rust-web-server 17.26.0

/// Automatic TLS certificate management via ACME (RFC 8555) / Let's Encrypt.
///
/// # Quick start
///
/// Set environment variables and call [`AcmeManager::provision_if_needed`] at
/// startup before starting the TLS server:
///
/// ```bash
/// RWS_CONFIG_ACME_DOMAINS=example.com,www.example.com
/// RWS_CONFIG_ACME_EMAIL=admin@example.com
/// ```
///
/// ```rust,no_run
/// use rust_web_server::acme::{AcmeConfig, AcmeManager};
///
/// # async fn run() {
/// if let Some(cfg) = AcmeConfig::from_env() {
///     let mgr = AcmeManager::new(cfg);
///     mgr.provision_if_needed().await.unwrap();
///     // TLS cert is now at the paths in RWS_CONFIG_TLS_CERT_FILE / KEY_FILE.
///     // Spawn renewal in background:
///     tokio::spawn(mgr.run_renewal_loop());
/// }
/// # }
/// ```
///
/// # Environment variables
///
/// | Variable | Default | Description |
/// |---|---|---|
/// | `RWS_CONFIG_ACME_DOMAINS` | — | Comma-separated domain list (required to activate ACME) |
/// | `RWS_CONFIG_ACME_EMAIL` | — | Contact email sent to the CA |
/// | `RWS_CONFIG_ACME_STAGING` | `false` | `true` = Let's Encrypt staging (for testing) |
/// | `RWS_CONFIG_ACME_DIRECTORY` | LE production URL | Custom ACME directory URL |
/// | `RWS_CONFIG_ACME_CERT_PATH` | `RWS_CONFIG_TLS_CERT_FILE` | Where to write the certificate chain |
/// | `RWS_CONFIG_ACME_KEY_PATH` | `RWS_CONFIG_TLS_KEY_FILE` | Where to write the certificate private key |
/// | `RWS_CONFIG_ACME_CHALLENGE_PORT` | `80` | Port for the HTTP-01 challenge server |
/// | `RWS_CONFIG_ACME_RENEW_BEFORE_DAYS` | `30` | Renew when fewer than this many days remain |
/// | `RWS_CONFIG_ACME_ACCOUNT_KEY_PATH` | `acme_account.key` | Persist the ACME account key |

mod crypto;
mod client;

#[cfg(test)]
mod tests;

use crate::entry_point::Config;
use client::AcmeHttpClient;
use crypto::AccountKey;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

// ── Config ────────────────────────────────────────────────────────────────────

/// ACME provisioning configuration.
#[derive(Clone, Debug)]
pub struct AcmeConfig {
    /// DNS names to include in the certificate (first entry is CN).
    pub domains: Vec<String>,
    /// Contact email address for the CA.
    pub email: String,
    /// ACME directory URL. Defaults to Let's Encrypt production.
    pub directory_url: String,
    /// Path to write the provisioned certificate chain (PEM).
    pub cert_path: String,
    /// Path to write the certificate's private key (PEM).
    pub key_path: String,
    /// Port for the temporary HTTP-01 challenge server (default 80).
    pub challenge_port: u16,
    /// Renew the certificate when fewer than this many days remain.
    pub renew_before_days: i64,
    /// Path to persist the ACME account key between restarts.
    pub account_key_path: String,
}

impl AcmeConfig {
    /// Let's Encrypt production ACME directory.
    pub const LETSENCRYPT: &'static str =
        "https://acme-v02.api.letsencrypt.org/directory";
    /// Let's Encrypt staging ACME directory (no real certificates; for testing).
    pub const LETSENCRYPT_STAGING: &'static str =
        "https://acme-staging-v02.api.letsencrypt.org/directory";

