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package satp

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/subtle"
	"encoding/binary"
	"errors"
	"strconv"
)

const (
	AES_CTR_KD_SALT_LENGTH = aes.BlockSize - 2 // two bytes spare for counter
)

type AESCTRKeyDerivation struct {
	cipher cipher.Block
	salt   []byte
	role   KDRole
}

func (kd *AESCTRKeyDerivation) generateCTR(dir Direction, usage KeyUsage, sequenceNumber uint32, ctr []byte) {
	ctr[aes.BlockSize-1] = 0
	ctr[aes.BlockSize-2] = 0
	subtle.ConstantTimeCopy(1, ctr[:len(kd.salt)], kd.salt)

	keyID := [8]byte{}
	binary.BigEndian.PutUint32(keyID[:4], getKDLabel(kd.role, dir, usage))
	binary.BigEndian.PutUint32(keyID[4:], sequenceNumber)

	// unfortunately crypto.xorBytes is not exported...
	offset := len(kd.salt) - len(keyID)
	n := len(keyID)
	for i := 0; i < n; i++ {
		ctr[offset+i] = ctr[offset+i] ^ keyID[i]
	}
}

func (kd *AESCTRKeyDerivation) Generate(dir Direction, usage KeyUsage, sequenceNumber uint32, out []byte) error {
	// this needs to stay in this function so that the go compiler can detect that it doesn't escape
	// from this function and whence will get allocated on the stack
	ctr := [aes.BlockSize]byte{}
	kd.generateCTR(dir, usage, sequenceNumber, ctr[:])
	for i := range out { // unfortunately there is no memset-style function in go...
		out[i] = 0
	}
	c := cipher.NewCTR(kd.cipher, ctr[:])
	c.XORKeyStream(out, out)
	return nil
}

func NewAESCTRKeyDerivation(key, salt []byte, role KDRole) (KeyDerivation, error) {
	if len(salt) != AES_CTR_KD_SALT_LENGTH {
		return nil, errors.New("invalid salt size: " + strconv.Itoa(len(salt)))
	}

	kd := &AESCTRKeyDerivation{salt: salt, role: role}

	var err error
	if kd.cipher, err = aes.NewCipher(key); err != nil {
		return nil, err
	}
	return kd, nil
}