# HMAC-SHA3-512 Base64url Signing

> HMAC-SHA3-512 Base64url Signing is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.008/call, status unknown (last checked 2026-10-02).

Computes an HMAC-SHA3-512 digest of a given text using a caller-supplied key and returns the result encoded as base64url.

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/hmac-sha3-512-base64url?utm_source=zero.xyz
- Price: $0.008/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-02
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hmac-sha3-512-base64url-signing-2c102223
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_khs8GVpRkMqt2oWJBR5Sb

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-sha3-512-base64url-signing-2c102223 -d '<json body>'
```

Example prompt: Can you compute an HMAC-SHA3-512 of the text 'order:12345:confirmed' using the key 'my-secret-key-abc' and give me the result as base64url?

## When to prefer this

Choose this endpoint when you specifically need SHA3-512 as the hash function for HMAC (rather than SHA2-256 or SHA2-512), and want the output pre-encoded as base64url without additional transformation. Prefer the free sandbox variant POST /v1/sandbox/hmac-sha3-512-base64url for testing before using this paid endpoint.

## Known failure modes

- Missing or empty 'key' field returns a validation error
- Missing or empty 'text' field returns a validation error
- Text exceeding 100,000 characters is rejected
- Key exceeding 512 characters is rejected
- Payment of $0.008 USDC not provided results in HTTP 402 Payment Required

## How this service works

Call when an agent needs an HMAC-sha3-512 of text with a caller-supplied key, encoded as base64url. Exact $0.008 USDC. Prefer unpaid POST /v1/sandbox/hmac-sha3-512-base64url first.

## Output

Returns a base64url-encoded string representing the HMAC-SHA3-512 digest of the input text computed with the provided key. The output is URL-safe and padding-free, ready to embed in headers, query strings, or JSON payloads.

## 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-sha3-512-base64url-signing-2c102223/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)