    /// Build config from environment variables. Returns `None` when
    /// `RWS_CONFIG_ACME_DOMAINS` is not set or empty.
    pub fn from_env() -> Option<Self> {
        let domains_str = std::env::var(Config::RWS_CONFIG_ACME_DOMAINS).ok()?;
        if domains_str.trim().is_empty() { return None; }

        let domains: Vec<String> = domains_str
            .split(',')
            .map(|s| s.trim().to_string())
            .filter(|s| !s.is_empty())
            .collect();
        if domains.is_empty() { return None; }

        let email = std::env::var(Config::RWS_CONFIG_ACME_EMAIL).unwrap_or_default();
        let staging = std::env::var(Config::RWS_CONFIG_ACME_STAGING)
            .map(|v| v.eq_ignore_ascii_case("true"))
            .unwrap_or(false);
        let directory_url = if staging {
            Self::LETSENCRYPT_STAGING.to_string()
        } else {
            std::env::var(Config::RWS_CONFIG_ACME_DIRECTORY)
                .unwrap_or_else(|_| Self::LETSENCRYPT.to_string())
        };

        let cert_path = std::env::var(Config::RWS_CONFIG_ACME_CERT_PATH)
            .unwrap_or_else(|_| {
                std::env::var(Config::RWS_CONFIG_TLS_CERT_FILE)
                    .unwrap_or_else(|_| "cert.pem".to_string())
            });
        let key_path = std::env::var(Config::RWS_CONFIG_ACME_KEY_PATH)
            .unwrap_or_else(|_| {
                std::env::var(Config::RWS_CONFIG_TLS_KEY_FILE)
                    .unwrap_or_else(|_| "key.pem".to_string())
            });

        let challenge_port = std::env::var(Config::RWS_CONFIG_ACME_CHALLENGE_PORT)
            .ok()
            .and_then(|v| v.parse().ok())
            .unwrap_or(80u16);

        let renew_before_days = std::env::var(Config::RWS_CONFIG_ACME_RENEW_BEFORE_DAYS)
            .ok()
            .and_then(|v| v.parse().ok())
            .unwrap_or(30i64);

        let account_key_path = std::env::var(Config::RWS_CONFIG_ACME_ACCOUNT_KEY_PATH)
            .unwrap_or_else(|_| "acme_account.key".to_string());

        Some(AcmeConfig {
            domains,
            email,
            directory_url,
            cert_path,
            key_path,
            challenge_port,
            renew_before_days,
            account_key_path,
        })
    }
}

// ── Manager ───────────────────────────────────────────────────────────────────

/// Manages the ACME lifecycle: initial provisioning and automatic renewal.
pub struct AcmeManager {
    config: AcmeConfig,
}

impl AcmeManager {
    pub fn new(config: AcmeConfig) -> Self {
        AcmeManager { config }
    }

    /// Provision a certificate only if none exists or it expires soon.
    pub async fn provision_if_needed(&self) -> Result<(), String> {
        match crypto::cert_days_until_expiry(&self.config.cert_path) {
            Some(d) if d > self.config.renew_before_days => {
                println!(
                    "[ACME] Certificate at '{}' is valid for {d} more days — no action needed.",
                    self.config.cert_path
                );
                return Ok(());
            }
            Some(d) => {
                println!("[ACME] Certificate expires in {d} days, renewing...");
            }
            None => {
                println!(
                    "[ACME] No valid certificate found at '{}', provisioning...",
                    self.config.cert_path
                );
            }
        }
        self.provision().await
    }

    /// Provision a new certificate unconditionally.
    pub async fn provision(&self) -> Result<(), String> {
        println!("[ACME] Provisioning certificate for: {:?}", self.config.domains);

        let http = AcmeHttpClient::new()?;

        // 1. Load or generate account key.
        let account_key = self.load_or_create_account_key()?;

        // 2. Fetch ACME directory.
        let dir = fetch_directory(&http, &self.config.directory_url).await?;

        // 3. Get initial nonce.
        let nonce = get_nonce(&http, &dir.new_nonce_url).await?;

        // 4. Create/find account; get account URL and fresh nonce.
        let (account_url, nonce) =
            create_account(&http, &account_key, &dir.new_account_url, &self.config.email, &nonce).await?;
        println!("[ACME] Account: {account_url}");

        // 5. Submit order.
        let (order, order_url, nonce) =
            new_order(&http, &account_key, &account_url, &dir.new_order_url, &self.config.domains, &nonce).await?;
        println!("[ACME] Order: {order_url}");

        // 6. Complete HTTP-01 challenge for each authorization.
        let mut nonce = nonce;
        for authz_url in &order.authorizations {
            nonce = self.complete_authorization(&http, &account_key, &account_url, authz_url, &nonce).await?;
        }

