First commit in rust

backchannel-server/Cargo.toml

@@ -0,0 +1,36 @@
+[package]
+name    = "backchannel-server"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "backchannel-server"
+path = "src/main.rs"
+
+[dependencies]
+backchannel-common = { path = "../backchannel-common" }
+
+serde            = { workspace = true }
+serde_json       = { workspace = true }
+uuid             = { workspace = true }
+chrono           = { workspace = true }
+thiserror        = { workspace = true }
+anyhow           = { workspace = true }
+tracing          = { workspace = true }
+tracing-subscriber = { workspace = true }
+base64           = { workspace = true }
+rand             = { workspace = true }
+argon2           = { workspace = true }
+ed25519-dalek    = { workspace = true }
+tokio            = { workspace = true }
+tokio-tungstenite = { workspace = true }
+
+sqlx         = { version = "0.8", features = ["sqlite", "runtime-tokio-rustls", "macros", "migrate", "chrono", "uuid"] }
+jsonwebtoken = "9"
+rustls         = { version = "0.23", features = ["ring"] }
+rustls-pemfile = "2"
+tokio-rustls   = "0.26"
+futures-util   = "0.3"
+axum           = "0.7"
+rust-embed     = "8"
+mime_guess     = "2"

backchannel-server/src/auth/mod.rs

@@ -0,0 +1,2 @@
+pub mod password;
+pub mod token;

backchannel-server/src/auth/password.rs

@@ -0,0 +1,31 @@
+use argon2::{
+    password_hash::{rand_core::OsRng, PasswordHash, PasswordHasher, PasswordVerifier, SaltString},
+    Argon2,
+};
+
+use crate::error::{Result, ServerError};
+
+/// Hash a plaintext password using Argon2id.
+///
+/// The returned string is the PHC-format encoded hash, suitable for storage in
+/// the `users.password_hash` column. Includes the salt, so no separate salt
+/// storage is needed.
+pub fn hash(password: &str) -> Result<String> {
+    let salt = SaltString::generate(&mut OsRng);
+    let hash = Argon2::default()
+        .hash_password(password.as_bytes(), &salt)?
+        .to_string();
+    Ok(hash)
+}
+
+/// Verify a plaintext password against a stored PHC-format hash.
+///
+/// Returns `Ok(())` on success, `Err(ServerError::Unauthorized)` on mismatch.
+pub fn verify(password: &str, stored_hash: &str) -> Result<()> {
+    let parsed = PasswordHash::new(stored_hash)
+        .map_err(|e| ServerError::PasswordHash(e.to_string()))?;
+
+    Argon2::default()
+        .verify_password(password.as_bytes(), &parsed)
+        .map_err(|_| ServerError::Unauthorized)
+}

backchannel-server/src/auth/token.rs

@@ -0,0 +1,110 @@
+use chrono::Utc;
+use jsonwebtoken::{decode, encode, Algorithm, DecodingKey, EncodingKey, Header, Validation};
+use serde::{Deserialize, Serialize};
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+
+/// Claims embedded in every BackChannel JWT.
+#[derive(Debug, Serialize, Deserialize, Clone)]
+pub struct Claims {
+    /// Subject: user UUID as a string.
+    pub sub: String,
+    pub username: String,
+    /// JWT ID — stored in the `sessions` table for revocation support.
+    pub jti: String,
+    /// Issued-at (Unix seconds).
+    pub iat: i64,
+    /// Expiry (Unix seconds).
+    pub exp: i64,
+}
+
+impl Claims {
+    pub fn user_id(&self) -> Result<Uuid> {
+        Uuid::parse_str(&self.sub).map_err(|e| ServerError::Internal(e.to_string()))
+    }
+}
+
+const TOKEN_TTL_SECS: i64 = 7 * 24 * 3600; // 7 days
+
+/// Issue a signed JWT and persist its JTI in the `sessions` table.
+pub async fn issue(
+    pool: &SqlitePool,
+    user_id: Uuid,
+    username: &str,
+    secret: &[u8],
+) -> Result<String> {
+    let now = Utc::now().timestamp();
+    let jti = Uuid::new_v4().to_string();
+
+    let claims = Claims {
+        sub: user_id.to_string(),
+        username: username.to_string(),
+        jti: jti.clone(),
+        iat: now,
+        exp: now + TOKEN_TTL_SECS,
+    };
+
+    let token = encode(
+        &Header::default(), // HS256
+        &claims,
+        &EncodingKey::from_secret(secret),
+    )?;
+
+    // Persist JTI for revocation checks.
+    let expires_at = Utc::now()
+        .checked_add_signed(chrono::Duration::seconds(TOKEN_TTL_SECS))
+        .unwrap_or_else(Utc::now)
+        .to_rfc3339();
+
+    sqlx::query(
+        "INSERT INTO sessions (jti, user_id, expires_at) VALUES (?, ?, ?)",
+    )
+    .bind(&jti)
+    .bind(user_id.to_string())
+    .bind(expires_at)
+    .execute(pool)
+    .await?;
+
+    Ok(token)
+}
+
+/// Decode and validate a JWT, then check the `sessions` table for revocation.
+pub async fn validate(pool: &SqlitePool, token: &str, secret: &[u8]) -> Result<Claims> {
+    let mut validation = Validation::new(Algorithm::HS256);
+    validation.validate_exp = true;
+
+    let token_data = decode::<Claims>(
+        token,
+        &DecodingKey::from_secret(secret),
+        &validation,
+    )
+    .map_err(|_| ServerError::Unauthorized)?;
+
+    let claims = token_data.claims;
+
+    // Check revocation.
+    let row: Option<(i64,)> =
+        sqlx::query_as("SELECT revoked FROM sessions WHERE jti = ?")
+            .bind(&claims.jti)
+            .fetch_optional(pool)
+            .await?;
+
+    match row {
+        None => return Err(ServerError::Unauthorized), // JTI unknown
+        Some((revoked,)) if revoked != 0 => return Err(ServerError::Unauthorized),
+        _ => {}
+    }
+
+    Ok(claims)
+}
+
+/// Mark a JWT as revoked (logout / forced expiry).
+pub async fn revoke(pool: &SqlitePool, jti: &str) -> Result<()> {
+    sqlx::query("UPDATE sessions SET revoked = 1 WHERE jti = ?")
+        .bind(jti)
+        .execute(pool)
+        .await?;
+    Ok(())
+}

backchannel-server/src/config.rs

@@ -0,0 +1,84 @@
+use anyhow::Result;
+
+/// Runtime configuration for the BackChannel server.
+/// Loaded from environment variables with sensible development defaults.
+pub struct ServerConfig {
+    /// TCP address to bind, e.g. `0.0.0.0:7777`
+    pub bind_addr: String,
+
+    /// HTTP address for the embedded web UI, e.g. `0.0.0.0:8080`
+    pub http_bind_addr: String,
+
+    /// SQLite connection string, e.g. `sqlite:backchannel.db`
+    pub db_url: String,
+
+    /// Raw bytes for HMAC-SHA256 JWT signing. In production, set
+    /// `BC_JWT_SECRET` to a long, random hex string.
+    pub jwt_secret: Vec<u8>,
+
+    /// Enable TLS on the WebSocket listener. Requires `tls_cert_path`
+    /// and `tls_key_path` when true.
+    pub tls_enabled: bool,
+
+    /// Path to the PEM-encoded TLS certificate chain.
+    pub tls_cert_path: Option<String>,
+
+    /// Path to the PEM-encoded private key.
+    pub tls_key_path: Option<String>,
+}
+
+impl ServerConfig {
+    /// Load configuration from environment variables.
+    ///
+    /// | Variable         | Default                     |
+    /// |------------------|-----------------------------|
+    /// | `BC_BIND_ADDR`   | `0.0.0.0:7777`              |
+    /// | `BC_HTTP_ADDR`   | `0.0.0.0:8080`              |
+    /// | `BC_DB_URL`      | `sqlite:backchannel.db`     |
+    /// | `BC_JWT_SECRET`  | *insecure dev placeholder*  |
+    /// | `BC_TLS`         | `false`                     |
+    /// | `BC_TLS_CERT`    | *(none)*                    |
+    /// | `BC_TLS_KEY`     | *(none)*                    |
+    pub fn from_env() -> Result<Self> {
+        let bind_addr = std::env::var("BC_BIND_ADDR")
+            .unwrap_or_else(|_| "0.0.0.0:7777".into());
+        let http_bind_addr = std::env::var("BC_HTTP_ADDR")
+            .unwrap_or_else(|_| "0.0.0.0:8080".into());
+
+        let db_url = std::env::var("BC_DB_URL")
+            .unwrap_or_else(|_| "sqlite:backchannel.db".into());
+
+        let jwt_secret = match std::env::var("BC_JWT_SECRET") {
+            Ok(s) => s.into_bytes(),
+            Err(_) => {
+                tracing::warn!(
+                    "BC_JWT_SECRET not set — using insecure dev placeholder. \
+                     Set this env var before deploying."
+                );
+                b"CHANGE-ME-dev-only-jwt-secret-32b".to_vec()
+            }
+        };
+
+        let tls_enabled = std::env::var("BC_TLS")
+            .map(|v| v.eq_ignore_ascii_case("true") || v == "1")
+            .unwrap_or(false);
+
+        let tls_cert_path = std::env::var("BC_TLS_CERT").ok();
+        let tls_key_path  = std::env::var("BC_TLS_KEY").ok();
+
+        if tls_enabled {
+            anyhow::ensure!(tls_cert_path.is_some(), "BC_TLS_CERT must be set when BC_TLS=true");
+            anyhow::ensure!(tls_key_path.is_some(),  "BC_TLS_KEY must be set when BC_TLS=true");
+        }
+
+        Ok(Self {
+            bind_addr,
+            http_bind_addr,
+            db_url,
+            jwt_secret,
+            tls_enabled,
+            tls_cert_path,
+            tls_key_path,
+        })
+    }
+}

