[Client] Add OAuth metadata discovery and challenge parsing

src/Client/Auth/AuthorizationChallenge.php

@@ -0,0 +1,99 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Auth;
+
+use Psr\Http\Message\ResponseInterface;
+
+/**
+ * The parsed `WWW-Authenticate` challenge a protected resource answers with.
+ *
+ * @internal
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class AuthorizationChallenge
+{
+    /**
+     * @param array<string, string> $parameters the challenge's auth-param list, keys lowercased
+     */
+    private function __construct(public readonly array $parameters)
+    {
+    }
+
+    public static function fromResponse(ResponseInterface $response): self
+    {
+        return new self(self::parseBearerParameters($response->getHeaderLine('WWW-Authenticate')));
+    }
+
+    /**
+     * The URL of the resource's metadata document, when the challenge named one.
+     */
+    public function getResourceMetadataUrl(): ?string
+    {
+        return $this->parameters['resource_metadata'] ?? null;
+    }
+
+    /**
+     * The scopes the rejected request needed.
+     *
+     * @return string[]
+     */
+    public function getScopes(): array
+    {
+        return preg_split('/\s+/', $this->parameters['scope'] ?? '', -1, \PREG_SPLIT_NO_EMPTY) ?: [];
+    }
+
+    public function getError(): ?string
+    {
+        return $this->parameters['error'] ?? null;
+    }
+
+    /**
+     * Read the Bearer challenge out of a field value that may list several challenges.
+     *
+     * @return array<string, string>
+     */
+    private static function parseBearerParameters(string $header): array
+    {
+        if (!preg_match_all('/(?:[^",]|"(?:[^"\\\\]|\\\\.)*")+/s', $header, $members)) {
+            return [];
+        }
+
+        $parameters = null;
+
+        foreach ($members[0] as $member) {
+            if (preg_match('/^\s*([^\s=,"]+)(?:\s++(?!=)|\s*$)(.*)$/s', $member, $scheme)) {
+                if (null !== $parameters) {
+                    break;
+                }
+
+                if (0 !== strcasecmp($scheme[1], 'Bearer')) {
+                    continue;
+                }
+
+                $parameters = [];
+                $member = $scheme[2];
+            }
+
+            if (null === $parameters) {
+                continue;
+            }
+
+            if (preg_match('/^\s*([^\s=,"]+)\s*=\s*(?:"((?:[^"\\\\]|\\\\.)*)"|((?!=)[^,\s]*))\s*$/s', $member, $parameter)) {
+                // RFC 9110 quoted-pair: only the backslash goes, unlike stripslashes() which maps \0 to NUL.
+                $parameters[strtolower($parameter[1])] = $parameter[3] ?? (preg_replace('/\\\\(.)/s', '$1', $parameter[2] ?? '') ?? '');
+            }
+        }
+
+        return $parameters ?? [];
+    }
+}

