feat(auth): support RS256 + OIDC discovery (JWKS auto-fetch)

Add Jwt__Algorithm config to choose between HS256 (shared symmetric key,
existing behavior, default) and RS256 (Authority-based OIDC discovery,
public-key auto-fetch with periodic refresh).

RS256 mode makes the server compatible with any standard OAuth 2.1 / OIDC
provider (Logto, ZITADEL, Keycloak, Auth0) without requiring a shared
secret. HS256 mode remains the default for minimal self-built AS setups.

Auth/JwtBearerSetup.cs

@@ -8,13 +8,19 @@ namespace ObsidianMcp.Auth;
 public static class JwtBearerSetup
 {
     /// <summary>
-    /// 配置 HS256 JWT Bearer 认证。
-    /// 支持 Current + Previous 双密钥，方便密钥轮换过渡期。
+    /// 配置 JWT Bearer 认证。根据 JwtOptions.Algorithm 选择：
+    /// <list type="bullet">
+    ///   <item><c>HS256</c>：与 AS 共享对称密钥（Current + Previous 双密钥用于轮换）。</item>
+    ///   <item><c>RS256</c>：委托 ASP.NET Core 通过 OIDC discovery 拉 JWKS，
+    ///     自动刷新公钥（默认 24h），AS 端密钥轮换无需重启本服务。</item>
+    /// </list>
     /// </summary>
     public static IServiceCollection AddObsidianJwtBearer(
         this IServiceCollection services,
         JwtOptions opts)
     {
+        var algorithm = (opts.Algorithm ?? "HS256").Trim().ToUpperInvariant();
+
         services
             .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
             .AddJwtBearer(options =>
@@ -23,7 +29,7 @@ public static class JwtBearerSetup
                 // ClaimTypes.NameIdentifier 之类的长 URI，下游 FindFirst("sub") 取不到。
                 options.MapInboundClaims = false;
 
-                options.TokenValidationParameters = new TokenValidationParameters
+                var tvp = new TokenValidationParameters
                 {
                     ValidateIssuer = true,
                     ValidIssuer = opts.Issuer,
@@ -32,25 +38,46 @@ public static class JwtBearerSetup
                     ValidAudience = opts.Audience,
 
                     ValidateIssuerSigningKey = true,
-                    // Current 必须有值；Previous 可选（密钥轮换过渡期）。
-                    // ToList 物化一次，避免每次验签都重建 SymmetricSecurityKey。
-                    IssuerSigningKeys = BuildSigningKeys(opts).ToList(),
-
                     ValidateLifetime = true,
                     ClockSkew = TimeSpan.FromMinutes(2),
 
-                    // scope claim 的 claim type 直接保持原样，User.FindAll("scope") 能取到。
                     NameClaimType = "sub",
                 };
+
+                if (algorithm == "RS256")
+                {
+                    if (string.IsNullOrWhiteSpace(opts.Issuer))
+                        throw new InvalidOperationException(
+                            "Jwt:Issuer 未配置，RS256 模式无法启动。");
+
+                    // Authority = Issuer 时，JwtBearer 会自动从
+                    // <Issuer>/.well-known/openid-configuration 拉 OIDC 元数据，
+                    // 再从其中的 jwks_uri 拉公钥并周期性刷新。IssuerSigningKeys
+                    // 由 ConfigurationManager 在验签时动态注入，无需在此设置。
+                    options.Authority = opts.Issuer;
+                    options.RequireHttpsMetadata = !IsLocalhostUrl(opts.Issuer);
+                }
+                else if (algorithm == "HS256")
+                {
+                    tvp.IssuerSigningKeys = BuildHs256Keys(opts).ToList();
+                }
+                else
+                {
+                    throw new InvalidOperationException(
+                        $"Jwt:Algorithm '{opts.Algorithm}' 不支持。可选值：HS256, RS256");
+                }
+
+                options.TokenValidationParameters = tvp;
             });
 
