// Package security provides the security primitives used across the backend:
// symmetric encryption for secrets at rest, hashing for access tokens and
// passwords, and the prompt-injection / least-privilege controls applied by
// the agent runtime.
package security

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"io"

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

// Crypto wraps an AES-GCM cipher derived from a master key. It is used to
// encrypt model API keys at rest.
type Crypto struct {
	aead cipher.AEAD
}

// NewCrypto initialises an AES-GCM cipher from a 32-byte master key.
func NewCrypto(masterKey string) (*Crypto, error) {
	if len(masterKey) != 32 {
		return nil, fmt.Errorf("master key must be 32 bytes, got %d", len(masterKey))
	}
	block, err := aes.NewCipher([]byte(masterKey))
	if err != nil {
		return nil, fmt.Errorf("create aes cipher: %w", err)
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("create gcm: %w", err)
	}
	return &Crypto{aead: aead}, nil
}

// Encrypt returns a base64-encoded ciphertext of the plaintext. A random
// nonce is prepended to the ciphertext.
func (c *Crypto) Encrypt(plaintext string) (string, error) {
	if plaintext == "" {
		return "", nil
	}
	nonce := make([]byte, c.aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", fmt.Errorf("read nonce: %w", err)
	}
	ct := c.aead.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ct), nil
}

// Decrypt reverses Encrypt. An empty input yields an empty output.
func (c *Crypto) Decrypt(b64 string) (string, error) {
	if b64 == "" {
		return "", nil
	}
	raw, err := base64.StdEncoding.DecodeString(b64)
	if err != nil {
		return "", fmt.Errorf("base64 decode: %w", err)
	}
	ns := c.aead.NonceSize()
	if len(raw) < ns {
		return "", errors.New("ciphertext too short")
	}
	pt, err := c.aead.Open(nil, raw[:ns], raw[ns:], nil)
	if err != nil {
		return "", fmt.Errorf("gcm open: %w", err)
	}
	return string(pt), nil
}

// HashToken returns a hex SHA-256 digest of an app access token. Tokens are
// random and high-entropy, so a plain hash (no salt) is sufficient and keeps
// lookups O(1) on the unique index.
func HashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:])
}

// HashPassword returns a bcrypt hash of a password.
func HashPassword(password string) (string, error) {
	b, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
	if err != nil {
		return "", fmt.Errorf("bcrypt: %w", err)
	}
	return string(b), nil
}

// VerifyPassword checks a bcrypt hash against a plaintext password.
func VerifyPassword(hash, password string) bool {
	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(password)) == nil
}