        // 7. Generate certificate key + CSR.
        let cert_key = rcgen::KeyPair::generate_for(&rcgen::PKCS_ECDSA_P256_SHA256)
            .map_err(|e| format!("cert key generation failed: {e}"))?;
        let mut params = rcgen::CertificateParams::new(self.config.domains.clone())
            .map_err(|e| format!("CSR params error: {e}"))?;
        // Let's Encrypt sets validity; override subject to be clean.
        params.distinguished_name = rcgen::DistinguishedName::new();
        let csr = params.serialize_request(&cert_key)
            .map_err(|e| format!("CSR serialize error: {e}"))?;
        let csr_b64 = crypto::base64url(csr.der());

        // 8. Finalize order with CSR.
        let nonce = finalize_order(&http, &account_key, &account_url, &order.finalize_url, &csr_b64, &nonce).await?;

        // 9. Poll order until certificate URL is available.
        let (cert_url, nonce) = poll_order(&http, &account_key, &account_url, &order_url, &nonce).await?;
        println!("[ACME] Certificate URL: {cert_url}");

        // 10. Download certificate chain.
        let cert_pem = download_cert(&http, &account_key, &account_url, &cert_url, &nonce).await?;

        // 11. Write cert + key.
        std::fs::write(&self.config.cert_path, &cert_pem)
            .map_err(|e| format!("failed to write cert to '{}': {e}", self.config.cert_path))?;
        std::fs::write(&self.config.key_path, cert_key.serialize_pem())
            .map_err(|e| format!("failed to write key to '{}': {e}", self.config.key_path))?;

        println!(
            "[ACME] Certificate written to '{}' and '{}'.",
            self.config.cert_path, self.config.key_path
        );
        Ok(())
    }

    /// Background renewal loop. Checks every 12 hours; renews when fewer than
    /// `renew_before_days` remain. After successful renewal, sends SIGHUP to
    /// self so the running server reloads the TLS certificate.
    pub async fn run_renewal_loop(self) {
        loop {
            tokio::time::sleep(tokio::time::Duration::from_secs(12 * 3600)).await;
            match self.provision_if_needed().await {
                Ok(()) => {
                    #[cfg(unix)]
                    {
                        // Signal the process to reload TLS cert (run_tls reloads on SIGHUP).
                        unsafe { libc::kill(libc::getpid(), libc::SIGHUP); }
                    }
                }
                Err(e) => eprintln!("[ACME] Renewal failed: {e}"),
            }
        }
    }

    // ── private helpers ───────────────────────────────────────────────────────

    fn load_or_create_account_key(&self) -> Result<AccountKey, String> {
        let path = &self.config.account_key_path;
        if let Ok(der) = std::fs::read(path) {
            let key = AccountKey::from_pkcs8(&der)
                .map_err(|e| format!("failed to load account key from '{path}': {e}"))?;
            println!("[ACME] Loaded account key from '{path}'.");
            return Ok(key);
        }
        let (key, der) = AccountKey::generate()
            .map_err(|e| format!("account key generation failed: {e}"))?;
        std::fs::write(path, &der)
            .map_err(|e| format!("failed to save account key to '{path}': {e}"))?;
        println!("[ACME] Generated new account key at '{path}'.");
        Ok(key)
    }

    async fn complete_authorization(
        &self,
        http: &AcmeHttpClient,
        key: &AccountKey,
        account_url: &str,
        authz_url: &str,
        nonce: &str,
    ) -> Result<String, String> {
        let (authz, nonce) = get_authorization(http, key, account_url, authz_url, nonce).await?;

        if authz.status == "valid" {
            return Ok(nonce);
        }

        let challenge = authz.challenges.iter()
            .find(|c| c.challenge_type == "http-01")
            .ok_or_else(|| format!("no HTTP-01 challenge in authorization {authz_url}"))?;

        let key_auth = format!(
            "{}.{}",
            challenge.token,
            crypto::key_thumbprint(key)?
        );

        println!("[ACME] Starting HTTP-01 challenge server on port {}...", self.config.challenge_port);

        // Start challenge server.
        let (shutdown_tx, shutdown_rx) = tokio::sync::oneshot::channel::<()>();
        let token = challenge.token.clone();
        let ka = key_auth.clone();
        let port = self.config.challenge_port;
        let handle = tokio::spawn(async move {
            if let Err(e) = run_challenge_server(port, token, ka, shutdown_rx).await {
                eprintln!("[ACME] Challenge server error: {e}");
            }
        });

