before paying $0.006 USDC per call. Prefer this over SHA2-based HMAC endpoints when SHA3 is mandated by your security policy or API contract.

## Known failure modes

- Empty or missing key field returns a validation error
- Empty or missing text field returns a validation error
- Text exceeding 100,000 characters is rejected
- Key exceeding 512 characters is rejected
- Payment failure or insufficient USDC balance returns a 402 error
- Malformed request body returns a 400 error

## How this service works

Call when an agent needs an HMAC-sha3-256 of text with a caller-supplied key, encoded as base64url. Exact $0.006 USDC. Prefer unpaid POST /v1/sandbox/hmac-sha3-256-base64url first.

## Output

Returns a base64url-encoded HMAC-SHA3-256 digest of the provided text, computed using the caller-supplied key. The output is a compact, URL-safe string suitable for embedding in headers, query parameters, or tokens.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "key": {
   "type": "string",
   "examples": [
    "secret"
   ],
   "maxLength": 512,
   "minLength": 1,
   "description": "HMAC secret. Required on HMAC SKUs. Omit it for plain digests and checksums. The key is not stored."
  },
  "text": {
   "type": "string",
   "examples": [
    "hello"
   ],
   "maxLength": 100000,
   "minLength": 1,
   "description": "Message to digest. For raw-hex SKUs this is hex bytes; for utf16 or latin1 SKUs it is decoded with that encoding. Otherwise it is UTF-8 text."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/hmac-sha3-256-base64url-signing-service-10013e3b/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agentshelf.syntexa.ch](https://www.zero.xyz/host/agentshelf.syntexa.ch/llms.txt)