Verify native Dosh Ed25519 user auth

src/native.rs

@@ -3,7 +3,7 @@ use crate::config::{ServerConfig, expand_tilde};
 use crate::crypto;
 use anyhow::{Context, Result, bail};
 use base64::Engine;
-use base64::engine::general_purpose::URL_SAFE_NO_PAD;
+use base64::engine::general_purpose::{STANDARD, URL_SAFE_NO_PAD};
 use ed25519_dalek::{Signature, Signer, SigningKey, Verifier, VerifyingKey};
 use serde::{Deserialize, Serialize};
 use std::fs;
@@ -95,6 +95,24 @@ pub struct NativeAuthOk {
     pub policy_flags: Vec<String>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct AuthorizedKey {
+    pub algorithm: String,
+    pub key: Vec<u8>,
+    pub options: AuthorizedKeyOptions,
+    pub comment: Option<String>,
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct AuthorizedKeyOptions {
+    pub from: Option<String>,
+    pub force_command: Option<String>,
+    pub restricted: bool,
+    pub no_port_forwarding: bool,
+    pub permitopen: Vec<String>,
+    pub unsupported: Vec<String>,
+}
+
 pub fn load_or_create_host_key(config: &ServerConfig) -> Result<SigningKey> {
     let path = expand_tilde(&config.host_key);
     if path.exists() {
@@ -233,6 +251,296 @@ pub fn verify_server_hello(client: &NativeClientHello, server: &NativeServerHell
     verify_host_signature(&server.host_key, &transcript, &server.host_signature)
 }
 
+pub fn user_auth_transcript(
+    client: &NativeClientHello,
+    server: &NativeServerHello,
+    auth: &NativeUserAuth,
+) -> Result<Vec<u8>> {
+    let mut unsigned = auth.clone();
+    unsigned.signature.clear();
+    let mut out = b"dosh/native/user-auth/v1".to_vec();
+    out.extend(bincode::serialize(client)?);
+    out.extend(bincode::serialize(server)?);
+    out.extend(bincode::serialize(&unsigned)?);
+    Ok(out)
+}
+
+pub fn sign_user_auth(
+    signing_key: &SigningKey,
+    client: &NativeClientHello,
+    server: &NativeServerHello,
+    requested_forwardings: Vec<ForwardingRequest>,
+) -> Result<NativeUserAuth> {
+    let verifying = VerifyingKey::from(signing_key);
+    let mut auth = NativeUserAuth {
+        public_key_algorithm: "ssh-ed25519".to_string(),
+        public_key: verifying.to_bytes().to_vec(),
+        signature: Vec::new(),
+        requested_forwardings,
+    };
+    let transcript = user_auth_transcript(client, server, &auth)?;
+    auth.signature = signing_key.sign(&transcript).to_bytes().to_vec();
+    Ok(auth)
+}
+
+pub fn verify_native_user_auth(
+    client: &NativeClientHello,
+    server: &NativeServerHello,
+    auth: &NativeUserAuth,
+    authorized_keys: &[AuthorizedKey],
+) -> Result<AuthorizedKey> {
+    anyhow::ensure!(
+        auth.public_key_algorithm == "ssh-ed25519",
+        "unsupported native user key algorithm {}",
+        auth.public_key_algorithm
+    );
+    anyhow::ensure!(
+        auth.public_key.len() == 32,
+        "ssh-ed25519 public key must be 32 bytes"
+    );
+    anyhow::ensure!(
+        auth.signature.len() == 64,
+        "ssh-ed25519 signature must be 64 bytes"
+    );
+    let authorized = authorized_keys
+        .iter()
+        .find(|key| key.algorithm == "ssh-ed25519" && key.key == auth.public_key)
+        .ok_or_else(|| anyhow::anyhow!("native user key is not authorized"))?;
+    authorized.ensure_native_allowed(auth)?;
+
+    let mut public_key = [0u8; 32];
+    public_key.copy_from_slice(&auth.public_key);
+    let verifying_key = VerifyingKey::from_bytes(&public_key).context("parse user public key")?;
+    let mut signature = [0u8; 64];
+    signature.copy_from_slice(&auth.signature);
+    let signature = Signature::from_bytes(&signature);