backchannel-server/src/db/channels.rs

@@ -0,0 +1,78 @@
+use chrono::Utc;
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+
+#[derive(Debug, sqlx::FromRow)]
+pub struct ChannelRow {
+    pub id: String,
+    pub name: String,
+    pub topic: Option<String>,
+    pub created_by: String,
+    pub created_at: String,
+}
+
+impl ChannelRow {
+    pub fn uuid(&self) -> Result<Uuid> {
+        Uuid::parse_str(&self.id).map_err(|e| ServerError::Internal(e.to_string()))
+    }
+}
+
+pub async fn create(
+    pool: &SqlitePool,
+    name: &str,
+    topic: Option<&str>,
+    created_by: Uuid,
+) -> Result<Uuid> {
+    let id = Uuid::new_v4();
+    let now = Utc::now().to_rfc3339();
+
+    sqlx::query(
+        "INSERT INTO channels (id, name, topic, created_by, created_at) VALUES (?, ?, ?, ?, ?)",
+    )
+    .bind(id.to_string())
+    .bind(name)
+    .bind(topic)
+    .bind(created_by.to_string())
+    .bind(now)
+    .execute(pool)
+    .await?;
+
+    Ok(id)
+}
+
+pub async fn list(pool: &SqlitePool) -> Result<Vec<ChannelRow>> {
+    let rows = sqlx::query_as::<_, ChannelRow>(
+        "SELECT id, name, topic, created_by, created_at FROM channels ORDER BY created_at ASC",
+    )
+    .fetch_all(pool)
+    .await?;
+    Ok(rows)
+}
+
+pub async fn find_by_id(pool: &SqlitePool, id: Uuid) -> Result<Option<ChannelRow>> {
+    let row = sqlx::query_as::<_, ChannelRow>(
+        "SELECT id, name, topic, created_by, created_at FROM channels WHERE id = ?",
+    )
+    .bind(id.to_string())
+    .fetch_optional(pool)
+    .await?;
+    Ok(row)
+}
+
+pub async fn delete(pool: &SqlitePool, id: Uuid) -> Result<bool> {
+    let result = sqlx::query("DELETE FROM channels WHERE id = ?")
+        .bind(id.to_string())
+        .execute(pool)
+        .await?;
+    Ok(result.rows_affected() > 0)
+}
+
+pub async fn name_exists(pool: &SqlitePool, name: &str) -> Result<bool> {
+    let count: (i64,) = sqlx::query_as("SELECT COUNT(*) FROM channels WHERE name = ?")
+        .bind(name)
+        .fetch_one(pool)
+        .await?;
+    Ok(count.0 > 0)
+}

backchannel-server/src/db/keys.rs

@@ -0,0 +1,17 @@
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::Result;
+
+/// Look up the registered Ed25519 identity public key for a user.
+///
+/// Returns `None` if the user does not exist or has no key on record
+/// (the latter shouldn't happen after registration, but is handled defensively).
+pub async fn get_identity_key(pool: &SqlitePool, user_id: Uuid) -> Result<Option<String>> {
+    let row: Option<(String,)> =
+        sqlx::query_as("SELECT identity_pubkey FROM users WHERE id = ?")
+            .bind(user_id.to_string())
+            .fetch_optional(pool)
+            .await?;
+    Ok(row.map(|(k,)| k))
+}

backchannel-server/src/db/messages.rs

@@ -0,0 +1,210 @@
+use backchannel_common::protocol::server::{HistoricChannelMessage, HistoricDm};
+use chrono::{DateTime, Utc};
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+
+#[derive(Debug, sqlx::FromRow)]
+struct ChannelMessageRow {
+    pub id: String,
+    pub channel_id: String,
+    pub author_id: String,
+    pub author_username: String,
+    pub content: String,
+    pub created_at: String,
+}
+
+#[derive(Debug, sqlx::FromRow)]
+struct DmRow {
+    pub id: String,
+    pub sender_id: String,
+    pub ciphertext: String,
+    pub nonce: String,
+    pub created_at: String,
+}
+
+pub async fn insert_channel_message(
+    pool: &SqlitePool,
+    channel_id: Uuid,
+    author_id: Uuid,
+    content: &str,
+) -> Result<(Uuid, DateTime<Utc>)> {
+    let id = Uuid::new_v4();
+    let now = Utc::now();
+    let now_str = now.to_rfc3339();
+
+    sqlx::query(
+        "INSERT INTO channel_messages (id, channel_id, author_id, content, created_at)
+         VALUES (?, ?, ?, ?, ?)",
+    )
+    .bind(id.to_string())
+    .bind(channel_id.to_string())
+    .bind(author_id.to_string())
+    .bind(content)
+    .bind(&now_str)
+    .execute(pool)
+    .await?;
+
+    Ok((id, now))
+}
+
+pub async fn fetch_channel_history(
+    pool: &SqlitePool,
+    channel_id: Uuid,
+    before_message_id: Option<Uuid>,
+    limit: u32,
+) -> Result<Vec<HistoricChannelMessage>> {
+    let limit = limit.min(100) as i64;
+
+    let rows: Vec<ChannelMessageRow> = if let Some(before_id) = before_message_id {
+        // Fetch messages older than `before_id` by joining to get its created_at
+        sqlx::query_as::<_, ChannelMessageRow>(
+            "SELECT m.id, m.channel_id, m.author_id, u.username AS author_username,
+                    m.content, m.created_at
+             FROM channel_messages m
+             JOIN users u ON u.id = m.author_id
+             WHERE m.channel_id = ?
+               AND m.created_at < (SELECT created_at FROM channel_messages WHERE id = ?)
+             ORDER BY m.created_at DESC
+             LIMIT ?",
+        )
+        .bind(channel_id.to_string())
+        .bind(before_id.to_string())
+        .bind(limit)
+        .fetch_all(pool)
+        .await?
+    } else {
+        sqlx::query_as::<_, ChannelMessageRow>(
+            "SELECT m.id, m.channel_id, m.author_id, u.username AS author_username,
+                    m.content, m.created_at
+             FROM channel_messages m
+             JOIN users u ON u.id = m.author_id
+             WHERE m.channel_id = ?
+             ORDER BY m.created_at DESC
+             LIMIT ?",
+        )
+        .bind(channel_id.to_string())
+        .bind(limit)
+        .fetch_all(pool)
+        .await?
+    };
+
+    let mut messages: Vec<HistoricChannelMessage> = rows
+        .into_iter()
+        .map(|r| {
+            let ts = r
+                .created_at
+                .parse::<DateTime<Utc>>()
+                .unwrap_or_else(|_| Utc::now());
+            Ok(HistoricChannelMessage {
+                message_id: Uuid::parse_str(&r.id)
+                    .map_err(|e| ServerError::Internal(e.to_string()))?,
+                author_id: Uuid::parse_str(&r.author_id)
+                    .map_err(|e| ServerError::Internal(e.to_string()))?,
+                author_username: r.author_username,
+                content: r.content,
+                timestamp: ts,
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
+
+    // Return in chronological order (oldest first).
+    messages.reverse();
+    Ok(messages)
+}
+
+pub async fn insert_dm(
+    pool: &SqlitePool,
+    sender_id: Uuid,
+    recipient_id: Uuid,
+    ciphertext: &str,
+    nonce: &str,
+) -> Result<(Uuid, DateTime<Utc>)> {
+    let id = Uuid::new_v4();
+    let now = Utc::now();
+    let now_str = now.to_rfc3339();
+
+    sqlx::query(
+        "INSERT INTO direct_messages (id, sender_id, recipient_id, ciphertext, nonce, created_at)
+         VALUES (?, ?, ?, ?, ?, ?)",
+    )
+    .bind(id.to_string())
+    .bind(sender_id.to_string())
+    .bind(recipient_id.to_string())
+    .bind(ciphertext)
+    .bind(nonce)
+    .bind(&now_str)
+    .execute(pool)
+    .await?;
+
+    Ok((id, now))
+}
+
+pub async fn fetch_dm_history(
+    pool: &SqlitePool,
+    user_a: Uuid,
+    user_b: Uuid,
+    before_message_id: Option<Uuid>,
+    limit: u32,
+) -> Result<Vec<HistoricDm>> {
+    let limit = limit.min(100) as i64;
+
+    let rows: Vec<DmRow> = if let Some(before_id) = before_message_id {
+        sqlx::query_as::<_, DmRow>(
+            "SELECT id, sender_id, ciphertext, nonce, created_at
+             FROM direct_messages
+             WHERE ((sender_id = ? AND recipient_id = ?)
+                 OR (sender_id = ? AND recipient_id = ?))
+               AND created_at < (SELECT created_at FROM direct_messages WHERE id = ?)
+             ORDER BY created_at DESC
+             LIMIT ?",
+        )
+        .bind(user_a.to_string())
+        .bind(user_b.to_string())
+        .bind(user_b.to_string())
+        .bind(user_a.to_string())
+        .bind(before_id.to_string())
+        .bind(limit)
+        .fetch_all(pool)
+        .await?
+    } else {
+        sqlx::query_as::<_, DmRow>(
+            "SELECT id, sender_id, ciphertext, nonce, created_at
+             FROM direct_messages
+             WHERE (sender_id = ? AND recipient_id = ?)
+                OR (sender_id = ? AND recipient_id = ?)
+             ORDER BY created_at DESC
+             LIMIT ?",
+        )
+        .bind(user_a.to_string())
+        .bind(user_b.to_string())
+        .bind(user_b.to_string())
+        .bind(user_a.to_string())
+        .bind(limit)
+        .fetch_all(pool)
+        .await?
+    };
+
+    let mut messages: Vec<HistoricDm> = rows
+        .into_iter()
+        .map(|r| {
+            let ts = r
+                .created_at
+                .parse::<DateTime<Utc>>()
+                .unwrap_or_else(|_| Utc::now());
+            Ok(HistoricDm {
+                message_id: Uuid::parse_str(&r.id)
+                    .map_err(|e| ServerError::Internal(e.to_string()))?,
+                sender_id: Uuid::parse_str(&r.sender_id)
+                    .map_err(|e| ServerError::Internal(e.to_string()))?,
+                ciphertext: r.ciphertext,
+                nonce: r.nonce,
+                timestamp: ts,
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
+
+    messages.reverse();
+    Ok(messages)
+}