        // Small delay to let the server bind before signalling ACME.
        tokio::time::sleep(tokio::time::Duration::from_millis(200)).await;

        // Signal challenge ready.
        let nonce = match signal_challenge(http, key, account_url, &challenge.url, &nonce).await {
            Ok(n) => n,
            Err(e) => {
                let _ = shutdown_tx.send(());
                let _ = handle.await;
                return Err(e);
            }
        };

        // Poll for validation (up to 60 seconds).
        let poll_result = poll_authorization_valid(http, key, account_url, authz_url, &nonce).await;

        // Stop the challenge server regardless of poll outcome.
        let _ = shutdown_tx.send(());
        let _ = handle.await;

        let nonce = poll_result?;

        println!("[ACME] Authorization validated for '{}'.", authz.identifier_value);
        Ok(nonce)
    }
}

// ── ACME protocol functions ───────────────────────────────────────────────────

struct Directory {
    new_nonce_url: String,
    new_account_url: String,
    new_order_url: String,
}

struct Order {
    authorizations: Vec<String>,
    finalize_url: String,
}

struct Authorization {
    status: String,
    identifier_value: String,
    challenges: Vec<Challenge>,
}

struct Challenge {
    challenge_type: String,
    url: String,
    token: String,
}

async fn fetch_directory(http: &AcmeHttpClient, url: &str) -> Result<Directory, String> {
    let resp = http.get(url).await?;
    if resp.status != 200 {
        return Err(format!("directory fetch failed: HTTP {}: {}", resp.status, resp.body));
    }
    let new_nonce_url = json_str(&resp.body, "newNonce")
        .ok_or("directory missing newNonce")?;
    let new_account_url = json_str(&resp.body, "newAccount")
        .ok_or("directory missing newAccount")?;
    let new_order_url = json_str(&resp.body, "newOrder")
        .ok_or("directory missing newOrder")?;
    Ok(Directory { new_nonce_url, new_account_url, new_order_url })
}

async fn get_nonce(http: &AcmeHttpClient, url: &str) -> Result<String, String> {
    let resp = http.head(url).await?;
    resp.nonce().ok_or_else(|| "Replay-Nonce missing from newNonce response".to_string())
}

async fn create_account(
    http: &AcmeHttpClient,
    key: &AccountKey,
    url: &str,
    email: &str,
    nonce: &str,
) -> Result<(String, String), String> {
    let payload = if email.is_empty() {
        r#"{"termsOfServiceAgreed":true}"#.to_string()
    } else {
        format!(r#"{{"termsOfServiceAgreed":true,"contact":["mailto:{email}"]}}"#)
    };
    let body = crypto::build_jws(key, nonce, url, None, Some(&payload))?;
    let resp = http.post_jws(url, &body).await?;
    if resp.status != 200 && resp.status != 201 {
        return Err(format!("newAccount failed: HTTP {}: {}", resp.status, resp.body));
    }
    let account_url = resp.location()
        .ok_or("newAccount response missing Location header")?;
    let nonce = resp.nonce()
        .ok_or("newAccount response missing Replay-Nonce")?;
    Ok((account_url, nonce))
}

async fn new_order(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    url: &str,
    domains: &[String],
    nonce: &str,
) -> Result<(Order, String, String), String> {
    let ids: Vec<String> = domains.iter()
        .map(|d| format!(r#"{{"type":"dns","value":"{d}"}}"#))
        .collect();
    let payload = format!(r#"{{"identifiers":[{}]}}"#, ids.join(","));
    let body = crypto::build_jws(key, nonce, url, Some(account_url), Some(&payload))?;
    let resp = http.post_jws(url, &body).await?;
    if resp.status != 201 {
        return Err(format!("newOrder failed: HTTP {}: {}", resp.status, resp.body));
    }
    let order_url = resp.location()
        .ok_or("newOrder response missing Location header")?;
    let nonce = resp.nonce()
        .ok_or("newOrder response missing Replay-Nonce")?;
    let authorizations = json_array_strings(&resp.body, "authorizations");
    let finalize_url = json_str(&resp.body, "finalize")
        .ok_or("newOrder response missing finalize URL")?;
    Ok((Order { authorizations, finalize_url }, order_url, nonce))
}

