package main

import (
	"crypto/hmac"
	cryptoRand "crypto/rand"
	"crypto/sha1"
	"crypto/sha256"
	mathRand "math/rand"
	"strings"
)

func getSHA1Digest(data []byte) []byte {
	array := sha1.Sum(data)
	return array[:]
}

func getSHA2Digest(data []byte) []byte {
	array := sha256.Sum256(data)
	return array[:]
}

func HKDF(data []byte) []byte {
	h := hmac.New(sha1.New, []byte(strings.Repeat("\x00", 64)))
	h.Write(data)
	h1 := h.Sum(nil)
	h2 := make([]byte, 0)
	res := make([]byte, 0)

	for i := 0; i < 2; i++ {
		h = hmac.New(sha1.New, h1)
		h.Write(h2)
		h.Write([]byte{byte(i) + 1})
		h2 = h.Sum(nil)
		res = append(res, h2...)
	}

	return res[:0x24]
}

func genStreamKeys(server bool, data []byte) (sendAESKey, sendHMACKey, receiveAESKey, receiveHMACKey []byte) {
	const magic2 = "On the client side, this is the send key; on the server side, it is the receive key."
	const magic3 = "On the client side, this is the receive key; on the server side, it is the send key."

	var txEnc, rxEnc []byte

	txEnc = append(data, []byte(strings.Repeat("\x00", 40))...)
	rxEnc = append(data, []byte(strings.Repeat("\x00", 40))...)

	if server {
		txEnc = append(txEnc, magic3...)
		rxEnc = append(rxEnc, magic2...)
	} else {
		txEnc = append(txEnc, magic2...)
		rxEnc = append(rxEnc, magic3...)
	}

	txEnc = append(txEnc, []byte(strings.Repeat("\xF2", 40))...)
	rxEnc = append(rxEnc, []byte(strings.Repeat("\xF2", 40))...)

	txEnc = getSHA1Digest(txEnc)[:16]
	rxEnc = getSHA1Digest(rxEnc)[:16]

	sendKey := HKDF(txEnc)
	sendAESKey = sendKey[:16]
	sendHMACKey = sendKey[16:]

	receiveKey := HKDF(rxEnc)
	receiveAESKey = receiveKey[:16]
	receiveHMACKey = receiveKey[16:]

	return sendAESKey, sendHMACKey, receiveAESKey, receiveHMACKey
}

func genPasswordValidatorPriv(username, password string, salt []byte) []byte {
	if len(salt) != 16 {
		panic("salt must be 16 bytes")
	}

	hash := getSHA2Digest([]byte(username + ":" + password))
	return getSHA2Digest(append(salt, hash...))
}

func genRandomBytes(n int) ([]byte, error) {
	b := make([]byte, n)

	var err error
	if CfgGetSwitch("crypt-predictable-rng") {
		_, err = mathRand.Read(b)
	} else {
		_, err = cryptoRand.Read(b)
	}

	if err != nil {
		return nil, err
	}

	return b, nil
}

func init() {
	CfgRegisterSwitch("crypt-predictable-rng", "disable secure rng and use a pseudorandom preseeded rng")
	mathRand.Seed(300)
}