backchannel-server/src/db/migrations/001_initial.sql

@@ -0,0 +1,85 @@
+-- ── Users ─────────────────────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS users (
+    id              TEXT NOT NULL PRIMARY KEY,
+    username        TEXT NOT NULL UNIQUE,
+    password_hash   TEXT NOT NULL,
+    identity_pubkey TEXT NOT NULL,
+    created_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    updated_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+-- ── Roles ─────────────────────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS roles (
+    id           TEXT NOT NULL PRIMARY KEY,
+    name         TEXT NOT NULL UNIQUE,
+    permissions  INTEGER NOT NULL DEFAULT 0,
+    created_at   TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+-- Seed built-in roles.
+-- admin: all bits set (i64::MAX in SQLite integer storage).
+-- member: READ_MESSAGES (bit 0) | SEND_MESSAGES (bit 1) = 3
+INSERT OR IGNORE INTO roles (id, name, permissions) VALUES
+    ('00000000-0000-0000-0000-000000000001', 'admin',  9223372036854775807),
+    ('00000000-0000-0000-0000-000000000002', 'member', 3);
+
+-- ── User ↔ Role membership ─────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS user_roles (
+    user_id     TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    role_id     TEXT NOT NULL REFERENCES roles(id) ON DELETE CASCADE,
+    assigned_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    PRIMARY KEY (user_id, role_id)
+);
+
+-- ── Channels ───────────────────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS channels (
+    id         TEXT NOT NULL PRIMARY KEY,
+    name       TEXT NOT NULL UNIQUE,
+    topic      TEXT,
+    created_by TEXT NOT NULL,
+    created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+-- Seed a default channel owned by a system placeholder user.
+-- The created_by value references no real user; foreign key is intentionally
+-- left loose here since the system user is never inserted.
+INSERT OR IGNORE INTO channels (id, name, created_by) VALUES
+    ('00000000-0000-0000-0000-000000000010', 'general', '00000000-0000-0000-0000-000000000000');
+
+-- ── Channel messages ───────────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS channel_messages (
+    id         TEXT NOT NULL PRIMARY KEY,
+    channel_id TEXT NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    author_id  TEXT NOT NULL REFERENCES users(id),
+    content    TEXT NOT NULL,
+    created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_channel_messages_channel_created
+    ON channel_messages (channel_id, created_at DESC);
+
+-- ── Direct messages ────────────────────────────────────────────────────────────
+-- Ciphertext is stored opaquely. The server cannot decrypt DM content.
+CREATE TABLE IF NOT EXISTS direct_messages (
+    id           TEXT NOT NULL PRIMARY KEY,
+    sender_id    TEXT NOT NULL REFERENCES users(id),
+    recipient_id TEXT NOT NULL REFERENCES users(id),
+    ciphertext   TEXT NOT NULL,
+    nonce        TEXT NOT NULL,
+    created_at   TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_dm_pair_created
+    ON direct_messages (sender_id, recipient_id, created_at DESC);
+
+-- ── Sessions ───────────────────────────────────────────────────────────────────
+-- JWT IDs stored here enable soft revocation (logout / forced expiry).
+CREATE TABLE IF NOT EXISTS sessions (
+    jti        TEXT NOT NULL PRIMARY KEY,
+    user_id    TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    expires_at TEXT NOT NULL,
+    revoked    INTEGER NOT NULL DEFAULT 0
+);
+
+CREATE INDEX IF NOT EXISTS idx_sessions_user ON sessions (user_id);

backchannel-server/src/db/mod.rs

@@ -0,0 +1,5 @@
+pub mod channels;
+pub mod keys;
+pub mod messages;
+pub mod roles;
+pub mod users;