src/Client/Auth/AuthorizationServerMetadata.php

@@ -0,0 +1,145 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Client\Auth;
+
+use Mcp\Exception\InvalidArgumentException;
+use Mcp\Server\Transport\Http\OAuth\SecureUrl;
+
+/**
+ * An authorization server's metadata document (RFC 8414 or OpenID Connect discovery).
+ *
+ * @internal
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class AuthorizationServerMetadata
+{
+    /**
+     * @param string   $issuer                            the issuer identifier, which must match how the document was found
+     * @param ?string  $authorizationEndpoint             where the user is sent to approve the request, when a supported grant uses it
+     * @param string   $tokenEndpoint                     where codes and refresh tokens are exchanged
+     * @param ?string  $registrationEndpoint              where a client can register itself, when the server allows it
+     * @param string[] $scopesSupported                   scopes the server knows about
+     * @param string[] $grantTypesSupported               grant types the server accepts
+     * @param string[] $tokenEndpointAuthMethodsSupported client authentication methods the token endpoint accepts
+     * @param string[] $codeChallengeMethodsSupported     PKCE challenge methods the server accepts
+     * @param bool     $clientIdMetadataDocumentSupported whether a URL may be used as the client id
+     * @param bool     $issuerParameterSupported          whether authorization responses carry an "iss" parameter
+     */
+    public function __construct(
+        public readonly string $issuer,
+        public readonly ?string $authorizationEndpoint,
+        public readonly string $tokenEndpoint,
+        public readonly ?string $registrationEndpoint = null,
+        public readonly array $scopesSupported = [],
+        public readonly array $grantTypesSupported = [],
+        public readonly array $tokenEndpointAuthMethodsSupported = [],
+        public readonly array $codeChallengeMethodsSupported = [],
+        public readonly bool $clientIdMetadataDocumentSupported = false,
+        public readonly bool $issuerParameterSupported = false,
+    ) {
+        if ('' === trim($issuer)) {
+            throw new InvalidArgumentException('The authorization server metadata must name an issuer.');
+        }
+    }
+
+    /**
+     * @param array<string, mixed> $data
+     */
+    public static function fromArray(array $data): self
+    {
+        $grantTypes = self::strings($data['grant_types_supported'] ?? null);
+        $requiredFields = ['issuer', 'token_endpoint'];
+
+        // RFC 8414 section 2 requires this endpoint only when a supported grant uses it.
+        if ([] === $grantTypes || [] !== array_intersect($grantTypes, ['authorization_code', 'implicit'])) {
+            $requiredFields[] = 'authorization_endpoint';
+        }
+
+        foreach ($requiredFields as $required) {
+            if (!isset($data[$required]) || !\is_string($data[$required])) {
+                throw new InvalidArgumentException(\sprintf('The authorization server metadata is missing a string "%s".', $required));
+            }
+        }
+
+        foreach (['authorization_endpoint', 'token_endpoint', 'registration_endpoint'] as $endpoint) {
+            if (\is_string($data[$endpoint] ?? null) && !self::isTransportSecure($data[$endpoint])) {
+                throw new InvalidArgumentException(\sprintf('The authorization server metadata declares "%s" as "%s", which is neither an HTTPS URL nor a loopback address.', $endpoint, $data[$endpoint]));
+            }
+        }
+
+        /* @var array{issuer: string, token_endpoint: string} $data */
+        return new self(
+            $data['issuer'],
+            \is_string($data['authorization_endpoint'] ?? null) ? $data['authorization_endpoint'] : null,
+            $data['token_endpoint'],
+            \is_string($data['registration_endpoint'] ?? null) ? $data['registration_endpoint'] : null,
+            self::strings($data['scopes_supported'] ?? null),
+            $grantTypes,
+            self::strings($data['token_endpoint_auth_methods_supported'] ?? null),
+            self::strings($data['code_challenge_methods_supported'] ?? null),
+            true === ($data['client_id_metadata_document_supported'] ?? null),
+            true === ($data['authorization_response_iss_parameter_supported'] ?? null),
+        );
+    }
+
+    /**
+     * The fixed endpoints a 2025-03-26 era server without metadata is assumed to expose.
+     *
+     * @throws InvalidArgumentException when the issuer is neither https nor loopback
+     */
+    public static function forLegacyServer(string $issuer): self
+    {
+        if (!self::isTransportSecure($issuer)) {
+            throw new InvalidArgumentException(\sprintf('The legacy authorization server "%s" is neither an HTTPS URL nor a loopback address.', $issuer));
+        }
+
+        $base = rtrim($issuer, '/');
+
+        return new self($issuer, $base.'/authorize', $base.'/token', $base.'/register');
+    }
+
+    public function supportsGrant(string $grantType): bool
+    {
+        // RFC 8414 section 2 also defaults to implicit, which OAuth 2.1 removed.
+        return [] === $this->grantTypesSupported
+            ? 'authorization_code' === $grantType
+            : \in_array($grantType, $this->grantTypesSupported, true);
+    }
+
+    public function supportsScope(string $scope): bool
+    {
+        return \in_array($scope, $this->scopesSupported, true);
+    }
+
+    /**
+     * Whether a URL is https or loopback, the only endpoints credentials may go to.
+     */
+    public static function isTransportSecure(string $url): bool
+    {
+        try {
+            SecureUrl::parse($url, 'URL');
+
+            return true;
+        } catch (InvalidArgumentException) {
+            return false;
+        }
+    }
+
+    /**
+     * @return string[]
+     */
+    private static function strings(mixed $value): array
+    {
+        return \is_array($value) ? array_values(array_filter($value, 'is_string')) : [];
+    }
+}