package utils

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"github.com/agl/ed25519/extra25519"
	"golang.org/x/crypto/curve25519"
	"golang.org/x/crypto/ed25519"
	"io/ioutil"
	"math"
	"math/big"
)

const (
	// InvalidPrivateKeyFileError is a library error, thrown when the given key file fials to load
	InvalidPrivateKeyFileError = Error("InvalidPrivateKeyFileError")

	// RicochetKeySize - tor onion services currently use rsa key sizes of 1024 bits
	RicochetKeySize = 1024
)

// GetRandNumber is a helper function which returns a random integer, this is
// currently mostly used to generate messageids
func GetRandNumber() *big.Int {
	num, err := rand.Int(rand.Reader, big.NewInt(math.MaxUint32))
	// If we can't generate random numbers then panicking is probably
	// the best option.
	CheckError(err)
	return num
}

// EDH implements diffie hellman using curve25519 keys derived from ed25519 keys
// NOTE: This uses a 3rd party library extra25519 as the key conversion is not in the core golang lib
// as such this definitely needs further review.
func EDH(privateKey ed25519.PrivateKey, remotePublicKey ed25519.PublicKey) [32]byte {
	var privKeyBytes [64]byte
	var remotePubKeyBytes [32]byte
	copy(privKeyBytes[:], privateKey[:])
	copy(remotePubKeyBytes[:], remotePublicKey[:])
	var secret, curve25519priv, curve25519pub [32]byte
	extra25519.PrivateKeyToCurve25519(&curve25519priv, &privKeyBytes)
	extra25519.PublicKeyToCurve25519(&curve25519pub, &remotePubKeyBytes)
	curve25519.ScalarMult(&secret, &curve25519priv, &curve25519pub)
	return secret
}

// GeneratePrivateKeyV3 cryptographically creats a new ed25519 key pair.
func GeneratePrivateKeyV3() (ed25519.PublicKey, ed25519.PrivateKey, error) {
	return ed25519.GenerateKey(rand.Reader)
}

// LoadPrivateKeyFromFile loads a private key from a file...
func LoadPrivateKeyFromFile(filename string) (*rsa.PrivateKey, error) {
	pemData, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}

	return ParsePrivateKey(pemData)
}

// ParsePrivateKey Convert a private key string to a usable private key
func ParsePrivateKey(pemData []byte) (*rsa.PrivateKey, error) {
	block, _ := pem.Decode(pemData)
	if block == nil || block.Type != "RSA PRIVATE KEY" {
		return nil, InvalidPrivateKeyFileError
	}

	return x509.ParsePKCS1PrivateKey(block.Bytes)
}

// PrivateKeyToString turns a private key into storable string
func PrivateKeyToString(privateKey *rsa.PrivateKey) string {
	privateKeyBlock := pem.Block{
		Type:    "RSA PRIVATE KEY",
		Headers: nil,
		Bytes:   x509.MarshalPKCS1PrivateKey(privateKey),
	}

	return string(pem.EncodeToMemory(&privateKeyBlock))
}