package util

import (
	"fmt"
	"math/big"
	"crypto/dsa"
	"encoding/asn1"
	"encoding/pem"
	"crypto/rand"
	"encoding/base64"
	"crypto/sha1"
	"bytes"
)

var privateKeyDer = []byte(`-----BEGIN DSA PRIVATE KEY-----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-----END DSA PRIVATE KEY-----`)

var publicKeyDer = []byte(`-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----`)

var privateKey *dsa.PrivateKey
var publicKey *dsa.PublicKey

var println = fmt.Println

func init() {

	block, _ := pem.Decode(privateKeyDer)

	// initialize keypair
	privateKey, _ = ParsePKCS8PrivateKey(block.Bytes)
	publicKey = &privateKey.PublicKey

	println(privateKey)
	println(publicKey)
}

func Sign(data []byte) (signature string) {
	_, _, signatureByte := sign(data, privateKey)

	return base64.StdEncoding.EncodeToString(signatureByte)
}

func putLength(buf *bytes.Buffer, len int)  {
	if len < 128 {
		buf.WriteByte(byte(len))
	} else if len < 256 {
		buf.WriteRune(rune(-127))
		buf.WriteByte(byte(len))
	} else if len < 65536 {
		buf.WriteRune(rune(-126))
		buf.WriteByte(byte(len >> 8))
		buf.WriteByte(byte(len))
	} else if len < 16777216 {
		buf.WriteRune(rune(-125))
		buf.WriteByte(byte(len >> 16))
		buf.WriteByte(byte(len >> 8))
		buf.WriteByte(byte(len))
	} else {
		buf.WriteRune(rune(-124))
		buf.WriteByte(byte(len >> 24))
		buf.WriteByte(byte(len >> 16))
		buf.WriteByte(byte(len >> 8))
		buf.WriteByte(byte(len))
	}
}


// golang cannot parse PKCS8 DSA private key right now
// we need to make an struct for unmarshal
type PKCS8DSAPrivateKey struct {
	E1, P, Q, G, Y, X *big.Int
}

func ParsePKCS8PrivateKey(der []byte) (privateKey *dsa.PrivateKey, err error) {

	myPrivateKey := PKCS8DSAPrivateKey{}

	asn1.Unmarshal(der, &myPrivateKey)

	privateKey = new(dsa.PrivateKey)

	dsaPublicKey := new(dsa.PublicKey)
	dsaParameter := new(dsa.Parameters)

	dsaParameter.G = myPrivateKey.G
	dsaParameter.P = myPrivateKey.P
	dsaParameter.Q = myPrivateKey.Q

	dsaPublicKey.Y = myPrivateKey.Y
	dsaPublicKey.Parameters = *dsaParameter

	privateKey.X = myPrivateKey.X
	privateKey.PublicKey = *dsaPublicKey

	return
}

func sign(data []byte, privatekey *dsa.PrivateKey) (r *big.Int, s *big.Int, signstr []byte) {

	sha1 := sha1.New()
	sha1.Write(data)
	signhash := sha1.Sum(nil)

	r, s, err := dsa.Sign(rand.Reader, privatekey, signhash[:])
	if err != nil {
		fmt.Println(err)
	}

	der := genDerValueFromRS(r, s)

	return r, s, der
}

func genDerValueFromRS(r, s *big.Int) (derValue []byte) {
	rsBuf := bytes.NewBuffer(make([]byte, 0))

	rsBuf.WriteRune(2)
	putLength(rsBuf, len(r.Bytes()))
	rsBuf.Write(r.Bytes())

	rsBuf.WriteRune(2)
	putLength(rsBuf, len(s.Bytes()))
	rsBuf.Write(s.Bytes())

	buf := bytes.NewBuffer([]byte{})

	buf.WriteRune(48)
	putLength(buf, len(rsBuf.Bytes()))
	buf.Write(rsBuf.Bytes())

	fmt.Println("6666: ", base64.StdEncoding.EncodeToString(buf.Bytes()))

	return buf.Bytes()
}