Facilitator Authentication

Overview

The PayAI facilitator authenticates merchants using short-lived JWTs signed with Ed25519 (the EdDSA algorithm). If you use the @payai/facilitator TypeScript package, this is handled automatically. This guide explains the underlying protocol so you can implement authentication in any language without depending on PayAI packages.

Authentication is only required beyond the free tier (1,000 settlements/month). Create a merchant account at merchant.payai.network to get your API keys.

API key structure

Your API key has two parts, available from the merchant dashboard:

Part	Environment Variable	Description
API Key ID	`PAYAI_API_KEY_ID`	A string identifier for your key
API Key Secret	`PAYAI_API_KEY_SECRET`	An Ed25519 private key in PKCS#8/DER format, base64-encoded

The secret may be prefixed with payai_sk_ as shown in the dashboard. Strip this prefix before use — the remaining string is a standard base64-encoded PKCS#8 DER key.

Protocol steps

1. Normalize the API key secret

If the secret starts with payai_sk_, remove that prefix. The result is a base64-encoded Ed25519 private key in PKCS#8/DER format.

2. Build the JWT header

{
  "alg": "EdDSA",
  "typ": "JWT",
  "kid": "<your_api_key_id>"
}

3. Build the JWT payload

{
  "sub": "<your_api_key_id>",
  "iss": "payai-merchant",
  "iat": 1709700000,
  "exp": 1709700120,
  "jti": "550e8400-e29b-41d4-a716-446655440000"
}

Claim	Description
`sub`	Your API Key ID
`iss`	Always `"payai-merchant"`
`iat`	Current time as a Unix timestamp (seconds)
`exp`	Expiration — `iat + 120` (2 minutes recommended)
`jti`	A random UUID v4 for replay protection

4. Encode and sign

Base64url-encode the header JSON -> headerB64
Base64url-encode the payload JSON -> payloadB64
Form the signing input: headerB64 + "." + payloadB64
Sign the UTF-8 bytes of the signing input with your Ed25519 private key
Base64url-encode the 64-byte signature -> signatureB64
The JWT is: headerB64.payloadB64.signatureB64

Base64url encoding uses the standard Base64 alphabet with + replaced by -, / replaced by _, and no = padding.

5. Send the request

Include the JWT as a Bearer token on all facilitator requests:

Authorization: Bearer <jwt>

This header is required on all authenticated facilitator endpoints: POST /verify, POST /settle, and GET /supported.

Token caching

JWTs are valid for the full exp - iat window (default: 120 seconds). To avoid signing on every request, cache the token and refresh it ~30 seconds before expiry.

Code examples

The following examples implement the full authentication flow with no PayAI-specific dependencies.

TypeScript
Python
Go
Rust

Uses the Web Crypto API — works in Node.js 18+, Deno, Bun, and browsers with zero dependencies.

// payai-auth.ts — zero dependencies, Web Crypto API only

function base64UrlEncode(data: Uint8Array): string {
  let binary = "";
  for (let i = 0; i < data.byteLength; i++) {
    binary += String.fromCharCode(data[i]);
  }
  return btoa(binary)
    .replace(/\+/g, "-")
    .replace(/\//g, "_")
    .replace(/=+$/, "");
}

function base64UrlEncodeString(str: string): string {
  return base64UrlEncode(new TextEncoder().encode(str));
}

function base64ToUint8Array(base64: string): Uint8Array {
  const standardized = base64.replace(/-/g, "+").replace(/_/g, "/");
  const binary = atob(standardized);
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < binary.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }
  return bytes;
}

function normalizeSecret(secret: string): string {
  const trimmed = secret.trim();
  return trimmed.startsWith("payai_sk_")
    ? trimmed.slice("payai_sk_".length)
    : trimmed;
}

export async function generatePayAIJwt(
  apiKeyId: string,
  apiKeySecret: string,
): Promise<string> {
  const now = Math.floor(Date.now() / 1000);

  const header = JSON.stringify({
    alg: "EdDSA",
    typ: "JWT",
    kid: apiKeyId,
  });
  const payload = JSON.stringify({
    sub: apiKeyId,
    iss: "payai-merchant",
    iat: now,
    exp: now + 120,
    jti: crypto.randomUUID(),
  });

  const headerB64 = base64UrlEncodeString(header);
  const payloadB64 = base64UrlEncodeString(payload);
  const message = `${headerB64}.${payloadB64}`;

  const keyBytes = base64ToUint8Array(normalizeSecret(apiKeySecret));
  const privateKey = await crypto.subtle.importKey(
    "pkcs8",
    keyBytes.buffer,
    { name: "Ed25519" },
    false,
    ["sign"],
  );

  const signature = await crypto.subtle.sign(
    "Ed25519",
    privateKey,
    new TextEncoder().encode(message),
  );

  return `${message}.${base64UrlEncode(new Uint8Array(signature))}`;
}

// --- Usage ---

const jwt = await generatePayAIJwt(
  "your-api-key-id",
  "payai_sk_your-api-key-secret",
);

const response = await fetch(
  "https://facilitator.payai.network/verify",
  {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      Authorization: `Bearer ${jwt}`,
    },
    body: JSON.stringify({
      x402Version: 2,
      paymentPayload: {
        /* ... */
      },
      paymentRequirements: {
        /* ... */
      },
    }),
  },
);

Requires the cryptography package:

pip install cryptography

# payai_auth.py
import base64
import json
import time
import uuid

from cryptography.hazmat.primitives.serialization import load_der_private_key


def base64url_encode(data: bytes) -> str:
    return base64.urlsafe_b64encode(data).rstrip(b"=").decode("ascii")


def normalize_secret(secret: str) -> str:
    s = secret.strip()
    return s.removeprefix("payai_sk_")


def generate_payai_jwt(api_key_id: str, api_key_secret: str) -> str:
    now = int(time.time())

    header = json.dumps(
        {"alg": "EdDSA", "typ": "JWT", "kid": api_key_id},
        separators=(",", ":"),
    )
    payload = json.dumps(
        {
            "sub": api_key_id,
            "iss": "payai-merchant",
            "iat": now,
            "exp": now + 120,
            "jti": str(uuid.uuid4()),
        },
        separators=(",", ":"),
    )

    header_b64 = base64url_encode(header.encode())
    payload_b64 = base64url_encode(payload.encode())
    message = f"{header_b64}.{payload_b64}"

    key_bytes = base64.b64decode(normalize_secret(api_key_secret))
    private_key = load_der_private_key(key_bytes, password=None)
    signature = private_key.sign(message.encode())

    return f"{message}.{base64url_encode(signature)}"


# --- Usage ---

if __name__ == "__main__":
    import requests

    jwt = generate_payai_jwt(
        "your-api-key-id",
        "payai_sk_your-api-key-secret",
    )

    response = requests.post(
        "https://facilitator.payai.network/verify",
        headers={
            "Content-Type": "application/json",
            "Authorization": f"Bearer {jwt}",
        },
        json={
            "x402Version": 2,
            "paymentPayload": {},   # ...
            "paymentRequirements": {},  # ...
        },
    )
    print(response.json())

Uses the Go standard library plus github.com/google/uuid for UUID generation.

package payaiauth

import (
	"crypto/ed25519"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"strings"
	"time"

	"github.com/google/uuid"
)

func base64URLEncode(data []byte) string {
	return strings.TrimRight(
		base64.URLEncoding.EncodeToString(data), "=",
	)
}

func normalizeSecret(secret string) string {
	s := strings.TrimSpace(secret)
	return strings.TrimPrefix(s, "payai_sk_")
}

func GeneratePayAIJwt(apiKeyID, apiKeySecret string) (string, error) {
	now := time.Now().Unix()

	header, _ := json.Marshal(map[string]string{
		"alg": "EdDSA",
		"typ": "JWT",
		"kid": apiKeyID,
	})
	payload, _ := json.Marshal(map[string]interface{}{
		"sub": apiKeyID,
		"iss": "payai-merchant",
		"iat": now,
		"exp": now + 120,
		"jti": uuid.New().String(),
	})

	headerB64 := base64URLEncode(header)
	payloadB64 := base64URLEncode(payload)
	message := headerB64 + "." + payloadB64

	keyBytes, err := base64.StdEncoding.DecodeString(
		normalizeSecret(apiKeySecret),
	)
	if err != nil {
		keyBytes, err = base64.URLEncoding.DecodeString(
			normalizeSecret(apiKeySecret),
		)
		if err != nil {
			return "", fmt.Errorf("decode API key secret: %w", err)
		}
	}

	parsed, err := x509.ParsePKCS8PrivateKey(keyBytes)
	if err != nil {
		return "", fmt.Errorf("parse PKCS#8 key: %w", err)
	}

	privateKey, ok := parsed.(ed25519.PrivateKey)
	if !ok {
		return "", fmt.Errorf("key is not Ed25519")
	}

	signature := ed25519.Sign(privateKey, []byte(message))
	return message + "." + base64URLEncode(signature), nil
}

Usage:

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"net/http"

	payaiauth "your-module/payaiauth"
)

func main() {
	jwt, err := payaiauth.GeneratePayAIJwt(
		"your-api-key-id",
		"payai_sk_your-api-key-secret",
	)
	if err != nil {
		panic(err)
	}

	body, _ := json.Marshal(map[string]interface{}{
		"x402Version":         2,
		"paymentPayload":      map[string]interface{}{},
		"paymentRequirements": map[string]interface{}{},
	})

	req, _ := http.NewRequest(
		"POST",
		"https://facilitator.payai.network/verify",
		bytes.NewReader(body),
	)
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Authorization", "Bearer "+jwt)

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	fmt.Println("Status:", resp.Status)
}

You can replace github.com/google/uuid with crypto/rand to generate a UUID v4 manually and avoid the external dependency entirely.

Add to Cargo.toml:

[dependencies]
ed25519-dalek = { version = "2", features = ["pkcs8"] }
base64 = "0.22"
serde_json = "1"
uuid = { version = "1", features = ["v4"] }

use base64::engine::general_purpose::{STANDARD, URL_SAFE_NO_PAD};
use base64::Engine;
use ed25519_dalek::pkcs8::DecodePrivateKey;
use ed25519_dalek::{Signer, SigningKey};
use serde_json::json;
use std::time::{SystemTime, UNIX_EPOCH};
use uuid::Uuid;

fn normalize_secret(secret: &str) -> &str {
    let s = secret.trim();
    s.strip_prefix("payai_sk_").unwrap_or(s)
}

pub fn generate_payai_jwt(
    api_key_id: &str,
    api_key_secret: &str,
) -> Result<String, Box<dyn std::error::Error>> {
    let now = SystemTime::now()
        .duration_since(UNIX_EPOCH)?
        .as_secs();

    let header = json!({
        "alg": "EdDSA",
        "typ": "JWT",
        "kid": api_key_id,
    });
    let payload = json!({
        "sub": api_key_id,
        "iss": "payai-merchant",
        "iat": now,
        "exp": now + 120,
        "jti": Uuid::new_v4().to_string(),
    });

    let header_b64 = URL_SAFE_NO_PAD.encode(
        header.to_string().as_bytes(),
    );
    let payload_b64 = URL_SAFE_NO_PAD.encode(
        payload.to_string().as_bytes(),
    );
    let message = format!("{header_b64}.{payload_b64}");

    let key_bytes = STANDARD.decode(
        normalize_secret(api_key_secret),
    )?;
    let signing_key = SigningKey::from_pkcs8_der(&key_bytes)?;
    let signature = signing_key.sign(message.as_bytes());

    Ok(format!(
        "{message}.{}",
        URL_SAFE_NO_PAD.encode(signature.to_bytes()),
    ))
}

// Usage:
// let jwt = generate_payai_jwt("your-api-key-id", "payai_sk_...")?;
// Set header: Authorization: Bearer {jwt}

Facilitator endpoints

All endpoints are at https://facilitator.payai.network and require the Authorization: Bearer <jwt> header when authenticated.

Endpoint	Method	Description
`/verify`	POST	Verify a payment payload against payment requirements
`/settle`	POST	Settle a verified payment on-chain
`/supported`	GET	List supported networks, tokens, and schemes

Request body (`/verify` and `/settle`)

{
  "x402Version": 2,
  "paymentPayload": { "..." },
  "paymentRequirements": { "..." }
}

For full payload schemas, see the x402 Reference.

Troubleshooting

Error	Cause	Fix
`401 Unauthorized`	Missing or expired JWT	Generate a fresh token (check `exp` claim)
Failed to parse key	Secret not valid PKCS#8/DER	Ensure you stripped the `payai_sk_` prefix and the remaining string is valid base64
Signature verification failed	Wrong key or corrupted secret	Verify your API Key ID and Secret match the merchant dashboard

Need help?

Join our Community

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PayAI

x402

x402 Merchants

x402 Clients

x402 Facilitator

x402 Reference

X402 Echo Merchant

Project Info

Overview

API key structure

Protocol steps

1. Normalize the API key secret

2. Build the JWT header

3. Build the JWT payload

4. Encode and sign

5. Send the request

Token caching

Code examples

Facilitator endpoints

Request body (`/verify` and `/settle`)

Troubleshooting

Need help?

Join our Community

PayAI

x402

x402 Merchants

x402 Clients

x402 Facilitator

x402 Reference

X402 Echo Merchant

Project Info

​Overview

​API key structure

​Protocol steps

​1. Normalize the API key secret

​2. Build the JWT header

​3. Build the JWT payload

​4. Encode and sign

​5. Send the request

​Token caching

​Code examples

​Facilitator endpoints

​Request body (/verify and /settle)

​Troubleshooting

​Need help?

Join our Community

Overview

API key structure

Protocol steps

1. Normalize the API key secret

2. Build the JWT header

3. Build the JWT payload

4. Encode and sign

5. Send the request

Token caching

Code examples

Facilitator endpoints

Request body (`/verify` and `/settle`)

Troubleshooting

Need help?