Verify the Masnet signature in Java

Requirements in Masnet

Before configuring the receiving system, prepare the destination in Masnet. The steps are the same for every system.

1. Open Company › Integrations (the Spanish interface labels it Empresa › Integraciones), go to the Destinations tab and select New destination. If the integration has no authorized data yet, Masnet asks you to authorize it first.
2. In Destination address (https) enter the receiver public URL. It must be https, port 443 or 8443 and a public DNS name: user information in the URL, fragments, IP addresses and internal names are not accepted. A query string is allowed; Masnet treats the whole URL as a credential.
3. In Format choose Standard JSON (application/json) or Structured CloudEvents (application/cloudevents+json). The event content is the same; see Webhooks.
4. Select Create destination. Masnet shows the signing secret (whsec_…) only once. Store it immediately in the receiver secret store; it cannot be read again.
5. Select Test destination and then Send test. The integration is activated only after a successful test; then use Activate destination.

Data the verifier needs

• The destination signing secret (whsec_…), stored outside the code.
• The exact body received, uninterpreted. In Spring receive the body as byte[] or String with @RequestBody, not as an already deserialized object.
• The webhook-id, webhook-timestamp and webhook-signature headers.
• A clock tolerance of 5 minutes, which is what the Standard Webhooks libraries apply and what is recommended for the receiver.

The signature is v1, followed by base64(HMAC-SHA256) computed with the binary key obtained by base64-decoding what follows whsec_, over the bytes webhook-id + . + webhook-timestamp + . + the exact body. Always verify over the received bytes, before parsing the JSON.

With the standardwebhooks library

Add the Maven dependency com.standardwebhooks:standardwebhooks.

// Maven: com.standardwebhooks:standardwebhooks:1.2.0
import com.standardwebhooks.Webhook;
import com.standardwebhooks.exceptions.EmptyWebhookSecretException;
import com.standardwebhooks.exceptions.WebhookVerificationException;
import java.util.List;
import java.util.Map;

/** `payload` is the unparsed request body; the library enforces a 5 minute tolerance. */
public final class JavaStandardWebhooks {
    public static boolean verify(String secret, String payload, Map<String, List<String>> headers) {
        try {
            new Webhook(secret).verify(payload, headers);
            return true;
        } catch (WebhookVerificationException e) {
            return false;
        } catch (EmptyWebhookSecretException e) {
            throw new IllegalStateException("The webhook secret is not configured", e);
        }
    }
}

Without dependencies

The class compares in constant time and accepts any of the signatures in the header.

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.Base64;
import java.util.Map;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

/** Verifies a Masnet webhook with the JDK only. `rawBody` is the exact bytes received. */
public final class JavaManual {
    public static boolean verify(String secret, Map<String, String> headers, byte[] rawBody, long nowSeconds, long tolerance) {
        String id = headers.get("webhook-id");
        String timestamp = headers.get("webhook-timestamp");
        String signatures = headers.get("webhook-signature");
        if (id == null || signatures == null || timestamp == null || !timestamp.matches("\\d+")) return false;
        if (Math.abs(nowSeconds - Long.parseLong(timestamp)) > tolerance) return false;
        try {
            byte[] key = Base64.getDecoder().decode(secret.replaceFirst("^whsec_", ""));
            Mac mac = Mac.getInstance("HmacSHA256");
            mac.init(new SecretKeySpec(key, "HmacSHA256"));
            mac.update((id + "." + timestamp + ".").getBytes(StandardCharsets.UTF_8));
            byte[] expected = mac.doFinal(rawBody);
            // During a rotation the header carries two signatures separated by a space; any match is valid.
            for (String part : signatures.split(" ")) {
                String[] pair = part.split(",", 2);
                if (pair.length != 2 || !pair[0].equals("v1")) continue;
                try {
                    if (MessageDigest.isEqual(Base64.getDecoder().decode(pair[1]), expected)) return true;
                } catch (IllegalArgumentException malformed) {
                    // ignore a malformed entry and keep looking
                }
            }
        } catch (Exception e) {
            return false;
        }
        return false;
    }
}

Operation: duplicates and ordering

• Idempotency. webhook-id identifies the event and is kept across retries and redeliveries. Record the identifiers already processed and discard repeats with a 2xx response.
• Ordering. Each resource (subject) carries an increasing sequence. Discard an event whose sequence is lower than or equal to the last one applied for that subject. Events of different resources have no order between them.
• Retries. Masnet retries any non-2xx response for up to 72 hours; the full schedule is in Webhooks.
• Secret rotation. When rotating with overlap, the previous secret stays valid for 24 hours and webhook-signature carries two v1,… signatures separated by a space. Accept the event if either one matches.

Troubleshooting

Masnet records the response code of each attempt in the delivery history. These are the effects of each receiver response: