package pki

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/x509"
	"encoding/asn1"
	"encoding/pem"
	"errors"
	"io"
	"math/big"
)

// This label is used as the type in the pem encoding of ECDSA private keys.
const PemLabelEcdsa = "EC PRIVATE KEY"

type (
	// This type handles the function calls to the ecdsa private key by
	// implementing the interface.
	EcdsaPrivateKey struct {
		private_key *ecdsa.PrivateKey
	}

	// EcdsaPublicKey is the specific public key type for ecdsa. It implements the
	// the PublicKey interface.
	EcdsaPublicKey struct {
		public_key *ecdsa.PublicKey
	}

	// This struct is used to marshal and parse the ecdsa signature.
	signatureEcdsa struct {
		R, S *big.Int
	}
)

// Create a new ECDSA private key using the specified curve.
// For available curves, please take a look at the crypto/elliptic package.
func NewPrivateKeyEcdsa(curve elliptic.Curve) (*EcdsaPrivateKey, error) {
	key, err := ecdsa.GenerateKey(curve, rand.Reader)
	if err != nil {
		return nil, err
	}
	return &EcdsaPrivateKey{key}, nil
}

// Load the private key from the asn1 representation.
func LoadPrivateKeyEcdsa(raw []byte) (*EcdsaPrivateKey, error) {
	key, err := x509.ParseECPrivateKey(raw)
	if err != nil {
		return nil, err
	}
	return &EcdsaPrivateKey{key}, nil
}

// Create a new public key from the private key.
func (pr EcdsaPrivateKey) Public() PublicKey {
	return &EcdsaPublicKey{pr.private_key.Public().(*ecdsa.PublicKey)}
}

// Sign a message using the private key and the provided hash function.
func (pr EcdsaPrivateKey) Sign(message []byte, hash crypto.Hash) ([]byte, error) {
	empty := make([]byte, 0)
	if !hash.Available() {
		return empty, errors.New("Hash method is not available!")
	}
	hashed_message := hash.New()
	hashed_message.Write(message)
	return pr.private_key.Sign(rand.Reader, hashed_message.Sum(nil), hash)
}

// This function returns the crypto.PrivateKey structure of the ECDSA key.
func (pr EcdsaPrivateKey) PrivateKey() crypto.PrivateKey {
	return pr.private_key
}

// This function implements the Pemmer interface to marshal the private key
// into a pem block.
func (pr EcdsaPrivateKey) MarshalPem() (io.WriterTo, error) {
	pem_block, err := pr.ToPem()
	if err != nil {
		return nil, err
	}
	return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
}

// This function implements ToPem to return the raw pem block.
func (pr EcdsaPrivateKey) ToPem() (pem.Block, error) {
	asn1, err := x509.MarshalECPrivateKey(pr.private_key)
	if err != nil {
		return pem.Block{}, err
	}
	return pem.Block{Type: PemLabelEcdsa, Bytes: asn1}, nil
}

// This functoin loads an ecdsa public key from the asn.1 representation.
func LoadPublicKeyEcdsa(raw []byte) (*EcdsaPublicKey, error) {
	raw_pub, err := x509.ParsePKIXPublicKey(raw)
	if err != nil {
		return nil, err
	}

	pub, ok := raw_pub.(*ecdsa.PublicKey)
	if !ok {
		return nil, errors.New("Not an ecdsa key!")
	}
	return &EcdsaPublicKey{pub}, nil
}

// This function implements the Pemmer interface to marshal the public key into
// a pem block.
func (pu *EcdsaPublicKey) MarshalPem() (io.WriterTo, error) {
	asn1, err := x509.MarshalPKIXPublicKey(pu.public_key)
	if err != nil {
		return nil, err
	}
	pem_block := pem.Block{Type: PemLabelPublic, Bytes: asn1}
	return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
}

// This function verifies a message using the public key, signature and hash
// function.
// The hash function must be the same as was used to create the signature.
func (pu *EcdsaPublicKey) Verify(message []byte, signature_raw []byte, hash crypto.Hash) (bool, error) {
	var sig signatureEcdsa
	_, err := asn1.Unmarshal(signature_raw, &sig)
	if err != nil {
		return false, err
	}
	hashed_message := hash.New()
	hashed_message.Write(message)
	return ecdsa.Verify(pu.public_key, hashed_message.Sum(nil), sig.R, sig.S), nil
}