MT-22022: Add webhook signature verification helper

README.md

@@ -279,6 +279,30 @@ Framework-specific (quick starts):
 
 See the full indexed list at [`examples/README.md`](examples/README.md).
 
+### Verifying webhook signatures
+
+Mailtrap signs every outbound webhook with HMAC-SHA256 and sends the lowercase hex digest in the `Mailtrap-Signature` header. Verify the signature against the raw request body using the `signing_secret` returned when you created the webhook:
+
+```php
+use Mailtrap\Helper\WebhookSignature;
+
+// $payload must be the unparsed request body bytes — do NOT re-serialize
+// the parsed JSON, as that may reorder keys and invalidate the signature.
+$rawBody = file_get_contents('php://input');
+$valid = WebhookSignature::verify(
+    $rawBody !== false ? $rawBody : '',
+    $_SERVER['HTTP_MAILTRAP_SIGNATURE'] ?? '',
+    $_ENV['MAILTRAP_WEBHOOK_SIGNING_SECRET'] ?? ''
+);
+
+if (!$valid) {
+    http_response_code(401);
+    exit;
+}
+```
+
+The helper performs a constant-time comparison and returns `false` (rather than raising) for empty, missing, or malformed signatures.
+
 ## Contributing
 
 Bug reports and pull requests are welcome on [GitHub](https://github.com/railsware/mailtrap-php). This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [code of conduct](CODE_OF_CONDUCT.md).

examples/webhooks/verify_signature.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+use Mailtrap\Helper\WebhookSignature;
+
+require __DIR__ . '/../../vendor/autoload.php';
+
+// --- Direct verification (e.g. for unit tests or custom routers) ----------
+$payload = '{"event":"delivery","message_id":"abc-123"}';
+$signingSecret = '8d9a3c0e7f5b2d4a6c1e9f8b3a7d5c2e';
+$signature = hash_hmac('sha256', $payload, $signingSecret);
+
+if (!WebhookSignature::verify($payload, $signature, $signingSecret)) {
+    fwrite(STDERR, "Signature verification failed!\n");
+    exit(1);
+}

src/Helper/WebhookSignature.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Mailtrap\Helper;
+
+/**
+ * Helpers for working with inbound Mailtrap webhooks.
+ *
+ * @see https://docs.mailtrap.io/email-api-smtp/advanced/webhooks#verifying-the-signature
+ */
+final class WebhookSignature
+{
+    /**
+     * Hex-encoded HMAC-SHA256 signature length (SHA-256 produces 32 bytes / 64 hex chars).
+     */
+    public const SIGNATURE_HEX_LENGTH = 64;
+
+    /**
+     * Verifies the HMAC-SHA256 signature of a Mailtrap webhook payload.
+     *
+     * Mailtrap signs every outbound webhook by computing
+     * `HMAC-SHA256(signing_secret, raw_request_body)` and sending the lowercase
+     * hex digest in the `Mailtrap-Signature` HTTP header. Compute the same
+     * digest on your side and compare it in constant time.
+     *
+     * The comparison is performed with {@see hash_equals()} to avoid timing
+     * side-channels.
+     *
+     * The method never raises on inputs that could plausibly arrive over the
+     * wire (empty strings, wrong-length signatures, non-hex characters, missing
+     * secret) — it simply returns `false`. This makes it safe to call directly
+     * from a request handler without wrapping in try/catch.
+     *
+     * @param string $payload       The raw request body, exactly as received.
+     *                              **Do not** parse and re-serialize the JSON —
+     *                              re-encoding may reorder keys or alter
+     *                              whitespace and invalidate the signature.
+     * @param string $signature     The value of the `Mailtrap-Signature` HTTP
+     *                              header (lowercase hex string).
+     * @param string $signingSecret The webhook's `signing_secret`, returned by
+     *                              the Webhooks API on webhook creation.
+     *
+     * @return bool `true` if the signature is valid for the given payload and
+     *              secret, `false` otherwise.
+     */
+    public static function verify(string $payload, string $signature, string $signingSecret): bool
+    {
+        if ($signature === '' || $signingSecret === '' || $payload === '') {
+            return false;
+        }
+
+        if (strlen($signature) !== self::SIGNATURE_HEX_LENGTH) {
+            return false;
+        }
+
+        $expected = hash_hmac('sha256', $payload, $signingSecret);
+
+        return hash_equals($expected, $signature);
+    }
+}

