# API Compatibility Reference

**Version**: 1.0.0
**Date**: 2026-06-26

This document defines the HTTP API contract between NexusGuard Server Core and Embedded Agent (ESP32).

## Overview

The embedded agent communicates with the server via two HTTP endpoints:

1. **Provisioning**: `POST /api/v1/provision` — Initial key exchange
2. **Heartbeat**: `POST /api/v1/heartbeat` — Periodic status + config sync

## 1. Provisioning

### Request

```
POST /api/v1/provision
Content-Type: application/json
```

```json
{
  "token": "string (required)",
  "hwid": "string (required)"
}
```

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `token` | string | Yes | Registration token (single-use) |
| `hwid` | string | Yes | Hardware ID (ESP32 MAC or serial) |

### Response (Success)

```
HTTP/1.1 200 OK
Content-Type: application/json
```

```json
{
  "encrypted_config": "base64-encoded-bytes"
}
```

| Field | Type | Description |
|-------|------|-------------|
| `encrypted_config` | string | AES-256-GCM encrypted ConfigPayload |

### Response (Error)

```json
{
  "error": "string"
}
```

| Status | Error | Description |
|--------|-------|-------------|
| 400 | `"invalid token format"` | Token is not valid UUID |
| 400 | `"token already used"` | Token was already consumed |
| 400 | `"invalid hardware id"` | HWID is empty or invalid |
| 403 | `"token expired"` | Token has expired (24h TTL) |
| 500 | `"provisioning failed"` | Server internal error |

### Decrypted ConfigPayload

After decrypting `encrypted_config`, the agent receives:

```json
{
  "device_id": "string (UUID)",
  "private_key": "string (hex, 64 chars)",
  "preshared_key": "string (hex, 64 chars)",
  "internal_ip": "string (e.g. 10.172.21.2)",
  "server_pub": "string (hex, 64 chars)",
  "endpoint": "string (e.g. italy-twenty.gl.at.ply.gg:59750)",
  "dns": "string (e.g. 1.1.1.1)",
  "allowed_ips": "string (e.g. 0.0.0.0/0)",
  "server_wg_ip": "string (e.g. 10.172.21.1)"
}
```

| Field | Type | Description |
|-------|------|-------------|
| `device_id` | string | Device UUID (use in heartbeat) |
| `private_key` | string | WireGuard private key (hex) |
| `preshared_key` | string | Pre-shared key (hex) |
| `internal_ip` | string | Device IP address |
| `server_pub` | string | Server WireGuard public key (hex) |
| `endpoint` | string | Server WireGuard endpoint |
| `dns` | string | DNS server IP |
| `allowed_ips` | string | Allowed IPs (e.g. "0.0.0.0/0" for full tunnel) |
| `server_wg_ip` | string | Server WireGuard IP |

## 2. Heartbeat

### Request

```
POST /api/v1/heartbeat
Content-Type: application/json
```

```json
{
  "device_id": "string (required)",
  "status": "string (optional)",
  "state": "string (optional)",
  "tunnel_up": "boolean (optional)",
  "last_handshake": "string (optional, ISO 8601)"
}
```

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `device_id` | string | Yes | Device UUID |
| `status` | string | No | "connected", "recovering", "stopped" |
| `state` | string | No | Free-form state description |
| `tunnel_up` | boolean | No | WireGuard tunnel status |
| `last_handshake` | string | No | ISO 8601 timestamp of last handshake |

### Response (Success)

```
HTTP/1.1 200 OK
Content-Type: application/json
```

```json
{
  "device_id": "string",
  "status": "ok",
  "internal_ip": "string",
  "private_key": "string (hex)",
  "preshared_key": "string (hex)",
  "server_pub": "string (hex)",
  "endpoint": "string",
  "server_wg_ip": "string",
  "dns": "string",
  "allowed_ips": "string",
  "forwards_hash": "string"
}
```

| Field | Type | Description |
|-------|------|-------------|
| `device_id` | string | Device UUID |
| `status` | string | "ok" or error |
| `internal_ip` | string | Device IP (may change) |
| `private_key` | string | WireGuard private key (may rotate) |
| `preshared_key` | string | Pre-shared key (may rotate) |
| `server_pub` | string | Server public key |
| `endpoint` | string | Server endpoint |
| `server_wg_ip` | string | Server WireGuard IP |
| `dns` | string | DNS server |
| `allowed_ips` | string | Allowed IPs |
| `forwards_hash` | string | Hash of port forwards (for change detection) |

### Response (Error)

```json
{
  "error": "string"
}
```

| Status | Error | Description |
|--------|-------|-------------|
| 400 | `"invalid device id"` | Device ID is not valid UUID |
| 403 | `"device is suspended"` | Device is suspended by admin |
| 500 | `"heartbeat failed"` | Server internal error |

