Add code challenge verifiers

src/CodeChallengeVerifiers/CodeChallengeVerifierInterface.php

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+<?php
+/**
+ * @author      Lukáš Unger <lookymsc@gmail.com>
+ * @copyright   Copyright (c) Lukáš Unger
+ * @license     http://mit-license.org/
+ *
+ * @link        https://github.com/thephpleague/oauth2-server
+ */
+
+namespace League\OAuth2\Server\CodeChallengeVerifiers;
+
+interface CodeChallengeVerifierInterface
+{
+    /**
+     * Return code challenge method.
+     *
+     * @return string
+     */
+    public function getMethod();
+
+    /**
+     * Verify the code challenge.
+     *
+     * @param string $codeVerifier
+     * @param string $codeChallenge
+     *
+     * @return bool
+     */
+    public function verifyCodeChallenge($codeVerifier, $codeChallenge);
+}

src/CodeChallengeVerifiers/PlainVerifier.php

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+<?php
+/**
+ * @author      Lukáš Unger <lookymsc@gmail.com>
+ * @copyright   Copyright (c) Lukáš Unger
+ * @license     http://mit-license.org/
+ *
+ * @link        https://github.com/thephpleague/oauth2-server
+ */
+
+namespace League\OAuth2\Server\CodeChallengeVerifiers;
+
+class PlainVerifier implements CodeChallengeVerifierInterface
+{
+    /**
+     * Return code challenge method.
+     *
+     * @return string
+     */
+    public function getMethod()
+    {
+        return 'plain';
+    }
+
+    /**
+     * Verify the code challenge.
+     *
+     * @param string $codeVerifier
+     * @param string $codeChallenge
+     *
+     * @return bool
+     */
+    public function verifyCodeChallenge($codeVerifier, $codeChallenge)
+    {
+        return hash_equals($codeVerifier, $codeChallenge);
+    }
+}

src/CodeChallengeVerifiers/S256Verifier.php

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+<?php
+/**
+ * @author      Lukáš Unger <lookymsc@gmail.com>
+ * @copyright   Copyright (c) Lukáš Unger
+ * @license     http://mit-license.org/
+ *
+ * @link        https://github.com/thephpleague/oauth2-server
+ */
+
+namespace League\OAuth2\Server\CodeChallengeVerifiers;
+
+class S256Verifier implements CodeChallengeVerifierInterface
+{
+    /**
+     * Return code challenge method.
+     *
+     * @return string
+     */
+    public function getMethod()
+    {
+        return 'S256';
+    }
+
+    /**
+     * Verify the code challenge.
+     *
+     * @param string $codeVerifier
+     * @param string $codeChallenge
+     *
+     * @return bool
+     */
+    public function verifyCodeChallenge($codeVerifier, $codeChallenge)
+    {
+        return hash_equals(
+            strtr(rtrim(base64_encode(hash('sha256', $codeVerifier, true)), '='), '+/', '-_'),
+            $codeChallenge
+        );
+    }
+}