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+# Sketch SSH Mutual Authentication System
+
+This document describes how the Sketch SSH system implements mutual authentication between the host machine and containers using certificate-based authentication.
+
+## Architecture Overview
+
+The Sketch SSH system uses a certificate-based approach to establish mutual trust between the host machine and containers. This ensures that:
+
+1. Host verifies container identity: The user can trust that they're connecting to the intended container
+2. Container verifies host identity: The container can trust that connections are coming from authorized host machines
+
+## Component Interaction Diagram
+
+```mermaid
+sequenceDiagram
+    participant User
+    participant Host as Host Machine (SSHTheater)
+    participant Container as Container (SSH Server)
+    participant Client as SSH Client (ssh/VSCode)
+
+    %% Setup phase
+    Note over Host,Container: Initialization Phase
+    Host->>Host: Generate Container CA keys
+    Host->>Host: Generate host certificate signed by Container CA
+    Host->>Host: Generate user identity keys
+    Host->>Host: Generate server identity keys
+    Host->>Container: Init Container with:<br/>- Server identity<br/>- User public key<br/>- Container CA public key<br/>- Host certificate
+    Container->>Container: Start SSH server with certificate verification
+
+    %% Connection phase
+    Note over Host,Container: Connection Phase
+    User->>Client: Request SSH connection to container
+    
+    Client->>Host: Get SSH config from ~/.ssh/config
+    Host->>Client: Config with:<br/>- Host identity path<br/>- Host certificate path
+    
+    Client->>Container: Connect with host certificate
+    
+    Container->>Container: Verify client certificate<br/>is signed by Container CA
+    
+    opt Mutual Authentication
+        Container->>Client: Challenge for host certificate
+        Client->>Container: Present host certificate
+        Container->>Container: Verify host certificate<br/>is for 'root' user<br/>from localhost only
+    end
+    
+    alt Valid Certificate
+        Container->>Client: Accept connection
+        Client->>User: Establish shell session
+    else Invalid Certificate
+        Container->>Client: Reject connection
+        Client->>User: Authentication failed
+    end
+```
+
+## Security Features
+
+### Certificate-Based Authentication
+
+- **Container CA**: A dedicated Certificate Authority for Sketch containers
+- **Host Certificates**: Signed by the Container CA, authorizing specific hosts to connect
+- **CA-Based Known Hosts**: Uses `@cert-authority localhost,127.0.0.1,[::1]` directive to trust only localhost certificates
+- **Certificate Constraints**:
+  - Valid only for 'root' user (the container user)
+  - Source address restricted to localhost (127.0.0.1, ::1)
+  - Limited validity period (30 days by default)
+
+### Security Benefits
+
+1. **Defense in Depth Security**:
+   - Multiple layers of protection:
+     - Certificates require CA signature
+     - CA authority is restricted to localhost addresses only
+     - Certificate constraints limit to localhost source addresses
+   - Even if the CA key is compromised, it can only be used for localhost connections
+   - Even if SSH ports are exposed, unauthorized connections are rejected
+
+2. **Localhost Restriction**:
+   - Host certificates only work from the local machine, preventing remote exploitation
+
+3. **Seamless User Experience**:
+   - Certificate management happens automatically, requiring no user intervention
+
+## Implementation Components
+
+- **SSHTheater**: Manages certificate generation and SSH configuration on the host
+- **Custom SSH Server**: Enforces certificate validation in the container
+- **SSH Client Configuration**: Automatically configured to use the right certificates
+
+## Key Files
+
+- `~/.config/sketch/container_ca`: Container CA private key
+- `~/.config/sketch/container_ca.pub`: Container CA public key
+- `~/.config/sketch/host_cert`: Host certificate signed by Container CA
+- `~/.config/sketch/container_user_identity`: User identity for SSH connections
+- `~/.config/sketch/container_server_identity`: Server identity for container SSH server
+
+## SSH Configuration Approach
+
+### Hybrid Configuration Model
+
+SSHTheater uses a hybrid approach to balance security and practicality:
+
+1. **Localhost-Restricted CA Trust** for host verification:
+   - A single `@cert-authority localhost,127.0.0.1,[::1]` entry in known_hosts
+   - Only trusts certificates for localhost addresses
+   - Prevents CA misuse even if keys are compromised
+   - Simplifies management of multiple containers
+   - Provides strong cryptographic verification
+
+2. **Individual Host Entries** in SSH config:
+   - Each container still needs a dedicated entry for port configuration
+   - This is necessary because SSH doesn't support wildcards for ports
+   - Container-specific entries use shared identity files and certificates
+
+This approach maintains the convenience of "ssh container-name" commands while leveraging the security benefits of certificate-based authentication.
+
+## Cleanup Behavior
+
+The system is designed to support multiple concurrent Sketch instances:
+
+- **Container-Specific Cleanup**: When a Sketch instance exits, it only removes its own container's entries from the SSH configuration
+- **Preserved CA Authority**: Certificate authority entries are preserved during cleanup since they may be used by other running containers
+- **Independent Host Certificates**: Each Sketch instance maintains its own host certificate
+
+This approach allows multiple Sketch instances to run concurrently while preserving the security benefits of certificate-based authentication.
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