## 3. Crypto Protocol

### Key Derivation

```
key = SHA256(hwid + salt)
```

- `hwid`: Hardware ID (ESP32 MAC or serial)
- `salt`: Server's `SERVER_SALT` environment variable
- Output: 32-byte AES key

### Encryption (Server Side)

```
plaintext = ConfigPayload JSON
nonce = random 12 bytes
ciphertext = AES-256-GCM-Encrypt(key, nonce, plaintext)
encrypted_config = nonce + ciphertext
```

### Decryption (Agent Side)

```
encrypted_config = response.encrypted_config (base64 decoded)
nonce = encrypted_config[0:12]
ciphertext = encrypted_config[12:]
plaintext = AES-256-GCM-Decrypt(key, nonce, ciphertext)
ConfigPayload = JSON.parse(plaintext)
```

### Implementation Notes

- Use `mbedtls_sha256()` for key derivation
- Use `mbedtls_gcm_*()` for AES-256-GCM
- Nonce size: 12 bytes (fixed for GCM)
- Tag size: 16 bytes (default for GCM)

## 4. Config Change Detection

### Heartbeat Hash Comparison

The server returns `forwards_hash` in heartbeat response. Agent should:

1. Store last received `forwards_hash` in NVS
2. Compare with current hash
3. If different, trigger config reload

```c
// Pseudo-code
char last_hash[64];
nvs_get_str(nvs_handle, "config_hash", last_hash, sizeof(last_hash));

if (strcmp(last_hash, response.forwards_hash) != 0) {
    // Config changed, rebuild tunnel
    tunnel_rebuild(&config);
    nvs_set_str(nvs_handle, "config_hash", response.forwards_hash);
}
```

## 5. Versioning

### Current Version

- API Version: v1 (implicit, no version in URL)
- Agent Version: 1.0.0

### Compatibility Policy

- **Breaking Changes**: Major version bump (v2)
- **New Fields**: Added without version bump (agent ignores unknown fields)
- **Deprecation**: 6-month notice before removal

### Agent Behavior

- Agent ignores unknown fields in server response
- Agent sends only required fields in request
- Agent handles missing optional fields gracefully

## 6. Examples

### Provisioning Flow

```c
// 1. Get hardware ID
char hwid[32];
get_hardware_id(hwid, sizeof(hwid));

// 2. Send provisioning request
char *response = http_post(server_url, "/api/v1/provision", 
    "{\"token\":\"%s\",\"hwid\":\"%s\"}", token, hwid);

// 3. Parse encrypted config
cJSON *json = cJSON_Parse(response);
char *encrypted = cJSON_GetObjectItem(json, "encrypted_config")->valuestring;

// 4. Decrypt config
wg_config_t config;
decrypt_config(encrypted, strlen(encrypted), hwid, salt, &config);

// 5. Store in NVS
config_save(&config);

// 6. Apply to WireGuard
tunnel_init(&config);
```

### Heartbeat Flow

```c
// 1. Get tunnel status
bool tunnel_up = tunnel_is_up();
int64_t last_hs = tunnel_get_last_handshake();

// 2. Send heartbeat
char *response = http_post(server_url, "/api/v1/heartbeat",
    "{\"device_id\":\"%s\",\"tunnel_up\":%s,\"last_handshake\":\"%s\"}",
    config.device_id, tunnel_up ? "true" : "false", 
    format_timestamp(last_hs));

// 3. Check for config changes
cJSON *json = cJSON_Parse(response);
const char *new_hash = cJSON_GetObjectItem(json, "forwards_hash")->valuestring;

if (strcmp(config.config_hash, new_hash) != 0) {
    // Config changed, reload
    load_config_from_response(json, &config);
    tunnel_rebuild(&config);
    config_save(&config);
}
```

## 7. Error Handling

### HTTP Retries

```c
#define MAX_RETRIES 3
#define RETRY_DELAY_MS 1000

for (int i = 0; i < MAX_RETRIES; i++) {
    esp_err_t err = http_post(...);
    if (err == ESP_OK) break;
    vTaskDelay(RETRY_DELAY_MS * (i + 1) / portTICK_PERIOD_MS);
}
```

### Timeout Settings

- Connect timeout: 10 seconds
- Read timeout: 30 seconds
- Write timeout: 10 seconds

## 8. Security Notes

- All communication over HTTPS (TLS 1.2+)
- Registration token is single-use (consumed on first provisioning)
- Device can re-provision by obtaining new token
- Keys never logged or transmitted in plaintext