+    let transcript = user_auth_transcript(client, server, auth)?;
+    verifying_key
+        .verify(&transcript, &signature)
+        .context("verify native user signature")?;
+    Ok(authorized.clone())
+}
+
+pub fn verify_native_user_auth_from_config(
+    config: &ServerConfig,
+    client: &NativeClientHello,
+    server: &NativeServerHello,
+    auth: &NativeUserAuth,
+) -> Result<AuthorizedKey> {
+    let authorized_keys = load_authorized_keys(&config.authorized_keys)?;
+    verify_native_user_auth(client, server, auth, &authorized_keys)
+}
+
+impl AuthorizedKey {
+    fn ensure_native_allowed(&self, auth: &NativeUserAuth) -> Result<()> {
+        if !self.options.unsupported.is_empty() {
+            bail!(
+                "authorized key has unsupported options: {}",
+                self.options.unsupported.join(",")
+            );
+        }
+        if self.options.force_command.is_some() {
+            bail!("authorized key command= is not supported for native Dosh terminal login");
+        }
+        if self.options.no_port_forwarding
+            && auth
+                .requested_forwardings
+                .iter()
+                .any(|forwarding| !matches!(forwarding.kind, ForwardingKind::Dynamic))
+        {
+            bail!("authorized key forbids port forwarding");
+        }
+        if !self.options.permitopen.is_empty() {
+            for forwarding in &auth.requested_forwardings {
+                if matches!(forwarding.kind, ForwardingKind::Local)
+                    && let (Some(host), Some(port)) =
+                        (forwarding.target_host.as_ref(), forwarding.target_port)
+                {
+                    let target = format!("{host}:{port}");
+                    if !self
+                        .options
+                        .permitopen
+                        .iter()
+                        .any(|permit| permit == &target)
+                    {
+                        bail!("authorized key does not permit opening {target}");
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+pub fn load_authorized_keys(paths: &[String]) -> Result<Vec<AuthorizedKey>> {
+    let mut out = Vec::new();
+    for path in paths {
+        let path = expand_tilde(path);
+        if !path.exists() {
+            continue;
+        }
+        let raw = fs::read_to_string(&path).with_context(|| format!("read {}", path.display()))?;
+        out.extend(
+            parse_authorized_keys(&raw).with_context(|| format!("parse {}", path.display()))?,
+        );
+    }
+    Ok(out)
+}
+
+pub fn parse_authorized_keys(raw: &str) -> Result<Vec<AuthorizedKey>> {
+    raw.lines()
+        .enumerate()
+        .filter_map(|(index, line)| {
+            let line = line.trim();
+            if line.is_empty() || line.starts_with('#') {
+                None
+            } else {
+                Some(parse_authorized_key_line(index + 1, line))
+            }
+        })
+        .collect()
+}
+
+fn parse_authorized_key_line(line_number: usize, line: &str) -> Result<AuthorizedKey> {
+    let fields = split_authorized_key_fields(line);
+    anyhow::ensure!(
+        fields.len() >= 2,
+        "authorized_keys:{line_number}: expected key fields"
+    );
+    let (options, key_type_index) = if fields[0].starts_with("ssh-") {
+        (AuthorizedKeyOptions::default(), 0)
+    } else {
+        (parse_authorized_key_options(&fields[0])?, 1)
+    };
+    let algorithm = fields
+        .get(key_type_index)
+        .with_context(|| format!("authorized_keys:{line_number}: missing key type"))?;
+    let key_blob = fields
+        .get(key_type_index + 1)