         return services;
     }
 
-    private static IEnumerable<SecurityKey> BuildSigningKeys(JwtOptions opts)
+    private static IEnumerable<SecurityKey> BuildHs256Keys(JwtOptions opts)
     {
         if (string.IsNullOrWhiteSpace(opts.SigningKey.Current))
-            throw new InvalidOperationException("Jwt:SigningKey:Current 未配置，服务无法启动。");
+            throw new InvalidOperationException(
+                "Jwt:SigningKey:Current 未配置，HS256 模式无法启动。");
 
         yield return new SymmetricSecurityKey(
             Encoding.UTF8.GetBytes(opts.SigningKey.Current));
@@ -59,4 +86,8 @@ public static class JwtBearerSetup
             yield return new SymmetricSecurityKey(
                 Encoding.UTF8.GetBytes(opts.SigningKey.Previous));
     }
+
+    private static bool IsLocalhostUrl(string url) =>
+        url.StartsWith("http://localhost", StringComparison.OrdinalIgnoreCase) ||
+        url.StartsWith("http://127.0.0.1", StringComparison.OrdinalIgnoreCase);
 }

Config/JwtOptions.cs

@@ -2,18 +2,30 @@ namespace ObsidianMcp.Config;
 
 /// <summary>
 /// JWT 验签配置。
-/// 环境变量：Jwt__Issuer, Jwt__Audience, Jwt__SigningKey__Current, Jwt__SigningKey__Previous
+/// 环境变量：Jwt__Algorithm, Jwt__Issuer, Jwt__Audience, Jwt__SigningKey__Current, Jwt__SigningKey__Previous
 /// </summary>
 public class JwtOptions
 {
     public const string Section = "Jwt";
 
+    /// <summary>
+    /// JWT 签名算法。
+    /// <list type="bullet">
+    ///   <item><c>HS256</c>（默认）：与 AS 共享对称密钥（SigningKey.Current 必填）。适合自建极简 AS。</item>
+    ///   <item><c>RS256</c>：从 Issuer 走 OIDC discovery 自动拉 JWKS（含自动刷新）。
+    ///     适合任何标准 OAuth 2.1 / OIDC AS（Logto / ZITADEL / Keycloak / Auth0 等）。
+    ///     要求 Issuer 暴露 <c>/.well-known/openid-configuration</c>。</item>
+    /// </list>
+    /// </summary>
+    public string Algorithm { get; set; } = "HS256";
+
     /// <summary>期望的 iss claim（你的 auth server 的 issuer URL），必须通过 env 注入</summary>
     public string Issuer { get; set; } = string.Empty;
 
     /// <summary>期望的 aud claim，默认 obsidian</summary>
     public string Audience { get; set; } = "obsidian";
 
+    /// <summary>HS256 模式使用；RS256 模式下忽略。</summary>
     public SigningKeyPair SigningKey { get; set; } = new();
 
     public class SigningKeyPair

README.md

@@ -4,7 +4,12 @@ Read and write an Obsidian vault via [MCP (Model Context Protocol)](https://mode
 gated by OAuth-issued JWT bearer tokens.
 
 This server pairs with any OAuth 2.1 + PKCE authorization server that can mint
-HS256 JWTs containing `aud`, `iss`, `sub`, `scope` claims. Bring your own AS.
+JWTs containing `aud`, `iss`, `sub`, `scope` claims. Two signing modes:
+
+- **HS256** (default) — shared symmetric key between AS and this server. Use for self-built minimal AS.
+- **RS256** — fetches JWKS automatically via OIDC discovery from `<Issuer>/.well-known/openid-configuration`. Use with any standard provider: [Logto](https://logto.io), [ZITADEL](https://zitadel.com), [Keycloak](https://www.keycloak.org), [Auth0](https://auth0.com), etc.
+
+See [Choosing an AS](#choosing-an-as) below for setup guidance.
 