backchannel-server/src/db/roles.rs

@@ -0,0 +1,86 @@
+use chrono::Utc;
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+
+#[derive(Debug, sqlx::FromRow)]
+pub struct RoleRow {
+    pub id: String,
+    pub name: String,
+    pub permissions: i64,
+    pub created_at: String,
+}
+
+impl RoleRow {
+    pub fn uuid(&self) -> Result<Uuid> {
+        Uuid::parse_str(&self.id).map_err(|e| ServerError::Internal(e.to_string()))
+    }
+
+    /// Returns the permissions bitmask as `u64` (SQLite stores it as `i64`).
+    pub fn permissions_u64(&self) -> u64 {
+        self.permissions as u64
+    }
+}
+
+pub async fn create(pool: &SqlitePool, name: &str, permissions: u64) -> Result<Uuid> {
+    let id = Uuid::new_v4();
+    let now = Utc::now().to_rfc3339();
+
+    sqlx::query("INSERT INTO roles (id, name, permissions, created_at) VALUES (?, ?, ?, ?)")
+        .bind(id.to_string())
+        .bind(name)
+        .bind(permissions as i64)
+        .bind(now)
+        .execute(pool)
+        .await?;
+
+    Ok(id)
+}
+
+pub async fn find_by_id(pool: &SqlitePool, id: Uuid) -> Result<Option<RoleRow>> {
+    let row = sqlx::query_as::<_, RoleRow>(
+        "SELECT id, name, permissions, created_at FROM roles WHERE id = ?",
+    )
+    .bind(id.to_string())
+    .fetch_optional(pool)
+    .await?;
+    Ok(row)
+}
+
+pub async fn assign(pool: &SqlitePool, user_id: Uuid, role_id: Uuid) -> Result<()> {
+    let now = Utc::now().to_rfc3339();
+    sqlx::query(
+        "INSERT OR IGNORE INTO user_roles (user_id, role_id, assigned_at) VALUES (?, ?, ?)",
+    )
+    .bind(user_id.to_string())
+    .bind(role_id.to_string())
+    .bind(now)
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
+pub async fn revoke(pool: &SqlitePool, user_id: Uuid, role_id: Uuid) -> Result<()> {
+    sqlx::query("DELETE FROM user_roles WHERE user_id = ? AND role_id = ?")
+        .bind(user_id.to_string())
+        .bind(role_id.to_string())
+        .execute(pool)
+        .await?;
+    Ok(())
+}
+
+/// Compute the effective permission bitmask for a user by OR-ing all their role permissions.
+pub async fn get_user_permissions(pool: &SqlitePool, user_id: Uuid) -> Result<u64> {
+    let rows: Vec<(i64,)> = sqlx::query_as(
+        "SELECT r.permissions FROM roles r
+         JOIN user_roles ur ON ur.role_id = r.id
+         WHERE ur.user_id = ?",
+    )
+    .bind(user_id.to_string())
+    .fetch_all(pool)
+    .await?;
+
+    let perms = rows.into_iter().fold(0u64, |acc, (p,)| acc | p as u64);
+    Ok(perms)
+}

backchannel-server/src/db/users.rs

@@ -0,0 +1,115 @@
+use chrono::Utc;
+use sqlx::SqlitePool;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+
+/// A row from the `users` table.
+#[derive(Debug, sqlx::FromRow)]
+pub struct UserRow {
+    pub id: String,
+    pub username: String,
+    pub password_hash: String,
+    pub identity_pubkey: String,
+    pub created_at: String,
+    pub updated_at: String,
+}
+
+impl UserRow {
+    pub fn uuid(&self) -> Result<Uuid> {
+        Uuid::parse_str(&self.id).map_err(|e| ServerError::Internal(e.to_string()))
+    }
+}
+
+/// Insert a new user. Returns the new row's UUID.
+pub async fn create(
+    pool: &SqlitePool,
+    username: &str,
+    password_hash: &str,
+    identity_pubkey: &str,
+) -> Result<Uuid> {
+    let id = Uuid::new_v4();
+    let now = Utc::now().to_rfc3339();
+
+    sqlx::query(
+        "INSERT INTO users (id, username, password_hash, identity_pubkey, created_at, updated_at)
+         VALUES (?, ?, ?, ?, ?, ?)",
+    )
+    .bind(id.to_string())
+    .bind(username)
+    .bind(password_hash)
+    .bind(identity_pubkey)
+    .bind(&now)
+    .bind(&now)
+    .execute(pool)
+    .await?;
+
+    Ok(id)
+}
+
+/// Find a user by username.
+pub async fn find_by_username(pool: &SqlitePool, username: &str) -> Result<Option<UserRow>> {
+    let row = sqlx::query_as::<_, UserRow>(
+        "SELECT id, username, password_hash, identity_pubkey, created_at, updated_at
+         FROM users WHERE username = ?",
+    )
+    .bind(username)
+    .fetch_optional(pool)
+    .await?;
+
+    Ok(row)
+}
+
+/// Find a user by UUID.
+pub async fn find_by_id(pool: &SqlitePool, id: Uuid) -> Result<Option<UserRow>> {
+    let row = sqlx::query_as::<_, UserRow>(
+        "SELECT id, username, password_hash, identity_pubkey, created_at, updated_at
+         FROM users WHERE id = ?",
+    )
+    .bind(id.to_string())
+    .fetch_optional(pool)
+    .await?;
+
+    Ok(row)
+}
+
+/// Update (or set for the first time) a user's Ed25519 identity public key.
+/// Called on every login to allow transparent key rotation.
+pub async fn update_identity_key(
+    pool: &SqlitePool,
+    user_id: Uuid,
+    identity_pubkey: &str,
+) -> Result<()> {
+    let now = Utc::now().to_rfc3339();
+    sqlx::query("UPDATE users SET identity_pubkey = ?, updated_at = ? WHERE id = ?")
+        .bind(identity_pubkey)
+        .bind(now)
+        .bind(user_id.to_string())
+        .execute(pool)
+        .await?;
+    Ok(())
+}
+
+/// Check whether a username is already taken.
+pub async fn username_exists(pool: &SqlitePool, username: &str) -> Result<bool> {
+    let count: (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM users WHERE username = ?")
+            .bind(username)
+            .fetch_one(pool)
+            .await?;
+    Ok(count.0 > 0)
+}
+
+/// Assign the built-in `member` role to a newly registered user.
+pub async fn assign_member_role(pool: &SqlitePool, user_id: Uuid) -> Result<()> {
+    let now = Utc::now().to_rfc3339();
+    sqlx::query(
+        "INSERT OR IGNORE INTO user_roles (user_id, role_id, assigned_at) VALUES (?, ?, ?)",
+    )
+    .bind(user_id.to_string())
+    .bind("00000000-0000-0000-0000-000000000002") // member role UUID
+    .bind(now)
+    .execute(pool)
+    .await?;
+    Ok(())
+}

backchannel-server/src/error.rs

@@ -0,0 +1,59 @@
+use thiserror::Error;
+
+pub type Result<T> = std::result::Result<T, ServerError>;
+
+#[derive(Debug, Error)]
+pub enum ServerError {
+    #[error("unauthorized")]
+    Unauthorized,
+
+    #[error("forbidden")]
+    Forbidden,
+
+    #[error("not found: {0}")]
+    NotFound(String),
+
+    #[error("bad request: {0}")]
+    BadRequest(String),
+
+    #[error("connection closed")]
+    ConnectionClosed,
+
+    #[error("database error: {0}")]
+    Database(#[from] sqlx::Error),
+
+    #[error("serialization error: {0}")]
+    Serialization(#[from] serde_json::Error),
+
+    #[error("JWT error: {0}")]
+    Jwt(#[from] jsonwebtoken::errors::Error),
+
+    #[error("password hash error: {0}")]
+    PasswordHash(String),
+
+    #[error("common error: {0}")]
+    Common(#[from] backchannel_common::BackchannelError),
+
+    #[error("internal error: {0}")]
+    Internal(String),
+}
+
+impl From<argon2::password_hash::Error> for ServerError {
+    fn from(e: argon2::password_hash::Error) -> Self {
+        ServerError::PasswordHash(e.to_string())
+    }
+}
+
+/// HTTP-style error code for `ServerMessage::Error`.
+impl ServerError {
+    pub fn code(&self) -> u16 {
+        match self {
+            ServerError::Unauthorized    => 401,
+            ServerError::Forbidden       => 403,
+            ServerError::NotFound(_)     => 404,
+            ServerError::BadRequest(_)   => 400,
+            ServerError::ConnectionClosed => 503,
+            _                            => 500,
+        }
+    }
+}

backchannel-server/src/handlers/auth.rs

@@ -0,0 +1,124 @@
+use std::sync::Arc;
+
+use backchannel_common::protocol::ServerMessage;
+use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation};
+
+use crate::auth::{password, token};
+use crate::db::users;
+use crate::error::{Result, ServerError};
+use crate::state::{AppState, ChannelBroadcast};
+use crate::ws::session::Session;
+
+pub async fn handle_login(
+    session: &mut Session,
+    state: &Arc<AppState>,
+    username: String,
+    password_plain: String,
+    identity_pubkey: String,
+) -> Result<()> {
+    let user = users::find_by_username(&state.db, &username)
+        .await?
+        .ok_or(ServerError::Unauthorized)?;
+
+    password::verify(&password_plain, &user.password_hash)?;
+    let user_id = user.uuid()?;
+
+    // Update identity key on every login to allow transparent key rotation.
+    users::update_identity_key(&state.db, user_id, &identity_pubkey).await?;
+
+    let jwt = token::issue(&state.db, user_id, &username, &state.jwt_secret).await?;
+    let jti = extract_jti(&jwt, &state.jwt_secret)?;
+
+    session.user_id  = Some(user_id);
+    session.username = Some(username.clone());
+    session.jti      = Some(jti);
+
+    state.register_session(session.conn_id, user_id, username.clone(), session.tx.clone()).await;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::UserOnline { user_id, username: username.clone() },
+    });
+
+    session.send(ServerMessage::AuthSuccess { user_id, username, session_token: jwt })
+}
+
+pub async fn handle_register(
+    session: &mut Session,
+    state: &Arc<AppState>,
+    username: String,
+    password_plain: String,
+    identity_pubkey: String,
+) -> Result<()> {
+    if username.len() < 2 || username.len() > 32 {
+        return Err(ServerError::BadRequest("Username must be 2–32 characters".into()));
+    }
+    if password_plain.len() < 8 {
+        return Err(ServerError::BadRequest("Password must be at least 8 characters".into()));
+    }
+    if users::username_exists(&state.db, &username).await? {
+        return Err(ServerError::BadRequest("Username already taken".into()));
+    }
+
+    let hash    = password::hash(&password_plain)?;
+    let user_id = users::create(&state.db, &username, &hash, &identity_pubkey).await?;
+    users::assign_member_role(&state.db, user_id).await?;
+
+    let jwt = token::issue(&state.db, user_id, &username, &state.jwt_secret).await?;
+    let jti = extract_jti(&jwt, &state.jwt_secret)?;
+
+    session.user_id  = Some(user_id);
+    session.username = Some(username.clone());
+    session.jti      = Some(jti);
+
+    state.register_session(session.conn_id, user_id, username.clone(), session.tx.clone()).await;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::UserOnline { user_id, username: username.clone() },
+    });
+
+    session.send(ServerMessage::AuthSuccess { user_id, username, session_token: jwt })
+}
+
+pub async fn handle_resume(
+    session: &mut Session,
+    state: &Arc<AppState>,
+    token_str: String,
+) -> Result<()> {
+    let claims  = token::validate(&state.db, &token_str, &state.jwt_secret).await?;
+    let user_id = claims.user_id()?;
+
+    session.user_id  = Some(user_id);
+    session.username = Some(claims.username.clone());
+    session.jti      = Some(claims.jti.clone());
+
+    state.register_session(session.conn_id, user_id, claims.username.clone(), session.tx.clone()).await;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::UserOnline { user_id, username: claims.username.clone() },
+    });
+
+    session.send(ServerMessage::AuthSuccess {
+        user_id,
+        username:      claims.username,
+        session_token: token_str,
+    })
+}
+
+pub async fn handle_logout(session: &mut Session, state: &Arc<AppState>) -> Result<()> {
+    session.require_auth()?;
+    if let Some(jti) = session.jti.take() {
+        token::revoke(&state.db, &jti).await?;
+    }
+    session.user_id  = None;
+    session.username = None;
+    Ok(())
+}
+
+/// Extract the JTI claim from a JWT without re-running expiry validation.
+/// Used immediately after `token::issue` to store the JTI in the session.
+fn extract_jti(jwt: &str, secret: &[u8]) -> Result<String> {
+    let mut v = Validation::new(Algorithm::HS256);
+    v.validate_exp = false;
+    let data = decode::<token::Claims>(jwt, &DecodingKey::from_secret(secret), &v)?;
+    Ok(data.claims.jti)
+}

backchannel-server/src/handlers/channels.rs

@@ -0,0 +1,107 @@
+use std::sync::Arc;
+
+use backchannel_common::protocol::ServerMessage;
+use backchannel_common::types::{has_permission, PermissionFlags};
+use uuid::Uuid;
+
+use crate::db::{channels, messages, roles};
+use crate::error::{Result, ServerError};
+use crate::state::{AppState, ChannelBroadcast};
+use crate::ws::session::Session;
+
+pub async fn handle_send(
+    session: &Session,
+    state: &Arc<AppState>,
+    channel_id: Uuid,
+    content: String,
+) -> Result<()> {
+    let user_id  = session.require_auth()?;
+    let username = session.require_username()?.to_string();
+
+    channels::find_by_id(&state.db, channel_id)
+        .await?
+        .ok_or_else(|| ServerError::NotFound("Channel not found".into()))?;
+
+    let perms = roles::get_user_permissions(&state.db, user_id).await?;
+    if !has_permission(perms, PermissionFlags::SEND_MESSAGES) {
+        return Err(ServerError::Forbidden);
+    }
+
+    let (message_id, timestamp) =
+        messages::insert_channel_message(&state.db, channel_id, user_id, &content).await?;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::ChannelMessage {
+            message_id,
+            channel_id,
+            author_id: user_id,
+            author_username: username,
+            content,
+            timestamp,
+        },
+    });
+
+    Ok(())
+}
+
+pub async fn handle_history(
+    session: &Session,
+    state: &Arc<AppState>,
+    channel_id: Uuid,
+    before_message_id: Option<Uuid>,
+    limit: u32,
+) -> Result<()> {
+    let user_id = session.require_auth()?;
+    let perms   = roles::get_user_permissions(&state.db, user_id).await?;
+    if !has_permission(perms, PermissionFlags::READ_MESSAGES) {
+        return Err(ServerError::Forbidden);
+    }
+
+    let msgs = messages::fetch_channel_history(&state.db, channel_id, before_message_id, limit).await?;
+    session.send(ServerMessage::ChannelHistory { channel_id, messages: msgs })
+}
+
+pub async fn handle_create(
+    session: &Session,
+    state: &Arc<AppState>,
+    name: String,
+    topic: Option<String>,
+) -> Result<()> {
+    let user_id = session.require_auth()?;
+    let perms   = roles::get_user_permissions(&state.db, user_id).await?;
+    if !has_permission(perms, PermissionFlags::MANAGE_CHANNELS) {
+        return Err(ServerError::Forbidden);
+    }
+    if channels::name_exists(&state.db, &name).await? {
+        return Err(ServerError::BadRequest("Channel name already exists".into()));
+    }
+
+    let channel_id = channels::create(&state.db, &name, topic.as_deref(), user_id).await?;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::ChannelCreated { channel_id, name, topic },
+    });
+
+    Ok(())
+}
+
+pub async fn handle_delete(
+    session: &Session,
+    state: &Arc<AppState>,
+    channel_id: Uuid,
+) -> Result<()> {
+    let user_id = session.require_auth()?;
+    let perms   = roles::get_user_permissions(&state.db, user_id).await?;
+    if !has_permission(perms, PermissionFlags::MANAGE_CHANNELS) {
+        return Err(ServerError::Forbidden);
+    }
+    if !channels::delete(&state.db, channel_id).await? {
+        return Err(ServerError::NotFound("Channel not found".into()));
+    }
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::ChannelDeleted { channel_id },
+    });
+
+    Ok(())
+}

backchannel-server/src/handlers/dms.rs

@@ -0,0 +1,115 @@
+use std::sync::Arc;
+
+use backchannel_common::protocol::ServerMessage;
+use uuid::Uuid;
+
+use crate::db::messages;
+use crate::error::Result;
+use crate::state::AppState;
+use crate::ws::session::Session;
+
+pub async fn handle_init_key_exchange(
+    session: &Session,
+    state: &Arc<AppState>,
+    recipient_id: Uuid,
+    sender_ephemeral_pubkey: String,
+) -> Result<()> {
+    let initiator_id       = session.require_auth()?;
+    let initiator_username = session.require_username()?.to_string();
+
+    // Store pending exchange in AppState (cleared once recipient accepts).
+    state
+        .pending_key_exchanges
+        .lock()
+        .await
+        .insert((initiator_id, recipient_id), sender_ephemeral_pubkey.clone());
+
+    // Relay to recipient if they're currently connected.
+    state
+        .send_to_user(
+            recipient_id,
+            &ServerMessage::DmKeyExchangeRequest {
+                initiator_id,
+                initiator_username,
+                sender_ephemeral_pubkey,
+            },
+        )
+        .await;
+
+    Ok(())
+}
+
+pub async fn handle_accept_key_exchange(
+    session: &Session,
+    state: &Arc<AppState>,
+    initiator_id: Uuid,
+    recipient_ephemeral_pubkey: String,
+) -> Result<()> {
+    let recipient_id       = session.require_auth()?;
+    let recipient_username = session.require_username()?.to_string();
+
+    // Clear the pending exchange.
+    state
+        .pending_key_exchanges
+        .lock()
+        .await
+        .remove(&(initiator_id, recipient_id));
+
+    // Relay response back to initiator.
+    state
+        .send_to_user(
+            initiator_id,
+            &ServerMessage::DmKeyExchangeResponse {
+                recipient_id,
+                recipient_username,
+                recipient_ephemeral_pubkey,
+            },
+        )
+        .await;
+
+    Ok(())
+}
+
+pub async fn handle_send_dm(
+    session: &Session,
+    state: &Arc<AppState>,
+    recipient_id: Uuid,
+    ciphertext: String,
+    nonce: String,
+) -> Result<()> {
+    let sender_id       = session.require_auth()?;
+    let sender_username = session.require_username()?.to_string();
+
+    let (message_id, timestamp) =
+        messages::insert_dm(&state.db, sender_id, recipient_id, &ciphertext, &nonce).await?;
+
+    let msg = ServerMessage::DirectMessage {
+        message_id,
+        sender_id,
+        sender_username,
+        ciphertext,
+        nonce,
+        timestamp,
+    };
+
+    // Deliver to recipient and to sender's other sessions (multi-client sync).
+    state.send_to_user(recipient_id, &msg).await;
+    state.send_to_user(sender_id, &msg).await;
+
+    Ok(())
+}
+
+pub async fn handle_dm_history(
+    session: &Session,
+    state: &Arc<AppState>,
+    peer_id: Uuid,
+    before_message_id: Option<Uuid>,
+    limit: u32,
+) -> Result<()> {
+    let user_id = session.require_auth()?;
+
+    let history =
+        messages::fetch_dm_history(&state.db, user_id, peer_id, before_message_id, limit).await?;
+
+    session.send(ServerMessage::DmHistory { peer_id, messages: history })
+}

backchannel-server/src/handlers/mod.rs

@@ -0,0 +1,4 @@
+pub mod auth;
+pub mod channels;
+pub mod dms;
+pub mod roles;

backchannel-server/src/handlers/roles.rs

@@ -0,0 +1,87 @@
+use std::sync::Arc;
+
+use backchannel_common::protocol::ServerMessage;
+use backchannel_common::types::{has_permission, PermissionFlags};
+use uuid::Uuid;
+
+use crate::db::{keys, roles};
+use crate::error::{Result, ServerError};
+use crate::state::{AppState, ChannelBroadcast};
+use crate::ws::session::Session;
+
+pub async fn handle_create(
+    session: &Session,
+    state: &Arc<AppState>,
+    name: String,
+    permissions: u64,
+) -> Result<()> {
+    let user_id = session.require_auth()?;
+    let perms   = roles::get_user_permissions(&state.db, user_id).await?;
+    if !has_permission(perms, PermissionFlags::MANAGE_ROLES) {
+        return Err(ServerError::Forbidden);
+    }
+
+    let role_id = roles::create(&state.db, &name, permissions).await?;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::RoleCreated { role_id, name, permissions },
+    });
+
+    Ok(())
+}
+
+pub async fn handle_assign(
+    session: &Session,
+    state: &Arc<AppState>,
+    user_id: Uuid,
+    role_id: Uuid,
+) -> Result<()> {
+    let caller_id = session.require_auth()?;
+    let perms     = roles::get_user_permissions(&state.db, caller_id).await?;
+    if !has_permission(perms, PermissionFlags::MANAGE_ROLES) {
+        return Err(ServerError::Forbidden);
+    }
+
+    roles::find_by_id(&state.db, role_id)
+        .await?
+        .ok_or_else(|| ServerError::NotFound("Role not found".into()))?;
+
+    roles::assign(&state.db, user_id, role_id).await?;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::RoleAssigned { user_id, role_id },
+    });
+
+    Ok(())
+}
+
+pub async fn handle_revoke(
+    session: &Session,
+    state: &Arc<AppState>,
+    user_id: Uuid,
+    role_id: Uuid,
+) -> Result<()> {
+    let caller_id = session.require_auth()?;
+    let perms     = roles::get_user_permissions(&state.db, caller_id).await?;
+    if !has_permission(perms, PermissionFlags::MANAGE_ROLES) {
+        return Err(ServerError::Forbidden);
+    }
+
+    roles::revoke(&state.db, user_id, role_id).await?;
+
+    let _ = state.channel_broadcast.send(ChannelBroadcast {
+        message: ServerMessage::RoleRevoked { user_id, role_id },
+    });
+
+    Ok(())
+}
+
+pub async fn handle_query_key(
+    session: &Session,
+    state: &Arc<AppState>,
+    user_id: Uuid,
+) -> Result<()> {
+    session.require_auth()?;
+    let pubkey = keys::get_identity_key(&state.db, user_id).await?;
+    session.send(ServerMessage::IdentityKeyResponse { user_id, identity_pubkey: pubkey })
+}

backchannel-server/src/http/mod.rs

@@ -0,0 +1,3 @@
+mod server;
+
+pub use server::serve;

backchannel-server/src/http/server.rs

@@ -0,0 +1,65 @@
+use anyhow::{Context, Result};
+use axum::body::Body;
+use axum::extract::Path;
+use axum::http::{header, HeaderValue, StatusCode};
+use axum::response::{IntoResponse, Response};
+use axum::routing::get;
+use axum::Router;
+use mime_guess::from_path;
+use rust_embed::RustEmbed;
+
+#[derive(RustEmbed)]
+#[folder = "../backchannel-web/dist"]
+struct WebAssets;
+
+pub async fn serve(bind_addr: &str) -> Result<()> {
+    let app = Router::new()
+        .route("/", get(index))
+        .route("/*path", get(asset_or_index));
+
+    let listener = tokio::net::TcpListener::bind(bind_addr)
+        .await
+        .with_context(|| format!("Failed to bind HTTP server to {bind_addr}"))?;
+
+    tracing::info!("BackChannel web UI listening on http://{bind_addr}");
+    axum::serve(listener, app).await.context("HTTP server failed")
+}
+
+async fn index() -> Response {
+    render_asset("index.html")
+}
+
+async fn asset_or_index(Path(path): Path<String>) -> Response {
+    if path.is_empty() {
+        return render_asset("index.html");
+    }
+
+    render_asset(&path)
+}
+
+fn render_asset(path: &str) -> Response {
+    let canonical = path.trim_start_matches('/');
+
+    if let Some(content) = WebAssets::get(canonical) {
+        return build_response(canonical, content.data.as_ref(), StatusCode::OK);
+    }
+
+    if let Some(index) = WebAssets::get("index.html") {
+        return build_response("index.html", index.data.as_ref(), StatusCode::OK);
+    }
+
+    (StatusCode::NOT_FOUND, "Web UI asset not found").into_response()
+}
+
+fn build_response(path: &str, body: &[u8], status: StatusCode) -> Response {
+    let mime = from_path(path).first_or_octet_stream();
+
+    let mut response = Response::new(Body::from(body.to_vec()));
+    *response.status_mut() = status;
+    response.headers_mut().insert(
+        header::CONTENT_TYPE,
+        HeaderValue::from_str(mime.as_ref()).unwrap_or(HeaderValue::from_static("application/octet-stream")),
+    );
+
+    response
+}

backchannel-server/src/main.rs

@@ -0,0 +1,62 @@
+mod auth;
+mod config;
+mod db;
+mod error;
+mod handlers;
+mod http;
+mod state;
+mod ws;
+
+use std::sync::Arc;
+
+use anyhow::Result;
+use sqlx::sqlite::{SqliteConnectOptions, SqlitePoolOptions};
+use std::str::FromStr;
+use tokio::sync::{broadcast, RwLock, Mutex};
+
+use state::AppState;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| "backchannel_server=debug,info".into()),
+        )
+        .init();
+
+    let config = config::ServerConfig::from_env()?;
+
+    let opts = SqliteConnectOptions::from_str(&config.db_url)?.create_if_missing(true);
+    let pool = SqlitePoolOptions::new()
+        .max_connections(5)
+        .connect_with(opts)
+        .await?;
+
+    sqlx::migrate!("./src/db/migrations").run(&pool).await?;
+    tracing::info!("Database migrations applied");
+
+    let (bc_tx, _) = broadcast::channel(1024);
+
+    let state = Arc::new(AppState {
+        sessions:               RwLock::new(std::collections::HashMap::new()),
+        user_sessions:          RwLock::new(std::collections::HashMap::new()),
+        pending_key_exchanges:  Mutex::new(std::collections::HashMap::new()),
+        channel_broadcast:      bc_tx,
+        db:                     pool,
+        jwt_secret:             config.jwt_secret,
+    });
+
+    tokio::try_join!(
+        ws::listen(
+            Arc::clone(&state),
+            &config.bind_addr,
+            config.tls_enabled,
+            config.tls_cert_path,
+            config.tls_key_path,
+        ),
+        http::serve(&config.http_bind_addr)
+    )?;
+
+    Ok(())
+}

backchannel-server/src/state.rs

@@ -0,0 +1,123 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use backchannel_common::protocol::ServerMessage;
+use tokio::sync::{broadcast, mpsc, Mutex, RwLock};
+use tokio_tungstenite::tungstenite::Message as WsMessage;
+use uuid::Uuid;
+
+/// A handle to send WebSocket frames directly to one connection.
+pub type WsSender = mpsc::UnboundedSender<WsMessage>;
+
+/// State for a single authenticated WebSocket connection.
+pub struct ConnectedSession {
+    pub user_id: Uuid,
+    pub username: String,
+    /// Sending on this channel pushes a frame to the client's write task.
+    pub tx: WsSender,
+}
+
+/// A channel event broadcast to all connected clients.
+///
+/// Channel messages are fanned out via a `tokio::sync::broadcast` channel
+/// so that the handler never has to hold a lock while iterating sessions.
+#[derive(Clone, Debug)]
+pub struct ChannelBroadcast {
+    pub message: ServerMessage,
+}
+
+/// The shared application state. Wrapped in `Arc` and passed to every task.
+pub struct AppState {
+    /// All live WebSocket sessions, keyed by a per-connection UUID.
+    /// One user may hold multiple connections.
+    pub sessions: RwLock<HashMap<Uuid, ConnectedSession>>,
+
+    /// Reverse index: user_id → set of connection UUIDs.
+    /// Allows sending to all clients of a specific user (e.g. for DMs).
+    pub user_sessions: RwLock<HashMap<Uuid, Vec<Uuid>>>,
+
+    /// In-flight DM key-exchange requests.
+    /// Key: (initiator_user_id, recipient_user_id)
+    /// Value: initiator's base64 X25519 ephemeral public key.
+    /// Cleared once the recipient accepts the exchange.
+    pub pending_key_exchanges: Mutex<HashMap<(Uuid, Uuid), String>>,
+
+    /// Server-wide broadcast for channel events. Each connection handler
+    /// calls `.subscribe()` to get its own receiver.
+    pub channel_broadcast: broadcast::Sender<ChannelBroadcast>,
+
+    /// SQLite connection pool.
+    pub db: sqlx::SqlitePool,
+
+    /// Bytes used as the HMAC secret for JWT signing.
+    pub jwt_secret: Vec<u8>,
+}
+
+impl AppState {
+    /// Look up all active sender handles for a given user ID.
+    /// Used when routing DMs or presence events to a specific user.
+    pub async fn senders_for_user(&self, user_id: Uuid) -> Vec<WsSender> {
+        let sessions = self.sessions.read().await;
+        let user_sessions = self.user_sessions.read().await;
+
+        user_sessions
+            .get(&user_id)
+            .map(|conn_ids| {
+                conn_ids
+                    .iter()
+                    .filter_map(|id| sessions.get(id).map(|s| s.tx.clone()))
+                    .collect()
+            })
+            .unwrap_or_default()
+    }
+
+    /// Send a `ServerMessage` to every connection belonging to `user_id`.
+    pub async fn send_to_user(&self, user_id: Uuid, msg: &ServerMessage) {
+        if let Ok(json) = serde_json::to_string(msg) {
+            for tx in self.senders_for_user(user_id).await {
+                let _ = tx.send(WsMessage::Text(json.clone()));
+            }
+        }
+    }
+
+    /// Register a new connection in the session maps.
+    pub async fn register_session(
+        self: &Arc<Self>,
+        conn_id: Uuid,
+        user_id: Uuid,
+        username: String,
+        tx: WsSender,
+    ) {
+        let session = ConnectedSession { user_id, username, tx };
+        self.sessions.write().await.insert(conn_id, session);
+        self.user_sessions
+            .write()
+            .await
+            .entry(user_id)
+            .or_default()
+            .push(conn_id);
+    }
+
+    /// Remove a connection from the session maps.
+    pub async fn remove_session(self: &Arc<Self>, conn_id: Uuid) -> Option<ConnectedSession> {
+        let session = self.sessions.write().await.remove(&conn_id)?;
+        let mut user_sessions = self.user_sessions.write().await;
+        if let Some(ids) = user_sessions.get_mut(&session.user_id) {
+            ids.retain(|id| id != &conn_id);
+            if ids.is_empty() {
+                user_sessions.remove(&session.user_id);
+            }
+        }
+        Some(session)
+    }
+
+    /// Returns true if the given user has at least one active connection.
+    pub async fn is_user_online(&self, user_id: Uuid) -> bool {
+        self.user_sessions
+            .read()
+            .await
+            .get(&user_id)
+            .map(|v| !v.is_empty())
+            .unwrap_or(false)
+    }
+}

backchannel-server/src/ws/handler.rs

@@ -0,0 +1,179 @@
+use std::io::BufReader;
+use std::net::SocketAddr;
+use std::sync::Arc;
+
+use anyhow::{Context, Result};
+use futures_util::{SinkExt, StreamExt};
+use tokio::net::TcpListener;
+use tokio::sync::{broadcast, mpsc};
+use tokio_tungstenite::{accept_async, tungstenite::Message as WsMessage, WebSocketStream};
+
+use backchannel_common::protocol::{ClientMessage, ServerMessage};
+
+use crate::state::{AppState, ChannelBroadcast};
+use crate::ws::{router, session::Session};
+
+/// Bind the TCP listener and accept connections in a loop.
+pub async fn run(
+    state: Arc<AppState>,
+    bind_addr: &str,
+    tls_enabled: bool,
+    tls_cert_path: Option<String>,
+    tls_key_path: Option<String>,
+) -> Result<()> {
+    let listener = TcpListener::bind(bind_addr)
+        .await
+        .with_context(|| format!("Failed to bind to {}", bind_addr))?;
+
+    tracing::info!("BackChannel server listening on ws{}://{}", if tls_enabled { "s" } else { "" }, bind_addr);
+
+    if tls_enabled {
+        let cert_path = tls_cert_path.expect("tls_cert_path required when TLS enabled");
+        let key_path  = tls_key_path.expect("tls_key_path required when TLS enabled");
+        let acceptor = build_tls_acceptor(&cert_path, &key_path)?;
+
+        loop {
+            let (stream, addr) = listener.accept().await?;
+            let state    = Arc::clone(&state);
+            let acceptor = acceptor.clone();
+
+            tokio::spawn(async move {
+                match acceptor.accept(stream).await {
+                    Ok(tls_stream) => match accept_async(tls_stream).await {
+                        Ok(ws)  => handle_ws(ws, state, addr).await,
+                        Err(e)  => tracing::debug!("WS handshake failed from {}: {}", addr, e),
+                    },
+                    Err(e) => tracing::debug!("TLS handshake failed from {}: {}", addr, e),
+                }
+            });
+        }
+    } else {
+        loop {
+            let (stream, addr) = listener.accept().await?;
+            let state = Arc::clone(&state);
+
+            tokio::spawn(async move {
+                match accept_async(stream).await {
+                    Ok(ws)  => handle_ws(ws, state, addr).await,
+                    Err(e)  => tracing::debug!("WS handshake failed from {}: {}", addr, e),
+                }
+            });
+        }
+    }
+}
+
+/// Per-connection WebSocket handler — generic over the underlying stream type
+/// so it works with both plain TCP and TLS.
+async fn handle_ws<S>(ws_stream: WebSocketStream<S>, state: Arc<AppState>, addr: SocketAddr)
+where
+    S: tokio::io::AsyncRead + tokio::io::AsyncWrite + Unpin + Send + 'static,
+{
+    tracing::debug!("Connection established from {}", addr);
+    let (mut ws_sink, mut ws_source) = ws_stream.split();
+
+    // Per-connection mpsc: any task can push to this sender; the write task
+    // drains it to the WebSocket sink.
+    let (tx, mut rx) = mpsc::unbounded_channel::<WsMessage>();
+
+    let mut session = Session::new(tx);
+    let conn_id = session.conn_id;
+
+    // Subscribe to the server-wide channel broadcast before the loop starts.
+    let mut bc_rx = state.channel_broadcast.subscribe();
+
+    // Write task: merges direct messages (mpsc) + broadcast events → WS sink.
+    let write_task = tokio::spawn(async move {
+        loop {
+            tokio::select! {
+                msg = rx.recv() => {
+                    match msg {
+                        Some(m) => { if ws_sink.send(m).await.is_err() { break; } }
+                        None    => break,
+                    }
+                }
+                event = bc_rx.recv() => {
+                    match event {
+                        Ok(ChannelBroadcast { message }) => {
+                            if let Ok(json) = serde_json::to_string(&message) {
+                                if ws_sink.send(WsMessage::Text(json)).await.is_err() { break; }
+                            }
+                        }
+                        Err(broadcast::error::RecvError::Lagged(n)) => {
+                            tracing::warn!("conn {} lagged by {} broadcast messages", conn_id, n);
+                        }
+                        Err(broadcast::error::RecvError::Closed) => break,
+                    }
+                }
+            }
+        }
+    });
+
+    // Read loop: receive, deserialize, dispatch.
+    while let Some(msg_result) = ws_source.next().await {
+        match msg_result {
+            Ok(WsMessage::Text(text)) => {
+                match serde_json::from_str::<ClientMessage>(&text) {
+                    Ok(client_msg) => {
+                        if let Err(e) = router::route(client_msg, &mut session, &state).await {
+                            let _ = session.send(ServerMessage::Error {
+                                code: e.code(),
+                                message: e.to_string(),
+                            });
+                        }
+                    }
+                    Err(_) => {
+                        let _ = session.send(ServerMessage::Error {
+                            code: 400,
+                            message: "Invalid message format".into(),
+                        });
+                    }
+                }
+            }
+            Ok(WsMessage::Close(_)) | Err(_) => break,
+            Ok(_) => {} // Ignore Ping/Pong/Binary frames.
+        }
+    }
+
+    tracing::debug!("Connection closed from {}", addr);
+
+    // Clean up session maps and broadcast offline status.
+    if let Some(removed) = state.remove_session(conn_id).await {
+        if !state.is_user_online(removed.user_id).await {
+            let _ = state.channel_broadcast.send(ChannelBroadcast {
+                message: ServerMessage::UserOffline {
+                    user_id: removed.user_id,
+                    username: removed.username,
+                },
+            });
+        }
+    }
+
+    write_task.abort();
+}
+
+/// Build a `tokio_rustls::TlsAcceptor` from PEM cert and key files.
+fn build_tls_acceptor(cert_path: &str, key_path: &str) -> Result<tokio_rustls::TlsAcceptor> {
+    use rustls::ServerConfig;
+    use rustls_pemfile::{certs, private_key};
+    use std::fs::File;
+
+    let cert_file = File::open(cert_path)
+        .with_context(|| format!("Cannot open TLS cert: {}", cert_path))?;
+    let key_file  = File::open(key_path)
+        .with_context(|| format!("Cannot open TLS key: {}", key_path))?;
+
+    let certs: Vec<_> = certs(&mut BufReader::new(cert_file))
+        .collect::<std::result::Result<_, _>>()
+        .context("Failed to parse TLS certificates")?;
+
+    let key = private_key(&mut BufReader::new(key_file))
+        .context("Failed to parse TLS private key")?
+        .context("No private key found in key file")?;
+
+    let config = ServerConfig::builder()
+        .with_no_client_auth()
+        .with_single_cert(certs, key)
+        .context("Invalid TLS certificate/key combination")?;
+
+    Ok(tokio_rustls::TlsAcceptor::from(Arc::new(config)))
+}