tests/Helper/WebhookSignatureTest.php

@@ -0,0 +1,150 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Mailtrap\Tests\Helper;
+
+use Mailtrap\Helper\WebhookSignature;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @covers \Mailtrap\Helper\WebhookSignature
+ *
+ * Class WebhookSignatureTest
+ */
+class WebhookSignatureTest extends TestCase
+{
+    // -----------------------------------------------------------------------
+    // Cross-SDK fixture
+    //
+    // The (payload, signing_secret, expected_signature) triple below is the
+    // canonical fixture shared verbatim by every official Mailtrap SDK
+    // (mailtrap-ruby, mailtrap-python, mailtrap-php, mailtrap-nodejs,
+    // mailtrap-java, mailtrap-dotnet). Any change here MUST be mirrored in
+    // the equivalent test files in the other SDKs so the helpers stay
+    // byte-for-byte compatible across languages.
+    // -----------------------------------------------------------------------
+    private const FIXTURE_PAYLOAD = '{"event":"delivery","sending_stream":"transactional","category":"welcome","message_id":"a8b1d8f6-1f8d-4a3c-9b2e-1a2b3c4d5e6f","email":"recipient@example.com","event_id":"f1e2d3c4-b5a6-7890-1234-567890abcdef","timestamp":1716070000}';
+    private const FIXTURE_SIGNING_SECRET = '8d9a3c0e7f5b2d4a6c1e9f8b3a7d5c2e';
+    private const FIXTURE_EXPECTED_SIGNATURE = '6d262e2611cd09be1f948382b5c611d63b0e585c4c9c5e40139d6ac3876d5433';
+
+    // --- 1. Valid signature for given payload + secret ---------------------
+    public function testReturnsTrueForValidSignaturePayloadAndSecret(): void
+    {
+        $this->assertTrue(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 2. Wrong secret ---------------------------------------------------
+    public function testReturnsFalseWithWrongSigningSecret(): void
+    {
+        $this->assertFalse(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                'ffffffffffffffffffffffffffffffff'
+            )
+        );
+    }
+
+    // --- 3. Payload tampered (one byte changed) ----------------------------
+    public function testReturnsFalseWhenPayloadIsTampered(): void
+    {
+        $tampered = str_replace('delivery', 'Delivery', self::FIXTURE_PAYLOAD);
+
+        $this->assertFalse(
+            WebhookSignature::verify(
+                $tampered,
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 4. Signature with wrong length ------------------------------------
+    public function testReturnsFalseWithoutRaisingWhenSignatureTooShort(): void
+    {
+        $tooShort = substr(self::FIXTURE_EXPECTED_SIGNATURE, 0, 31);
+
+        $this->assertFalse(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                $tooShort,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 5. Signature with non-hex characters ------------------------------
+    public function testReturnsFalseWithoutRaisingForNonHexSignature(): void
+    {
+        $notHex = str_repeat('z', WebhookSignature::SIGNATURE_HEX_LENGTH);
+
+        $this->assertFalse(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                $notHex,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 6. Empty signature string -----------------------------------------
+    public function testReturnsFalseForEmptySignature(): void
+    {
+        $this->assertFalse(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                '',
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 7. Empty signing_secret -------------------------------------------
+    public function testReturnsFalseForEmptySigningSecret(): void
+    {
+        $this->assertFalse(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                ''
+            )
+        );
+    }
+
+    // --- 8. Empty payload + non-empty signature ----------------------------
+    public function testReturnsFalseForEmptyPayload(): void
+    {
+        $this->assertFalse(
+            WebhookSignature::verify(
+                '',
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+
+    // --- 9. Known-good cross-SDK fixture -----------------------------------
+    public function testMatchesHardcodedHmacSha256DigestForSharedFixture(): void
+    {
+        // Recompute the digest in-place so a regression in PHP's hash
+        // extension or the fixture itself fails loudly: this is the
+        // byte-for-byte contract every other Mailtrap SDK must satisfy.
+        $computed = hash_hmac('sha256', self::FIXTURE_PAYLOAD, self::FIXTURE_SIGNING_SECRET);
+
+        $this->assertSame(self::FIXTURE_EXPECTED_SIGNATURE, $computed);
+        $this->assertTrue(
+            WebhookSignature::verify(
+                self::FIXTURE_PAYLOAD,
+                self::FIXTURE_EXPECTED_SIGNATURE,
+                self::FIXTURE_SIGNING_SECRET
+            )
+        );
+    }
+}