async fn get_authorization(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    authz_url: &str,
    nonce: &str,
) -> Result<(Authorization, String), String> {
    // POST-as-GET (empty payload)
    let body = crypto::build_jws(key, nonce, authz_url, Some(account_url), None)?;
    let resp = http.post_jws(authz_url, &body).await?;
    if resp.status != 200 {
        return Err(format!("authorization GET failed: HTTP {}: {}", resp.status, resp.body));
    }
    let nonce = resp.nonce().unwrap_or_default();
    let status = json_str(&resp.body, "status").unwrap_or_default();
    let identifier_value = json_str(&resp.body, "value").unwrap_or_default();
    let challenges = parse_challenges(&resp.body);
    Ok((Authorization { status, identifier_value, challenges }, nonce))
}

async fn signal_challenge(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    challenge_url: &str,
    nonce: &str,
) -> Result<String, String> {
    let body = crypto::build_jws(key, nonce, challenge_url, Some(account_url), Some("{}"))?;
    let resp = http.post_jws(challenge_url, &body).await?;
    if resp.status != 200 {
        return Err(format!("challenge signal failed: HTTP {}: {}", resp.status, resp.body));
    }
    Ok(resp.nonce().unwrap_or_default())
}

async fn poll_authorization_valid(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    authz_url: &str,
    start_nonce: &str,
) -> Result<String, String> {
    let mut nonce = start_nonce.to_string();
    for attempt in 0..20 {
        tokio::time::sleep(tokio::time::Duration::from_secs(3)).await;
        // If nonce is empty, fetch a fresh one from the authz URL
        let body = crypto::build_jws(key, &nonce, authz_url, Some(account_url), None)?;
        let resp = http.post_jws(authz_url, &body).await?;
        if let Some(n) = resp.nonce() { nonce = n; }
        let status = json_str(&resp.body, "status").unwrap_or_default();
        match status.as_str() {
            "valid" => return Ok(nonce),
            "invalid" => {
                let err = json_str(&resp.body, "detail").unwrap_or_else(|| resp.body.clone());
                return Err(format!("authorization became invalid: {err}"));
            }
            _ => {
                if attempt == 19 {
                    return Err(format!("authorization timed out with status '{status}'"));
                }
            }
        }
    }
    Err("authorization polling timed out".to_string())
}

async fn finalize_order(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    finalize_url: &str,
    csr_b64: &str,
    nonce: &str,
) -> Result<String, String> {
    let payload = format!(r#"{{"csr":"{csr_b64}"}}"#);
    let body = crypto::build_jws(key, nonce, finalize_url, Some(account_url), Some(&payload))?;
    let resp = http.post_jws(finalize_url, &body).await?;
    if resp.status != 200 {
        return Err(format!("finalize failed: HTTP {}: {}", resp.status, resp.body));
    }
    Ok(resp.nonce().unwrap_or_default())
}

async fn poll_order(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    order_url: &str,
    start_nonce: &str,
) -> Result<(String, String), String> {
    let mut nonce = start_nonce.to_string();
    for attempt in 0..20 {
        tokio::time::sleep(tokio::time::Duration::from_secs(3)).await;
        let body = crypto::build_jws(key, &nonce, order_url, Some(account_url), None)?;
        let resp = http.post_jws(order_url, &body).await?;
        if let Some(n) = resp.nonce() { nonce = n; }
        let status = json_str(&resp.body, "status").unwrap_or_default();
        match status.as_str() {
            "valid" => {
                let cert_url = json_str(&resp.body, "certificate")
                    .ok_or("order is valid but missing certificate URL")?;
                return Ok((cert_url, nonce));
            }
            "invalid" => {
                return Err(format!("order became invalid: {}", resp.body));
            }
            _ => {
                if attempt == 19 {
                    return Err(format!("order timed out with status '{status}'"));
                }
            }
        }
    }
    Err("order polling timed out".to_string())
}

async fn download_cert(
    http: &AcmeHttpClient,
    key: &AccountKey,
    account_url: &str,
    cert_url: &str,
    nonce: &str,
) -> Result<String, String> {
    let body = crypto::build_jws(key, nonce, cert_url, Some(account_url), None)?;
    let resp = http.post_jws(cert_url, &body).await?;
    if resp.status != 200 {
        return Err(format!("cert download failed: HTTP {}: {}", resp.status, resp.body));
    }
    Ok(resp.body)
}