backchannel-server/src/ws/mod.rs

@@ -0,0 +1,18 @@
+pub mod handler;
+pub mod router;
+pub mod session;
+
+use std::sync::Arc;
+use anyhow::Result;
+use crate::state::AppState;
+
+/// Bind the TCP listener and accept WebSocket connections.
+pub async fn listen(
+    state: Arc<AppState>,
+    bind_addr: &str,
+    tls_enabled: bool,
+    tls_cert_path: Option<String>,
+    tls_key_path: Option<String>,
+) -> Result<()> {
+    handler::run(state, bind_addr, tls_enabled, tls_cert_path, tls_key_path).await
+}

backchannel-server/src/ws/router.rs

@@ -0,0 +1,64 @@
+use std::sync::Arc;
+
+use backchannel_common::protocol::{ClientMessage, ServerMessage};
+
+use crate::error::Result;
+use crate::handlers;
+use crate::state::AppState;
+use crate::ws::session::Session;
+
+/// Dispatch a `ClientMessage` to the appropriate handler.
+pub async fn route(msg: ClientMessage, session: &mut Session, state: &Arc<AppState>) -> Result<()> {
+    use ClientMessage::*;
+    match msg {
+        Login { username, password, identity_pubkey } => {
+            handlers::auth::handle_login(session, state, username, password, identity_pubkey).await
+        }
+        Register { username, password, identity_pubkey } => {
+            handlers::auth::handle_register(session, state, username, password, identity_pubkey).await
+        }
+        ResumeSession { token } => {
+            handlers::auth::handle_resume(session, state, token).await
+        }
+        Logout => {
+            handlers::auth::handle_logout(session, state).await
+        }
+        SendChannelMessage { channel_id, content } => {
+            handlers::channels::handle_send(session, state, channel_id, content).await
+        }
+        FetchChannelHistory { channel_id, before_message_id, limit } => {
+            handlers::channels::handle_history(session, state, channel_id, before_message_id, limit).await
+        }
+        CreateChannel { name, topic } => {
+            handlers::channels::handle_create(session, state, name, topic).await
+        }
+        DeleteChannel { channel_id } => {
+            handlers::channels::handle_delete(session, state, channel_id).await
+        }
+        InitDmKeyExchange { recipient_id, sender_ephemeral_pubkey } => {
+            handlers::dms::handle_init_key_exchange(session, state, recipient_id, sender_ephemeral_pubkey).await
+        }
+        AcceptDmKeyExchange { initiator_id, recipient_ephemeral_pubkey } => {
+            handlers::dms::handle_accept_key_exchange(session, state, initiator_id, recipient_ephemeral_pubkey).await
+        }
+        SendDm { recipient_id, ciphertext, nonce } => {
+            handlers::dms::handle_send_dm(session, state, recipient_id, ciphertext, nonce).await
+        }
+        FetchDmHistory { peer_id, before_message_id, limit } => {
+            handlers::dms::handle_dm_history(session, state, peer_id, before_message_id, limit).await
+        }
+        CreateRole { name, permissions } => {
+            handlers::roles::handle_create(session, state, name, permissions).await
+        }
+        AssignRole { user_id, role_id } => {
+            handlers::roles::handle_assign(session, state, user_id, role_id).await
+        }
+        RevokeRole { user_id, role_id } => {
+            handlers::roles::handle_revoke(session, state, user_id, role_id).await
+        }
+        QueryIdentityKey { user_id } => {
+            handlers::roles::handle_query_key(session, state, user_id).await
+        }
+        Ping => session.send(ServerMessage::Pong),
+    }
+}

backchannel-server/src/ws/session.rs

@@ -0,0 +1,45 @@
+use backchannel_common::protocol::ServerMessage;
+use tokio_tungstenite::tungstenite::Message as WsMessage;
+use uuid::Uuid;
+
+use crate::error::{Result, ServerError};
+use crate::state::WsSender;
+
+/// Per-connection mutable state, owned exclusively by the connection's read task.
+pub struct Session {
+    pub conn_id: Uuid,
+    pub user_id: Option<Uuid>,
+    pub username: Option<String>,
+    /// JWT ID stored for revocation on logout.
+    pub jti: Option<String>,
+    /// Sender to the write task for this connection.
+    pub tx: WsSender,
+}
+
+impl Session {
+    pub fn new(tx: WsSender) -> Self {
+        Self {
+            conn_id: Uuid::new_v4(),
+            user_id: None,
+            username: None,
+            jti: None,
+            tx,
+        }
+    }
+
+    /// Serialize and send a `ServerMessage` to this connection's write task.
+    pub fn send(&self, msg: ServerMessage) -> Result<()> {
+        let json = serde_json::to_string(&msg)?;
+        self.tx
+            .send(WsMessage::Text(json))
+            .map_err(|_| ServerError::ConnectionClosed)
+    }
+
+    pub fn require_auth(&self) -> Result<Uuid> {
+        self.user_id.ok_or(ServerError::Unauthorized)
+    }
+
+    pub fn require_username(&self) -> Result<&str> {
+        self.username.as_deref().ok_or(ServerError::Unauthorized)
+    }
+}