+        .with_context(|| format!("authorized_keys:{line_number}: missing key blob"))?;
+    anyhow::ensure!(
+        algorithm == "ssh-ed25519",
+        "authorized_keys:{line_number}: unsupported key type {algorithm}"
+    );
+    let decoded = STANDARD
+        .decode(key_blob)
+        .with_context(|| format!("authorized_keys:{line_number}: decode key blob"))?;
+    let key = parse_ssh_ed25519_public_blob(&decoded)
+        .with_context(|| format!("authorized_keys:{line_number}: parse ssh-ed25519 key"))?;
+    let comment = if fields.len() > key_type_index + 2 {
+        Some(fields[key_type_index + 2..].join(" "))
+    } else {
+        None
+    };
+    Ok(AuthorizedKey {
+        algorithm: algorithm.to_string(),
+        key: key.to_vec(),
+        options,
+        comment,
+    })
+}
+
+fn split_authorized_key_fields(line: &str) -> Vec<String> {
+    line.split_whitespace().map(ToString::to_string).collect()
+}
+
+fn parse_authorized_key_options(raw: &str) -> Result<AuthorizedKeyOptions> {
+    let mut options = AuthorizedKeyOptions::default();
+    for option in split_options(raw)? {
+        if option == "restrict" {
+            options.restricted = true;
+        } else if option == "no-port-forwarding" {
+            options.no_port_forwarding = true;
+        } else if let Some(value) = option.strip_prefix("from=") {
+            options.from = Some(strip_quotes(value).to_string());
+        } else if let Some(value) = option.strip_prefix("command=") {
+            options.force_command = Some(strip_quotes(value).to_string());
+        } else if let Some(value) = option.strip_prefix("permitopen=") {
+            options.permitopen.push(strip_quotes(value).to_string());
+        } else {
+            options.unsupported.push(option);
+        }
+    }
+    Ok(options)
+}
+
+fn split_options(raw: &str) -> Result<Vec<String>> {
+    let mut out = Vec::new();
+    let mut current = String::new();
+    let mut quoted = false;
+    let mut escaped = false;
+    for ch in raw.chars() {
+        if escaped {
+            current.push(ch);
+            escaped = false;
+            continue;
+        }
+        if ch == '\\' && quoted {
+            escaped = true;
+            continue;
+        }
+        if ch == '"' {
+            quoted = !quoted;
+            current.push(ch);
+            continue;
+        }
+        if ch == ',' && !quoted {
+            out.push(current);
+            current = String::new();
+            continue;
+        }
+        current.push(ch);
+    }
+    anyhow::ensure!(!quoted, "unterminated authorized_keys option quote");
+    if !current.is_empty() {
+        out.push(current);
+    }
+    Ok(out)
+}
+
+fn strip_quotes(raw: &str) -> &str {
+    raw.strip_prefix('"')
+        .and_then(|value| value.strip_suffix('"'))
+        .unwrap_or(raw)
+}
+
+fn parse_ssh_ed25519_public_blob(blob: &[u8]) -> Result<[u8; 32]> {
+    let mut cursor = blob;
+    let key_type = read_ssh_string(&mut cursor)?;
+    anyhow::ensure!(key_type == b"ssh-ed25519", "key blob type mismatch");
+    let key = read_ssh_string(&mut cursor)?;
+    anyhow::ensure!(key.len() == 32, "ssh-ed25519 key must be 32 bytes");
+    anyhow::ensure!(cursor.is_empty(), "trailing data in ssh-ed25519 key blob");
+    let mut out = [0u8; 32];
+    out.copy_from_slice(key);
+    Ok(out)
+}
+
+fn read_ssh_string<'a>(cursor: &mut &'a [u8]) -> Result<&'a [u8]> {
+    anyhow::ensure!(cursor.len() >= 4, "truncated SSH string length");
+    let len = u32::from_be_bytes(cursor[..4].try_into().unwrap()) as usize;
+    *cursor = &cursor[4..];
+    anyhow::ensure!(cursor.len() >= len, "truncated SSH string body");
