# Implant Design

## Overview

The necropolis implant is a lightweight Go binary that connects to the libp2p network and
communicates with its operator entirely through direct p2p streams, with pubsub as a
fallback. It has no hardcoded server IPs or DNS names — only a cryptographic reference
to the operator's public key.

## Directory Structure

```
implant/
├── main.go              # Entry point
└── core/                # All implant runtime logic
    ├── agent.go         # Core lifecycle, beacon loop, command dispatch, handlers
    ├── shell.go         # Shell path selection (/bin/bash, cmd.exe, etc.)
    ├── shell_unix.go    # PTY-based interactive shell (Linux/macOS)
    ├── shell_windows.go # ConPTY-based interactive shell (Windows)
    ├── portfwd.go       # TCP port forwarding tunnel
    ├── socks.go         # SOCKS proxy tunnel (identical to portfwd currently)
    ├── ps.go            # Process listing (Linux /proc, Windows tasklist)
    ├── antivm.go        # VM detection framework (cross-platform, build-tag gated)
    ├── antivm_linux.go  # 50+ Linux-specific VM detection techniques
    ├── antivm_darwin.go # macOS-specific VM detection techniques
    ├── antivm_windows.go# Windows-specific VM detection techniques
    ├── antivm_stub.go   # No-op stub when compiled without `-tags=antivm`
    ├── embed_evasion.go        # Embeds valak.dll via //go:embed
    ├── evasion_windows.go      # Evasion loader + sleep obfuscation
    ├── valak.dll              # Zig evasion DLL (FreshyCalls, indirect syscalls, AMSI/ETW bypass, module stomping)
    ├── evasion_stub.go         # No-op stub when compiled without `-tags=evasion`
    ├── agent_relay.go          # Implant-to-implant relay advertisement and routing
    ├── embedded_pubkey.go        # Generated: operator's public key embedded at build time
    ├── embedded_implant_key.go   # Generated: implant's unique keypair embedded at build time
    ├── embedded_authtoken.go   # Generated: operator-specific auth token embedded at build time
    └── embedded_boxpubkey.go   # Generated: NaCl box public key embedded at build time
```

## Build Process

1. Operator runs `necropolis generate --os linux --arch amd64` (from CLI or standalone)
2. `server/core/generate.go` unpacks the embedded source tree from `server/core/embedsrc/implant_src.tar.gz`
3. Injects the operator's public key and a fresh implant Ed25519 keypair into the source tree
4. Optionally applies build tags (`-tags=antivm`, `-tags=evasion`) and quiet-mode stubs
5. Cross-compiles with `CGO_ENABLED=0` for the target OS/arch
6. When `--evasion` is set, embeds `valak.dll` (1.5 MB Zig kernel evasion DLL) into the
    binary via `//go:embed`. The DLL is loaded at runtime from memory and provides FreshyCalls
    syscall dispatch, AMSI/ETW bypass, module stomping, EDR callback removal.
7. Applies garble obfuscation (`--obfuscate`), UPX compression (`--upx`), and strip (`--quiet`)
8. The resulting binary is self-contained — no source tree, no external dependencies

Each build generates a unique implant keypair. The implant keeps the same PeerID across restarts.

## Core Lifecycle

### Startup

1. Load operator's public key from embedded `embedded_pubkey.go`
2. Load implant's private key from embedded `embedded_implant_key.go`
3. Derive libp2p PeerID from implant public key
4. Initialise libp2p host with:
   - TCP + WebSocket transports (no UDP/QUIC for sandbox compatibility)
   - AutoNAT + relay client for NAT traversal
   - DHT client for peer discovery
   - GossipSub pubsub
5. If `--peer` flag is provided, connect to operator directly
6. If `--wss` flag is provided, connect via WebSocket Secure (TLS over TCP/443)
7. Start DHT discovery loop to find operator via rendezvous namespace
8. Start DHT dead-drop poll loop (`pollDHTCmdLoop`) — polls `/necropolis/cmd/<id>/<nonce>` every 30s for signed command envelopes
9. Once connected, open persistent beacon stream (`/bc/1.0.0`) to operator
10. Register via signed `Z1` (beacon register with system metadata)
11. Start beacon loop and cover traffic loop

### Main Loop

```
beaconLoop (every 10-15s):
  Send Z1 (beacon register) on persistent /bc/1.0.0 stream
  Sleep(interval + random(jitter))

streamKeepaliveLoop (every 5s):
  Write MsgTypeCover on persistent stream to prevent relay idle timeout

coverTrafficLoop (every 4-7s):
  Publish random noise to the beacon pubsub topic

discoverOperatorLoop (every 15s):
  Find operator via DHT rendezvous
  If beacon stream is nil, reconnect

pollDHTCmdLoop (every 30s):
  Poll DHT dead-drop at /necropolis/cmd/<id>/<nonce> for signed command envelopes
  Process through existing command handler

commandStream handler:
  Read length-prefixed envelope from /bc/1.0.0/cmd stream
  Verify Ed25519 signature against embedded operator pubkey
  Dispatch by message type
  Send result on persistent beacon stream
```

