// Package demo seals a camera list so a release can carry it without carrying
// the credentials in any usable form.
//
// The need: a demo build that installs with the office cameras already set up,
// handed to people who should not be able to read the cameras' admin password
// out of the zip. "Encode it" does not do that - anything the installer can
// decode, anyone holding the installer can decode. So the bundle is encrypted
// with a key that is NOT in the package: a short unlock code, generated when
// the bundle is sealed, spoken or messaged to whoever runs setup, and typed
// once. Without it the file is noise.
//
// The code is random, not chosen, so it is used as key material directly
// (through SHA-256) rather than stretched with a KDF. A human-chosen
// passphrase would need argon2 and a dependency; 120 random bits do not.
package demo

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base32"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
)

// Magic identifies the file and the format version, so a future change can be
// told apart from corruption instead of failing as "authentication failed".
const magic = "BVDEMO1\n"

// Payload is what a sealed bundle carries. Two demo shapes exist:
//
//   - cameras only: the PC runs on its own with these cameras (the first
//     demo build);
//   - an enrolment code: the PC claims a real shop at head office and gets
//     its cameras from there, exactly as a customer install would, so the
//     demo exercises the whole product rather than a local copy of it. The
//     code is single-use, so one bundle is one install.
//
// A bundle from the first build is a bare JSON array; Decode accepts both.
type Payload struct {
	Cameras   []Camera `json:"cameras,omitempty"`
	EnrolCode string   `json:"enrol_code,omitempty"`
	CloudBase string   `json:"cloud_base,omitempty"`
}

// Decode reads either payload shape.
func Decode(plain []byte) (Payload, error) {
	var p Payload
	if len(plain) > 0 && plain[0] == '[' {
		return p, json.Unmarshal(plain, &p.Cameras)
	}
	return p, json.Unmarshal(plain, &p)
}

// Camera is one entry as the engine's Add Camera endpoint accepts it.
type Camera struct {
	ID       string `json:"id"`
	Label    string `json:"label,omitempty"`
	Host     string `json:"host"`
	Port     int    `json:"port"`
	Path     string `json:"path"`
	Username string `json:"username"`
	Password string `json:"password"`
	MaxWidth int    `json:"max_width,omitempty"`
}

// NewCode mints an unlock code: 15 random bytes as 24 base32 characters in
// four groups, the same shape as an installation code, for the same reason -
// it gets read down a phone.
func NewCode() (string, error) {
	raw := make([]byte, 15)
	if _, err := rand.Read(raw); err != nil {
		return "", err
	}
	s := base32.StdEncoding.WithPadding(base32.NoPadding).EncodeToString(raw)
	return fmt.Sprintf("%s-%s-%s-%s", s[0:6], s[6:12], s[12:18], s[18:24]), nil
}

// NormalizeCode makes the typed and the printed form hash the same: case,
// spaces and dashes are all noise a person adds or drops.
func NormalizeCode(code string) string {
	code = strings.ToUpper(code)
	code = strings.NewReplacer("-", "", " ", "", "\t", "", "\r", "", "\n", "").Replace(code)
	return code
}

func keyFor(code string) []byte {
	sum := sha256.Sum256([]byte("behavision-demo-bundle:" + NormalizeCode(code)))
	return sum[:]
}

// Seal encrypts plaintext under the code. Output is magic || nonce || ciphertext.
func Seal(code string, plaintext []byte) ([]byte, error) {
	block, err := aes.NewCipher(keyFor(code))
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return nil, err
	}
	out := append([]byte(magic), nonce...)
	return gcm.Seal(out, nonce, plaintext, []byte(magic)), nil
}

// ErrWrongCode is what a mistyped code looks like. GCM cannot tell a wrong key
// from a corrupted file, and neither can we, so both read as this.
var ErrWrongCode = errors.New("that unlock code does not open this bundle")

// Open decrypts a sealed bundle.
func Open(code string, sealed []byte) ([]byte, error) {
	if !strings.HasPrefix(string(sealed), magic) {
		return nil, errors.New("not a Behavision demo bundle")
	}
	body := sealed[len(magic):]
	block, err := aes.NewCipher(keyFor(code))
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	if len(body) < gcm.NonceSize() {
		return nil, errors.New("bundle is truncated")
	}
	nonce, ct := body[:gcm.NonceSize()], body[gcm.NonceSize():]
	plain, err := gcm.Open(nil, nonce, ct, []byte(magic))
	if err != nil {
		return nil, ErrWrongCode
	}
	return plain, nil
}