felhom-agent/internal/wgtunnel/key.go

// Package wgtunnel manages the host's WireGuard tunnel to the offsite endpoint (S3, doc 06 §3.3
// steps 1+5): pure-Go keygen, one-shot pubkey registration with the hub, consumption of the
// hub-served `wireguard` desired-state block, and the `wg-quick@wg-felhom` host service through
// the narrow-sudoers runner — the lanresolver/dnsmasq shape.
//
// SECRETS DISCIPLINE (elevated — the S1 session-log incident): the WG private key lives in
// exactly two places on the box (the StateDir key file and the installed conf, both 0600) plus
// the R-wrapped escrow blob. It is NEVER put on an argv, in a log/error, or read back via
// `wg show <if> dump` (whose interface line carries the private key — the ONLY wg read this
// package performs is `wg show wg-felhom latest-handshakes`).
package wgtunnel

import (
	"crypto/rand"
	"encoding/base64"
	"fmt"
	"os"
	"path/filepath"

	"golang.org/x/crypto/curve25519"
)

// keyFileName is the private-key file under <StateDir>/wg/. 0600, agent-owned. Never overwritten:
// a key file that exists may be the identity a customer's escrow blob carries.
const keyFileName = "private.key"

// clampPrivateKey applies the curve25519 clamping WireGuard requires. NOTE: x/crypto's X25519
// clamps the scalar internally (RFC 7748), so DERIVATION is clamp-invariant — what the explicit
// clamp guarantees is the STORED key file: the persisted bytes must be in canonical clamped form
// so an external `wg pubkey < private.key` agrees with the pubkey the agent registered (covered
// by TestEnsureKey_StoredKeyIsClamped).
func clampPrivateKey(b []byte) {
	b[0] &= 248
	b[31] &= 127
	b[31] |= 64
}

// derivePublic derives the WG public key (base64) from a 32-byte private key.
func derivePublic(priv []byte) (string, error) {
	pub, err := curve25519.X25519(priv, curve25519.Basepoint)
	if err != nil {
		return "", fmt.Errorf("wgtunnel: derive public key: %w", err)
	}
	return base64.StdEncoding.EncodeToString(pub), nil
}

// EnsureKey creates the keypair once (0600 file in a 0700 dir) or loads the existing one, and
// returns the PUBLIC key only — the private key never leaves the package except via KeyFilePath
// (rendering) and the escrow join, which read the file themselves. A corrupt key file is an
// ERROR, never an overwrite (it may be an escrowed identity).
func EnsureKey(stateDir string) (pub string, created bool, err error) {
	dir := filepath.Join(stateDir, "wg")
	path := filepath.Join(dir, keyFileName)

	if raw, rerr := os.ReadFile(path); rerr == nil {
		priv, derr := decodeKey(raw)
		if derr != nil {
			return "", false, fmt.Errorf("wgtunnel: existing key file %s is corrupt (%v) — refusing to overwrite; operator must resolve", path, derr)
		}
		pub, err = derivePublic(priv)
		return pub, false, err
	} else if !os.IsNotExist(rerr) {
		return "", false, fmt.Errorf("wgtunnel: reading key file: %w", rerr)
	}

	if err := os.MkdirAll(dir, 0o700); err != nil {
		return "", false, fmt.Errorf("wgtunnel: creating %s: %w", dir, err)
	}
	priv := make([]byte, 32)
	if _, err := rand.Read(priv); err != nil {
		return "", false, fmt.Errorf("wgtunnel: entropy: %w", err)
	}
	clampPrivateKey(priv)
	enc := base64.StdEncoding.EncodeToString(priv) + "\n"
	if err := os.WriteFile(path, []byte(enc), 0o600); err != nil {
		return "", false, fmt.Errorf("wgtunnel: writing key file: %w", err)
	}
	pub, err = derivePublic(priv)
	if err != nil {
		return "", false, err
	}
	return pub, true, nil
}

// KeyFilePath returns the private-key file location for a state dir (escrow join + conf render
// read it directly; the value of the file is never returned by this package's API).
func KeyFilePath(stateDir string) string {
	return filepath.Join(stateDir, "wg", keyFileName)
}

// decodeKey parses a key-file payload: base64 of exactly 32 bytes (trailing whitespace ok).
func decodeKey(raw []byte) ([]byte, error) {
	s := string(raw)
	for len(s) > 0 && (s[len(s)-1] == '\n' || s[len(s)-1] == '\r' || s[len(s)-1] == ' ') {
		s = s[:len(s)-1]
	}
	priv, err := base64.StdEncoding.DecodeString(s)
	if err != nil {
		return nil, fmt.Errorf("not valid base64: %v", err)
	}
	if len(priv) != 32 {
		return nil, fmt.Errorf("decodes to %d bytes, want 32", len(priv))
	}
	return priv, nil
}

// readPrivateKeyB64 loads + validates the private key file, returning the base64 string for
// conf rendering. Internal only; callers must never log the value.
func readPrivateKeyB64(stateDir string) (string, error) {
	raw, err := os.ReadFile(KeyFilePath(stateDir))
	if err != nil {
		return "", fmt.Errorf("wgtunnel: reading key file: %w", err)
	}
	priv, err := decodeKey(raw)
	if err != nil {
		return "", fmt.Errorf("wgtunnel: key file corrupt: %w", err)
	}
	return base64.StdEncoding.EncodeToString(priv), nil
}