### Session Mode (Shell, Portfwd, SOCKS)

```
on incoming stream:
  Read target/winsize from stream header
  Establish local connection / PTY
  Bidirectional io.Copy between libp2p stream and local resource
  On disconnect or Ctrl+]: cleanup and return
```

## Command Handlers

All command handlers follow the same pattern:

1. Deserialise protobuf request from envelope
2. Execute operation (local filesystem, process execution, etc.)
3. Serialise protobuf result
4. Send result via `sendResult()` — tries persistent stream first, falls back to pubsub

| Handler | Protobuf | Operation |
|---|---|---|
| `handlePs` | Z12 → Z13 | List processes via `/proc` (Linux) or `tasklist` (Windows) |
| `handleLs` | Z16 → Z17 | Read directory entries |
| `handleCd` | Z19 → Z21 | `os.Chdir()` |
| `handlePwd` | Z20 → Z21 | `os.Getwd()` |
| `handleExecute` | Z14 → Z15 | `exec.CommandContext()` with optional output capture |
| `handleDownload` | Z22 → Z23 | `os.ReadFile()` with 100MB limit |
| `handleUpload` | Z24 → Z25 | `os.WriteFile()` with optional overwrite and 100MB limit |
| `handleKill` | (none) → (none) | Log stack trace, `os.Exit(0)` |
| `handleScreenshot` | Z2 → Z3 | Returns "not implemented" stub |

## Platform Support

| Feature | Windows | Linux | macOS |
|---|---|---|---|
| TCP transport | ✓ | ✓ | ✓ |
| WebSocket transport | ✓ | ✓ | ✓ |
| Process list | ✓ (tasklist) | ✓ (/proc) | ✓ (stub) |
| File ops | ✓ | ✓ | ✓ |
| Interactive shell | ✓ (ConPTY) | ✓ (PTY) | ✓ (PTY) |
| Port forwarding | ✓ | ✓ | ✓ |
| VM detection | 50+ checks | 50+ checks | 8 checks |
| Kernel evasion | ✓ (DLL) | — | — |

## Evasion System

When compiled with `--evasion`, the implant includes a Zig DLL (`valak.dll`) built from
`evasion/` in the source tree. The DLL is embedded via `//go:embed valak.dll` at compile
time and loaded from memory via reflective PE loader at runtime. After loading, the DLL
stomps its `.text` section into a legitimate signed Microsoft DLL (e.g. `CryptoAPI.dll`,
`dwrite.dll`), frees the original allocation, and wipes its PE headers. Once stomped, the
code executes from inside a Microsoft-signed address range with no orphaned PE signature.
It provides kernel-level evasion primitives ported from the Tenshu C2 framework:

| Technique | Implementation | Persistence |
|---|---|---|---|
| Module stomping | `.text` copied into signed MS DLL range; original headers wiped | On load |
| Image header wiping | MZ/PE\0\0 zeroed at original allocation after stomp | On load |
| Sleep obfuscation (PAGE_NOACCESS) | DLL .text set PAGE_NOACCESS via VirtualProtect during idle | Per sleep cycle |
| Indirect syscalls | FreshyCalls + HAL's Gate SSN extraction, random ntdll gadget | Available via DLL call |
| ETW patch | Three-tier: NtTraceControl → HWBP → RET patch | On init |
| AMSI bypass | DR0 hardware breakpoint + VEH with SuspendThread/ResumeThread | Installed on init |
| EDR callback removal | SeDebugPrivilege + NtSetInformationProcess(40) | On init |
If the DLL fails to load, evasion is skipped. No fallback to pure Go implementations — the DLL is the only evasion path.

## Transport Configuration

The implant explicitly disables libp2p's default transports and enables only:

1. **WebSocket** — primary transport, works through most proxies
2. **WebSocket Secure** — WSS is a dial address (`/dns4/host/tcp/443/wss/p2p/peerid`), not a separate transport layer; it uses the same WebSocket transport over TLS
3. **TCP** — fallback for direct connections

No UDP, no QUIC, no multicast. This maximises sandbox/container compatibility.

The implant always connects with `network.WithAllowLimitedConn` to work through relay
circuits when direct connections are unavailable.

When started with `-wss`, the implant connects via `/dns4/<host>/tcp/443/wss/p2p/<peer-id>`
before falling back to DHT discovery and dead-drop polling.
