feat: Implement password authentication for server (#127)

ARCHITECTURE.md

@@ -197,7 +197,7 @@ The `bssh-server` binary provides a command-line interface for managing and oper
 **Subcommands**:
 - **run** - Start the SSH server (default when no subcommand specified)
 - **gen-config** - Generate a configuration file template with secure defaults
-- **hash-password** - Hash passwords for configuration using bcrypt
+- **hash-password** - Hash passwords for configuration using Argon2id (recommended)
 - **check-config** - Validate configuration files and display settings
 - **gen-host-key** - Generate SSH host keys (Ed25519 or RSA)
 - **version** - Show version and build information
@@ -321,6 +321,8 @@ The authentication subsystem (`src/server/auth/`) provides extensible authentica
 - `mod.rs` - Module exports and re-exports
 - `provider.rs` - `AuthProvider` trait definition
 - `publickey.rs` - `PublicKeyVerifier` implementation
+- `password.rs` - `PasswordVerifier` implementation with Argon2id hashing
+- `composite.rs` - `CompositeAuthProvider` combining multiple auth methods
 
 **AuthProvider Trait**:
 
@@ -356,14 +358,49 @@ Implements public key authentication by parsing OpenSSH authorized_keys files:
   - `no-pty`, `no-port-forwarding`, `no-agent-forwarding`, `no-X11-forwarding`
   - `environment="..."` - Set environment variables
 
+**PasswordVerifier**:
+
+Implements password authentication with secure password hashing:
+
+- **Argon2id hashing**: Uses the OWASP-recommended password hashing algorithm
+  - Memory cost: 19 MiB
+  - Time cost: 2 iterations
+  - Parallelism: 1
+
+- **User configuration**:
+  - External YAML file with user definitions
+  - Inline users in server configuration
+  - User attributes: name, password_hash, shell, home, env
+
+- **Security features**:
+  - Timing attack mitigation with constant-time verification
+  - Minimum verification time (100ms) regardless of user existence
+  - Dummy hash verification for non-existent users
+  - Secure memory cleanup using `zeroize` crate
+  - User enumeration protection
+
+- **Hash compatibility**:
+  - Argon2id (recommended, generated by `hash-password` command)
+  - bcrypt (supported for backward compatibility)
+
+**CompositeAuthProvider**:
+
+Combines multiple authentication methods into a single provider:
+
+- Delegates to `PublicKeyVerifier` for public key auth
+- Delegates to `PasswordVerifier` for password auth
+- Prioritizes password verifier for user info (more detailed)
+- Supports hot-reloading of password users via `reload_password_users()`
+
 **Security Features**:
 
 - **Username validation**: Prevents path traversal attacks (e.g., `../etc/passwd`)
 - **File permission checks** (Unix): Rejects world/group-writable files and symlinks
 - **Symlink protection**: Uses `symlink_metadata()` to detect and reject symlinks
 - **Parent directory validation**: Checks parent directory permissions
 - **Rate limiting**: Token bucket rate limiter for authentication attempts
-- **Timing attack mitigation**: Constant-time behavior in `user_exists()` check
+- **Timing attack mitigation**: Constant-time behavior in password verification and `user_exists()` check
+- **Secure memory handling**: Password strings cleared from memory after use via `zeroize`
 - **Comprehensive logging**: All authentication attempts are logged
 
 ## Data Flow

Cargo.toml

@@ -55,6 +55,7 @@ shell-words = "1.1.1"
 libc = "0.2"
 ipnetwork = "0.20"
 bcrypt = "0.16"
+argon2 = "0.5"
 rand = "0.8"
 ssh-key = { version = "0.6", features = ["std"] }
 

docs/architecture/server-configuration.md

@@ -118,7 +118,7 @@ auth:
     # Inline user definitions
     users:
       - name: testuser
-        password_hash: "$6$rounds=656000$..."  # openssl passwd -6
+        password_hash: "$argon2id$v=19$m=19456,t=2,p=1$..."  # bssh-server hash-password
         shell: /bin/bash
         home: /home/testuser
         env:
@@ -364,11 +364,19 @@ Generated keys have secure permissions (0600) and are in OpenSSH format.
 ### Hash Passwords
 
 ```bash
-# Interactive password hashing with bcrypt
+# Interactive password hashing with Argon2id (recommended)
 bssh-server hash-password
 ```
 
-This prompts for a password, confirms it, and outputs a bcrypt hash suitable for use in the configuration file.
+This prompts for a password, confirms it, and outputs an Argon2id hash suitable for use in the configuration file. Argon2id is the OWASP-recommended password hashing algorithm with memory-hard properties that resist GPU and ASIC attacks.
+
+The generated hash includes:
+- Algorithm: Argon2id (variant resistant to both side-channel and GPU attacks)
+- Memory cost: 19 MiB
+- Time cost: 2 iterations
+- Parallelism: 1
+
+Note: bcrypt hashes are also supported for backward compatibility with existing configurations.
 
 ### Validate Configuration
 

src/bin/bssh_server.rs

@@ -257,6 +257,7 @@ fn gen_config(output: Option<PathBuf>) -> Result<()> {
 
 /// Hash a password for configuration
 async fn hash_password() -> Result<()> {
+    use bssh::server::auth::hash_password as generate_hash;
     use rpassword::read_password;
 
     print!("Enter password: ");
@@ -269,7 +270,7 @@ async fn hash_password() -> Result<()> {
 
     // Warn about weak passwords (but still allow them)
     if password.len() < 8 {
-        println!("\n⚠ Warning: Password is shorter than 8 characters.");
+        println!("\n Warning: Password is shorter than 8 characters.");
         println!("   This is considered weak and may be easily compromised.");
         println!("   Consider using a longer password for better security.\n");
     }
@@ -282,8 +283,8 @@ async fn hash_password() -> Result<()> {
         anyhow::bail!("Passwords do not match");
     }
 
-    // Use bcrypt for password hashing (cost factor 12)
-    let hash = bcrypt::hash(&password, 12).context("Failed to hash password")?;
+    // Use Argon2id for password hashing (recommended algorithm)
+    let hash = generate_hash(&password).context("Failed to hash password")?;
 
     println!("\nPassword hash (use in configuration):");
     println!("{}", hash);
@@ -295,6 +296,8 @@ async fn hash_password() -> Result<()> {
     println!("    users:");
     println!("      - name: username");
     println!("        password_hash: \"{}\"", hash);
+    println!("\nNote: This hash uses Argon2id algorithm (recommended).");
+    println!("      bcrypt hashes are also supported for compatibility.");
 
     Ok(())
 }