package dsakey

import (
	"crypto/sha1"
	"crypto/dsa"
	"crypto/openpgp"
	"crypto/openpgp/packet"
	"crypto/rand"
	"crypto"
	"fmt"
	"io"
	"math/big"
)

const P = "C1773C0DEF5C1D75BA556137CBCE0F6EE534034FCE503D7ED1FF7A27E8638EAC3BD627C734E08D1D828B52C39EB602DC63D9544D1734A981AE2408F8037305B548EFE457E2A79EB511CFF11A0C3DB05CF64971A6AF3EF191D3EBA0841AAAC3BECF4B6CF199EDD59C732BA642A0074BAE1DC3CF724F830930C898B1865F597EF7"
const Q = "DCA9E7C9FDC18CB0B8E9A80E487F96438147EF75"
const G = "502FF28CC4D7BC1100123C9227994341C29773BFBD8D7E8FFED6D87A9D82FE573744AC8E4CCAE93E3A017A6388921CA5B0C9349B249EF87AB30AE01B3C9FD723001CB25E560CA6C25EDFC97613B41346D0597C2ECA2BED7BC6C9A032049B3FFF9AED462D09651A5995DB8E5E111384AC7B62CBAD827009269FC79D3E4E6D8AA3"

func PrivKey(r []byte) *dsa.PrivateKey {
	priv := new(dsa.PrivateKey)
	priv.Parameters.P,_ = new(big.Int).SetString(P, 16)
	priv.Parameters.Q,_ = new(big.Int).SetString(Q, 16)
	priv.Parameters.G,_ = new(big.Int).SetString(G, 16)

	// q > 2^159 prime
	// x = sha1(r)
	// if x == 0 then x = q - 1
	// if x == q then x = q - 2
	// if x > q  then x = x mod q

	x := new(big.Int)
	h := sha1.New()
	h.Write(r)
	x.SetBytes(h.Sum())
	if x.Sign() == 0 {
		x.Sub(priv.Q, big.NewInt(1))
	}
	switch x.Cmp(priv.Q) {
	case 0:
		x.Sub(priv.Q, big.NewInt(2))
	case 1:
		x.Sub(x, priv.Q)
	}
	priv.X = x
	priv.Y = new(big.Int)
	priv.Y.Exp(priv.G, x, priv.P)
	return priv
}

func GenKey() (priv *dsa.PrivateKey, err error) {
	x := make([]byte, len(Q)/2)
	_, err = io.ReadFull(rand.Reader, x)
	priv = PrivKey(x)
	return
}

// NewEntity returns an Entity that contains a fresh DSA private key with a
// single identity composed of the given full name, comment and email, any of
// which may be empty but must not contain any of "()<>\x00".
func NewEntity(priv *dsa.PrivateKey, currentTimeSecs int64, name, comment, email string) (e *openpgp.Entity, err error) {
	uid := packet.NewUserId(name, comment, email)
	if uid == nil {
		return nil, fmt.Errorf("NewEntity: invalid argument: user id field contained invalid characters")
	}
	t := uint32(currentTimeSecs)
	e = &openpgp.Entity{
		PrimaryKey: packet.NewDSAPublicKey(t, &priv.PublicKey, false /* not a subkey */ ),
		PrivateKey: packet.NewDSAPrivateKey(t, priv, false /* not a subkey */ ),
		Identities: make(map[string]*openpgp.Identity),
	}
	isPrimaryId := true
	e.Identities[uid.Id] = &openpgp.Identity{
		Name:   uid.Name,
		UserId: uid,
		SelfSignature: &packet.Signature{
			CreationTime: t,
			SigType:      packet.SigTypePositiveCert,
			PubKeyAlgo:   packet.PubKeyAlgoDSA,
			Hash:         crypto.SHA256,
			IsPrimaryId:  &isPrimaryId,
			FlagsValid:   true,
			FlagSign:     true,
			FlagCertify:  true,
			IssuerKeyId:  &e.PrimaryKey.KeyId,
		},
	}
/*
	e.Subkeys = make([]Subkey, 1)
	e.Subkeys[0] = Subkey{
		PublicKey:  packet.NewRSAPublicKey(t, &encryptingPriv.PublicKey, true),
		PrivateKey: packet.NewRSAPrivateKey(t, encryptingPriv, true),
		Sig: &packet.Signature{
			CreationTime:              t,
			SigType:                   packet.SigTypeSubkeyBinding,
			PubKeyAlgo:                packet.PubKeyAlgoRSA,
			Hash:                      crypto.SHA256,
			FlagsValid:                true,
			FlagEncryptStorage:        true,
			FlagEncryptCommunications: true,
			IssuerKeyId:               &e.PrimaryKey.KeyId,
		},
	}
*/
	return
}

// simple key generation for obligation issuer clients
func NewIssuerEntity(r []byte, denomination string) (e *openpgp.Entity, err error) {
	return NewEntity(PrivKey(r), 0, "Issuer", denomination, "")
}
// simple key generation for obligation holder clients
func NewHolderEntity(r []byte, issuer, denomination string) (e *openpgp.Entity, err error) {
	return NewEntity(PrivKey(r), 0, "Holder of " + issuer, denomination, "")
}

// check the issuer and denomination associated with the given pgp key
func CheckEntity(e *openpgp.Entity) (isIssuer bool, issuer, denomination string, err error) {
	// TODO: allow non-epoint uids
	if len(e.Identities) != 1 {
		err = fmt.Errorf("CheckEntity: expected one identity")
		return
	}
	for _, i := range e.Identities {
		denomination = i.UserId.Comment
		if i.UserId.Name == "Issuer" {
			isIssuer = true
			issuer = fmt.Sprintf("%X", e.PrimaryKey.Fingerprint)
			return
		}
		prefix := "Holder of "
		if i.UserId.Name[:len(prefix)] == prefix {
			issuer = i.UserId.Name[len(prefix):]
			return
		}
		break
	}
	err = fmt.Errorf("CheckENtity: invalid userid")
	return
}