 ## Architecture
 
@@ -47,10 +52,11 @@ prefixes (double underscore = nested section). Production values must be injecte
 | `Vault__Root` | `/vault` | yes | Vault root directory inside the container |
 | `Vault__Blacklist__0` | — | no | Extra path segments to deny (`.obsidian`, `.trash`, `.git` are always denied) |
 | `Vault__WriteWhitelist__0` | — | for write tools | Writable path entries (see below) |
+| `Jwt__Algorithm` | `HS256` | no | `HS256` or `RS256` |
 | `Jwt__Issuer` | — | **yes** | Expected `iss` claim — your AS's issuer URL |
 | `Jwt__Audience` | `obsidian` | no | Expected `aud` claim |
-| `Jwt__SigningKey__Current` | — | **yes** | HS256 signing key, shared with your AS |
-| `Jwt__SigningKey__Previous` | — | no | Previous key during rotation window |
+| `Jwt__SigningKey__Current` | — | HS256 only | HS256 signing key, shared with your AS |
+| `Jwt__SigningKey__Previous` | — | no | Previous HS256 key during rotation window |
 | `Mcp__OAuthDiscovery__Issuer` | — | **yes** | `/.well-known/oauth-authorization-server` `issuer` field |
 | `Mcp__OAuthDiscovery__AuthorizationEndpoint` | — | **yes** | Your AS's `/authorize` URL |
 | `Mcp__OAuthDiscovery__TokenEndpoint` | — | **yes** | Your AS's `/token` URL |
@@ -136,6 +142,35 @@ builds and pushes the image. It expects these repository Variables / Secrets:
 - `vars.IMAGE_OWNER` — registry owner/namespace
 - `secrets.PACKAGES_TOKEN` — registry push token
 
+## Choosing an AS
+
+Claude.ai chat enforces the full OAuth Authorization Code + PKCE flow against
+your MCP server's `/.well-known/oauth-authorization-server` endpoint — there
+is no bearer-token shortcut. Pick one of these paths:
+
+**Hosted (fastest start, recommended for new setups)** — RS256 mode:
+
+| Provider | Free tier | Notes |
+|---|---|---|
+| [Logto Cloud](https://logto.io) | 5000 MAU | Lightest, ~30 min setup |
+| [ZITADEL Cloud](https://zitadel.com) | 25k auths/month | More featureful, slightly heavier docs |
+
+Set `Jwt__Algorithm=RS256` and `Jwt__Issuer=<your-tenant-issuer-URL>`.
+Public keys are fetched automatically from `<Issuer>/.well-known/openid-configuration`.
+
+**Self-hosted, full-featured** — RS256 mode:
+[Keycloak](https://www.keycloak.org), [ZITADEL](https://github.com/zitadel/zitadel), [Logto](https://github.com/logto-io/logto), [Authentik](https://goauthentik.io).
+
+**Self-hosted, minimal** — HS256 mode:
+Write your own ~500 LoC AS that issues HS256 JWTs with the right claims. The
+MCP server's `Jwt__SigningKey__Current` and the AS's signing key must match.
+
+**Required AS features regardless of choice:**
+- OAuth 2.1 + PKCE (RFC 7636)
+- Dynamic Client Registration (RFC 7591) — so Claude.ai can self-register
+- `resource` parameter support (RFC 8707) — for audience-bound tokens
+- Custom scope support (`read:obsidian`, `write:obsidian`)
+
 ## Running tests
 
 ```bash

appsettings.json

@@ -16,7 +16,9 @@
   },
 
   // JWT 验签配置（生产值必须通过 env 覆盖）
+  // Algorithm: "HS256"（默认，与 AS 共享 SigningKey）或 "RS256"（从 Issuer 走 OIDC discovery 拉 JWKS）
   "Jwt": {
+    "Algorithm": "HS256",
     "Issuer": "",
     "Audience": "obsidian",
     "SigningKey": {