One sign-in for decePubClient: the root JWT exchanged for a persona's token

The first-party client signs in on /clientapi and exchanges that JWT on
/oauth/token (RFC 8693, subject_token_type jwt, avatar_id) for one
persona's Mastodon token. Only the seeded public application `decepub`
holds the grant; RootJwtSubjectToken validates the JWT through RootJwt,
which JwtBearer now shares (signature, lifetime, ban, deletion, session
stamp). The issued token names the avatar, never the root.

Owner decision recorded in ROADMAP; it supersedes "moving decePubClient
onto the Mastodon API is out of scope".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Services/RootJwt.cs

@@ -0,0 +1,53 @@
+using Microsoft.IdentityModel.JsonWebTokens;
+using Microsoft.IdentityModel.Tokens;
+
+using PrivaPub.Extensions;
+using PrivaPub.Models.User;
+using PrivaPub.StaticServices;
+
+using System.Security.Claims;
+using System.Text;
+
+namespace PrivaPub.Services
+{
+	// The /clientapi token: how it is validated (JwtBearer through AddPrivaPubAuth, and the persona token exchange on
+	// /oauth/token), and when a valid one can no longer be used. One place, so the two never drift apart.
+	public static class RootJwt
+	{
+		public static TokenValidationParameters Parameters(IConfiguration configuration) => new()
+		{
+			ValidateIssuer = true,
+			ValidateAudience = true,
+			ValidateLifetime = true,
+			ValidateIssuerSigningKey = true,
+			ValidIssuer = configuration["AppConfiguration:Jwt:Issuer"],
+			ValidAudience = configuration["AppConfiguration:Jwt:Audience"],
+			IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(configuration["AppConfiguration:Jwt:Key"]))
+		};
+
+		//why a token that validated can no longer be used, or default when it can: a token outlives a ban, a removal and a
+		//password recovery (RootSessions gives the root a new stamp), so the database has the last word
+		public static string Refusal(RootUser root, ClaimsPrincipal principal)
+		{
+			if (root is not { IsBanned: false, DeletedAt: null })
+				return "The account can no longer be used.";
+			if ((root.SessionStamp ?? string.Empty) != (principal.FindFirst(AuthTokenManager.SessionStamp)?.Value ?? string.Empty))
+				return "The account's sessions were ended.";
+			return default;
+		}
+
+		//the root a token belongs to, when the token is valid and can still be used; default otherwise, never an exception
+		public static async Task<RootUser> Root(string jwt, IConfiguration configuration, DbEntities dbEntities, CancellationToken token)
+		{
+			if (string.IsNullOrEmpty(jwt))
+				return default;
+			var result = await new JsonWebTokenHandler { MapInboundClaims = false }.ValidateTokenAsync(jwt, Parameters(configuration));
+			if (!result.IsValid)
+				return default;
+			var principal = new ClaimsPrincipal(result.ClaimsIdentity);
+			var rootId = principal.GetUserId();
+			var root = string.IsNullOrEmpty(rootId) ? default : await dbEntities.RootUsers.MatchID(rootId).ExecuteFirstAsync(token);
+			return Refusal(root, principal) == default ? root : default;
+		}
+	}
+}