# HMAC-SHA3-256 Base64 Signing

> HMAC-SHA3-256 Base64 Signing is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-29).

Computes an HMAC-SHA3-256 message authentication code for a given text using a caller-supplied key and returns the result encoded as base64.

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/hmac-sha3-256-base64?utm_source=zero.xyz
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hmac-sha3-256-base64-signing-97547599
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_2woLViOpTb76stzIyJIKp

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-base64-signing-97547599 -d '<json body>'
```

Example prompt: Compute an HMAC-SHA3-256 of the text 'order-id:98765' using the key 'mysecretkey2024' and give me the result as base64.

## When to prefer this

Choose this endpoint when you need HMAC-SHA3-256 specifically (not SHA-256, SHA-384, or other HMAC variants) and require the output as standard base64 encoding. Prefer it over the sandbox endpoint when the computation must be performed in a production billing context. Use the sibling SHA-384/base64url endpoint if you need the SHA-384 algorithm or base64url encoding instead of base64.

## Known failure modes

- Empty or missing key (minLength:1 violated) returns a validation error
- Empty or missing text (minLength:1 violated) returns a validation error
- Text exceeding 100,000 characters returns a payload-too-large error
- Key exceeding 512 characters returns a validation error
- Payment of exactly $0.005 USDC required; insufficient funds returns 402
- Network timeout on the paid endpoint — retry with the free sandbox endpoint at /v1/sandbox/hmac-sha3-256-base64

## How this service works

Call when an agent needs an HMAC-sha3-256 of text with a caller-supplied key, encoded as base64. Exact $0.005 USDC. Prefer unpaid POST /v1/sandbox/hmac-sha3-256-base64 first.

## Output

Returns a base64-encoded HMAC-SHA3-256 digest string computed from the provided text and key. The output is a standard base64 string representing the 256-bit message authentication code.

## 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-base64-signing-97547599/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)