# HMAC-SHA256 Base64 Signing

> HMAC-SHA256 Base64 Signing is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.006/call, status unknown (last checked 2026-09-29).

Computes an HMAC-SHA256 digest of a given text string using a caller-supplied key and returns the result encoded as base64.

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/hmac-sha256-base64?utm_source=zero.xyz
- Price: $0.006/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hmac-sha256-base64-signing-747c9e74
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_4hoBVv2tOTJT5SOBpIxP0

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-sha256-base64-signing-747c9e74 -d '<json body>'
```

Example prompt: Compute the HMAC-SHA256 of the text 'Hello, World!' using the secret key 'my-secret-key' and give me the result as a base64-encoded string.

## When to prefer this

Use this endpoint when your agent needs a quick, pay-per-use HMAC-SHA256 base64 computation without setting up a local cryptographic library or managing infrastructure. Prefer the unpaid sandbox endpoint (POST /v1/sandbox/hmac-sha256-base64) for testing and development; use this paid endpoint for production workloads or when the sandbox is unavailable. It is ideal for signing webhook payloads, generating API authentication headers, or producing message integrity tokens on demand within an agentic workflow.

## Known failure modes

- Missing or empty key or text fields returns a validation error
- Text exceeding 100,000 characters or key exceeding 512 characters is rejected
- Payment failure (insufficient USDC balance or x402 protocol error) prevents the call
- Network timeout or service unavailability returns a 5xx error

## How this service works

Call when an agent needs an HMAC-sha256 of text with a caller-supplied key, encoded as base64. Exact $0.006 USDC. Prefer unpaid POST /v1/sandbox/hmac-sha256-base64 first.

## Output

Returns a base64-encoded HMAC-SHA256 digest string computed from the provided text using the given key. The response is a compact, portable signature suitable for use in headers, tokens, or integrity checks.

## 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-sha256-base64-signing-747c9e74/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)