// Copyright 2018 DREP Foundation Ltd.
// This file is part of the drep-cli library.
//
// The drep-cli library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The drep-cli library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the drep-cli library. If not, see <http://www.gnu.org/licenses/>.

package crypto

import (
	"errors"
	"fmt"
	"github.com/drep-project/drepcli/crypto/secp256k1"
	"math/big"
)

// Ecrecover returns the uncompressed public key that created the given signature.
func Ecrecover(hash, sig []byte) ([]byte, error) {
	pub, err := SigToPub(hash, sig)
	if err != nil {
		return nil, err
	}
	bytes := (*secp256k1.PublicKey)(pub).SerializeUncompressed()
	return bytes, err
}

// SigToPub returns the public key that created the given signature.
func SigToPub(hash, sig []byte) (*secp256k1.PublicKey, error) {
	// Convert to btcec input format with 'recovery id' v at the beginning.
	btcsig := make([]byte, 65)
	btcsig[0] = sig[64] + 27
	copy(btcsig[1:], sig)

	pub, _, err := secp256k1.RecoverCompact(btcsig, hash)
	return pub, err
}

// Sign calculates an ECDSA signature.
//
// This function is susceptible to chosen plaintext attacks that can leak
// information about the private key that is used for signing. Callers must
// be aware that the given hash cannot be chosen by an adversery. Common
// solution is to hash any input before calculating the signature.
//
// The produced signature is in the [R || S || V] format where V is 0 or 1.
func Sign(hash []byte, prv *secp256k1.PrivateKey) ([]byte, error) {
	if len(hash) != 32 {
		return nil, fmt.Errorf("hash is required to be exactly 32 bytes (%d)", len(hash))
	}
	sig, err := secp256k1.SignCompact((*secp256k1.PrivateKey)(prv), hash, false)
	if err != nil {
		return nil, err
	}
	// Convert to Ethereum signature format with 'recovery id' v at the end.
	v := sig[0] - 27
	copy(sig, sig[1:])
	sig[64] = v
	return sig, nil
}

// VerifySignature checks that the given public key created signature over hash.
// The public key should be in compressed (33 bytes) or uncompressed (65 bytes) format.
// The signature should have the 64 byte [R || S] format.
func VerifySignature(pubkey, hash, signature []byte) bool {
	if len(signature) != 64 {
		return false
	}
	sig := &secp256k1.Signature{R: new(big.Int).SetBytes(signature[:32]), S: new(big.Int).SetBytes(signature[32:])}
	key, err := secp256k1.ParsePubKey(pubkey)
	if err != nil {
		return false
	}
	// Reject malleable signatures. libsecp256k1 does this check but btcec doesn't.
	if sig.S.Cmp(secp256k1halfN) > 0 {
		return false
	}
	return sig.Verify(hash, key)
}

// DecompressPubkey parses a public key in the 33-byte compressed format.
func DecompressPubkey(pubkey []byte) (*secp256k1.PublicKey, error) {
	if len(pubkey) != 33 {
		return nil, errors.New("invalid compressed public key length")
	}
	key, err := secp256k1.ParsePubKey(pubkey)
	if err != nil {
		return nil, err
	}
	return key, nil
}

// CompressPubkey encodes a public key to the 33-byte compressed format.
func CompressPubkey(pubkey *secp256k1.PublicKey) []byte {
	return pubkey.SerializeCompressed()
}