+    let (value, rest) = cursor.split_at(len);
+    *cursor = rest;
+    Ok(value)
+}
+
+#[cfg(test)]
+fn ssh_ed25519_authorized_key_line(signing_key: &SigningKey) -> String {
+    let verifying = VerifyingKey::from(signing_key);
+    let mut blob = Vec::new();
+    write_ssh_string(&mut blob, b"ssh-ed25519");
+    write_ssh_string(&mut blob, &verifying.to_bytes());
+    format!("ssh-ed25519 {} test-key", STANDARD.encode(blob))
+}
+
+#[cfg(test)]
+fn write_ssh_string(out: &mut Vec<u8>, value: &[u8]) {
+    out.extend_from_slice(&(value.len() as u32).to_be_bytes());
+    out.extend_from_slice(value);
+}
+
 pub fn known_host_line(host: &str, key: &HostPublicKey, source: &str, first_seen: u64) -> String {
     format!(
         "{} {} {} first-seen={} source={}",
@@ -482,7 +790,119 @@ mod tests {
     #[test]
     fn server_hello_signature_round_trips() {
         let signing = SigningKey::from_bytes(&[3u8; 32]);
-        let client = NativeClientHello {
+        let client = test_client_hello();
+        let mut server = test_server_hello(&signing);
+        sign_server_hello(&signing, &client, &mut server).unwrap();
+        verify_server_hello(&client, &server).unwrap();
+        server.auth_challenge = [7u8; 32];
+        assert!(verify_server_hello(&client, &server).is_err());
+    }
+
+    #[test]
+    fn authorized_keys_parses_ed25519() {
+        let signing = SigningKey::from_bytes(&[8u8; 32]);
+        let raw = ssh_ed25519_authorized_key_line(&signing);
+        let parsed = parse_authorized_keys(&raw).unwrap();
+        assert_eq!(parsed.len(), 1);
+        assert_eq!(parsed[0].algorithm, "ssh-ed25519");
+        assert_eq!(
+            parsed[0].key,
+            VerifyingKey::from(&signing).to_bytes().to_vec()
+        );
+        assert_eq!(parsed[0].comment.as_deref(), Some("test-key"));
+    }
+
+    #[test]
+    fn native_user_auth_accepts_authorized_key_and_rejects_removed_key() {
+        let host_signing = SigningKey::from_bytes(&[3u8; 32]);
+        let user_signing = SigningKey::from_bytes(&[9u8; 32]);
+        let other_signing = SigningKey::from_bytes(&[10u8; 32]);
+        let client = test_client_hello();
+        let mut server = test_server_hello(&host_signing);
+        sign_server_hello(&host_signing, &client, &mut server).unwrap();
+        let auth = sign_user_auth(&user_signing, &client, &server, Vec::new()).unwrap();
+        let authorized =
+            parse_authorized_keys(&ssh_ed25519_authorized_key_line(&user_signing)).unwrap();
+
+        verify_native_user_auth(&client, &server, &auth, &authorized).unwrap();
+
+        let removed =
+            parse_authorized_keys(&ssh_ed25519_authorized_key_line(&other_signing)).unwrap();
+        assert!(verify_native_user_auth(&client, &server, &auth, &removed).is_err());
+    }
+
+    #[test]
+    fn native_user_auth_rejects_tampered_transcript() {
+        let host_signing = SigningKey::from_bytes(&[3u8; 32]);
+        let user_signing = SigningKey::from_bytes(&[9u8; 32]);
+        let client = test_client_hello();
+        let mut server = test_server_hello(&host_signing);
+        sign_server_hello(&host_signing, &client, &mut server).unwrap();
+        let auth = sign_user_auth(&user_signing, &client, &server, Vec::new()).unwrap();
+        let authorized =
+            parse_authorized_keys(&ssh_ed25519_authorized_key_line(&user_signing)).unwrap();
+        let mut tampered = client.clone();
+        tampered.requested_session = "other".to_string();
+
+        assert!(verify_native_user_auth(&tampered, &server, &auth, &authorized).is_err());
+    }
+
+    #[test]
+    fn native_user_auth_fails_closed_on_unsupported_authorized_key_options() {