// ── HTTP-01 challenge server ──────────────────────────────────────────────────

async fn run_challenge_server(
    port: u16,
    token: String,
    key_auth: String,
    shutdown: tokio::sync::oneshot::Receiver<()>,
) -> Result<(), String> {
    use tokio::net::TcpListener;
    let listener = TcpListener::bind(format!("0.0.0.0:{port}"))
        .await
        .map_err(|e| format!("challenge server: bind port {port} failed: {e}"))?;

    tokio::select! {
        _ = shutdown => {}
        _ = serve_challenges(listener, token, key_auth) => {}
    }
    Ok(())
}

async fn serve_challenges(
    listener: tokio::net::TcpListener,
    token: String,
    key_auth: String,
) {
    loop {
        let Ok((mut stream, _)) = listener.accept().await else { break };
        let t = token.clone();
        let ka = key_auth.clone();
        tokio::spawn(async move {
            let mut buf = [0u8; 1024];
            let _ = stream.read(&mut buf).await;
            let req = String::from_utf8_lossy(&buf);
            let target = format!("/.well-known/acme-challenge/{}", t);
            let response = if req.contains(&target) {
                format!(
                    "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nContent-Length: {}\r\nConnection: close\r\n\r\n{}",
                    ka.len(), ka
                )
            } else {
                "HTTP/1.1 404 Not Found\r\nContent-Length: 0\r\nConnection: close\r\n\r\n".to_string()
            };
            let _ = stream.write_all(response.as_bytes()).await;
        });
    }
}

// ── minimal JSON field extractor ──────────────────────────────────────────────

fn json_str(json: &str, field: &str) -> Option<String> {
    let key = format!("\"{}\"", field);
    let start = json.find(&key)? + key.len();
    let rest = json[start..].trim_start_matches([' ', ':'].as_ref()).trim_start();
    if rest.starts_with('"') {
        let inner = &rest[1..];
        let end = inner.find('"')?;
        Some(inner[..end].to_string())
    } else {
        None
    }
}

fn json_array_strings(json: &str, field: &str) -> Vec<String> {
    let key = format!("\"{}\"", field);
    let start = match json.find(&key) {
        Some(s) => s + key.len(),
        None => return vec![],
    };
    let rest = json[start..].trim_start_matches([' ', ':'].as_ref()).trim_start();
    if !rest.starts_with('[') { return vec![]; }
    let arr_end = rest.find(']').unwrap_or(rest.len());
    let arr = &rest[1..arr_end];
    let mut result = Vec::new();
    let mut pos = 0;
    while let Some(q) = arr[pos..].find('"') {
        let abs = pos + q + 1;
        if let Some(end) = arr[abs..].find('"') {
            result.push(arr[abs..abs + end].to_string());
            pos = abs + end + 1;
        } else {
            break;
        }
    }
    result
}

fn parse_challenges(json: &str) -> Vec<Challenge> {
    // Find all objects in the "challenges" array.
    let marker = "\"challenges\"";
    let start = match json.find(marker) {
        Some(s) => s + marker.len(),
        None => return vec![],
    };
    let rest = json[start..].trim_start_matches([' ', ':'].as_ref()).trim_start();
    if !rest.starts_with('[') { return vec![]; }

    let mut result = Vec::new();
    let mut pos = 1usize; // skip '['
    while pos < rest.len() {
        let Some(obj_start) = rest[pos..].find('{') else { break };
        let abs = pos + obj_start;
        // Find the matching closing brace.
        let mut depth = 0usize;
        let mut obj_end = abs;
        for (i, ch) in rest[abs..].char_indices() {
            match ch {
                '{' => depth += 1,
                '}' => {
                    depth -= 1;
                    if depth == 0 { obj_end = abs + i + 1; break; }
                }
                _ => {}
            }
        }
        if obj_end <= abs { break; }
        let obj = &rest[abs..obj_end];
        if let (Some(ct), Some(url), Some(token)) = (
            json_str(obj, "type"),
            json_str(obj, "url"),
            json_str(obj, "token"),
        ) {
            result.push(Challenge { challenge_type: ct, url, token });
        }
        pos = obj_end;
    }
    result
}