+        let host_signing = SigningKey::from_bytes(&[3u8; 32]);
+        let user_signing = SigningKey::from_bytes(&[9u8; 32]);
+        let client = test_client_hello();
+        let mut server = test_server_hello(&host_signing);
+        sign_server_hello(&host_signing, &client, &mut server).unwrap();
+        let auth = sign_user_auth(&user_signing, &client, &server, Vec::new()).unwrap();
+        let raw = format!(
+            "no-agent-forwarding {}",
+            ssh_ed25519_authorized_key_line(&user_signing)
+        );
+        let authorized = parse_authorized_keys(&raw).unwrap();
+
+        assert!(verify_native_user_auth(&client, &server, &auth, &authorized).is_err());
+    }
+
+    #[test]
+    fn native_user_auth_enforces_no_port_forwarding_and_permitopen() {
+        let host_signing = SigningKey::from_bytes(&[3u8; 32]);
+        let user_signing = SigningKey::from_bytes(&[9u8; 32]);
+        let client = test_client_hello();
+        let mut server = test_server_hello(&host_signing);
+        sign_server_hello(&host_signing, &client, &mut server).unwrap();
+        let forward = ForwardingRequest {
+            kind: ForwardingKind::Local,
+            bind_host: None,
+            listen_port: 8080,
+            target_host: Some("127.0.0.1".to_string()),
+            target_port: Some(80),
+        };
+        let auth = sign_user_auth(&user_signing, &client, &server, vec![forward]).unwrap();
+        let no_forwarding = parse_authorized_keys(&format!(
+            "no-port-forwarding {}",
+            ssh_ed25519_authorized_key_line(&user_signing)
+        ))
+        .unwrap();
+        assert!(verify_native_user_auth(&client, &server, &auth, &no_forwarding).is_err());
+
+        let permitted = parse_authorized_keys(&format!(
+            "permitopen=\"127.0.0.1:80\" {}",
+            ssh_ed25519_authorized_key_line(&user_signing)
+        ))
+        .unwrap();
+        verify_native_user_auth(&client, &server, &auth, &permitted).unwrap();
+
+        let denied = parse_authorized_keys(&format!(
+            "permitopen=\"127.0.0.1:81\" {}",
+            ssh_ed25519_authorized_key_line(&user_signing)
+        ))
+        .unwrap();
+        assert!(verify_native_user_auth(&client, &server, &auth, &denied).is_err());
+    }
+
+    fn test_client_hello() -> NativeClientHello {
+        NativeClientHello {
             protocol_version: NATIVE_PROTOCOL_VERSION,
             client_random: [1u8; 32],
             client_ephemeral_public: [2u8; 32],
@@ -495,21 +915,20 @@ mod tests {
             supported_user_key_algorithms: vec!["ssh-ed25519".to_string()],
             cached_host_key_fingerprint: None,
             attach_ticket_envelope: None,
-        };
-        let mut server = NativeServerHello {
+        }
+    }
+
+    fn test_server_hello(signing: &SigningKey) -> NativeServerHello {
+        NativeServerHello {
             protocol_version: NATIVE_PROTOCOL_VERSION,
             server_random: [4u8; 32],
             server_ephemeral_public: [5u8; 32],
-            host_key: host_public_key(&signing),
+            host_key: host_public_key(signing),
             chosen_aead: "chacha20poly1305".to_string(),
             server_key_epoch: 1,
             auth_challenge: [6u8; 32],
             rate_limit_remaining: Some(30),
             host_signature: Vec::new(),
-        };
-        sign_server_hello(&signing, &client, &mut server).unwrap();
-        verify_server_hello(&client, &server).unwrap();
-        server.auth_challenge = [7u8; 32];
-        assert!(verify_server_hello(&client, &server).is_err());
+        }
     }
 }