// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package packet

import (
	"big"
	"crypto"
	"crypto/dsa"
	"crypto/openpgp/error"
	"crypto/openpgp/s2k"
	"crypto/rand"
	"crypto/rsa"
	"encoding/binary"
	"hash"
	"io"
	"os"
	"strconv"
)

// Signature represents a signature. See RFC 4880, section 5.2.
type Signature struct {
	SigType    SignatureType
	PubKeyAlgo PublicKeyAlgorithm
	Hash       crypto.Hash

	// HashSuffix is extra data that is hashed in after the signed data.
	HashSuffix []byte
	// HashTag contains the first two bytes of the hash for fast rejection
	// of bad signed data.
	HashTag      [2]byte
	CreationTime uint32 // Unix epoch time

	RSASignature     []byte
	DSASigR, DSASigS *big.Int

	// The following are optional so are nil when not included in the
	// signature.

	SigLifetimeSecs, KeyLifetimeSecs                        *uint32
	PreferredSymmetric, PreferredHash, PreferredCompression []uint8
	IssuerKeyId                                             *uint64
	IsPrimaryId                                             *bool

	// FlagsValid is set if any flags were given. See RFC 4880, section
	// 5.2.3.21 for details.
	FlagsValid                                                           bool
	FlagCertify, FlagSign, FlagEncryptCommunications, FlagEncryptStorage bool

	outSubpackets []outputSubpacket
}

func (sig *Signature) parse(r io.Reader) (err os.Error) {
	// RFC 4880, section 5.2.3
	var buf [5]byte
	_, err = readFull(r, buf[:1])
	if err != nil {
		return
	}
	if buf[0] != 4 {
		err = error.UnsupportedError("signature packet version " + strconv.Itoa(int(buf[0])))
		return
	}

	_, err = readFull(r, buf[:5])
	if err != nil {
		return
	}
	sig.SigType = SignatureType(buf[0])
	sig.PubKeyAlgo = PublicKeyAlgorithm(buf[1])
	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly, PubKeyAlgoDSA:
	default:
		err = error.UnsupportedError("public key algorithm " + strconv.Itoa(int(sig.PubKeyAlgo)))
		return
	}

	var ok bool
	sig.Hash, ok = s2k.HashIdToHash(buf[2])
	if !ok {
		return error.UnsupportedError("hash function " + strconv.Itoa(int(buf[2])))
	}

	hashedSubpacketsLength := int(buf[3])<<8 | int(buf[4])
	l := 6 + hashedSubpacketsLength
	sig.HashSuffix = make([]byte, l+6)
	sig.HashSuffix[0] = 4
	copy(sig.HashSuffix[1:], buf[:5])
	hashedSubpackets := sig.HashSuffix[6:l]
	_, err = readFull(r, hashedSubpackets)
	if err != nil {
		return
	}
	// See RFC 4880, section 5.2.4
	trailer := sig.HashSuffix[l:]
	trailer[0] = 4
	trailer[1] = 0xff
	trailer[2] = uint8(l >> 24)
	trailer[3] = uint8(l >> 16)
	trailer[4] = uint8(l >> 8)
	trailer[5] = uint8(l)

	err = parseSignatureSubpackets(sig, hashedSubpackets, true)
	if err != nil {
		return
	}

	_, err = readFull(r, buf[:2])
	if err != nil {
		return
	}
	unhashedSubpacketsLength := int(buf[0])<<8 | int(buf[1])
	unhashedSubpackets := make([]byte, unhashedSubpacketsLength)
	_, err = readFull(r, unhashedSubpackets)
	if err != nil {
		return
	}
	err = parseSignatureSubpackets(sig, unhashedSubpackets, false)
	if err != nil {
		return
	}

	_, err = readFull(r, sig.HashTag[:2])
	if err != nil {
		return
	}

	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		sig.RSASignature, _, err = readMPI(r)
	case PubKeyAlgoDSA:
		var rBytes, sBytes []byte
		rBytes, _, err = readMPI(r)
		sig.DSASigR = new(big.Int).SetBytes(rBytes)
		if err == nil {
			sBytes, _, err = readMPI(r)
			sig.DSASigS = new(big.Int).SetBytes(sBytes)
		}
	default:
		panic("unreachable")
	}
	return
}

// parseSignatureSubpackets parses subpackets of the main signature packet. See
// RFC 4880, section 5.2.3.1.
func parseSignatureSubpackets(sig *Signature, subpackets []byte, isHashed bool) (err os.Error) {
	for len(subpackets) > 0 {
		subpackets, err = parseSignatureSubpacket(sig, subpackets, isHashed)
		if err != nil {
			return
		}
	}

	if sig.CreationTime == 0 {
		err = error.StructuralError("no creation time in signature")
	}

	return
}

type signatureSubpacketType uint8

const (
	creationTimeSubpacket        signatureSubpacketType = 2
	signatureExpirationSubpacket signatureSubpacketType = 3
	keyExpirySubpacket           signatureSubpacketType = 9
	prefSymmetricAlgosSubpacket  signatureSubpacketType = 11
	issuerSubpacket              signatureSubpacketType = 16
	prefHashAlgosSubpacket       signatureSubpacketType = 21
	prefCompressionSubpacket     signatureSubpacketType = 22
	primaryUserIdSubpacket       signatureSubpacketType = 25
	keyFlagsSubpacket            signatureSubpacketType = 27
)

// parseSignatureSubpacket parses a single subpacket. len(subpacket) is >= 1.
func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (rest []byte, err os.Error) {
	// RFC 4880, section 5.2.3.1
	var length uint32
	switch {
	case subpacket[0] < 192:
		length = uint32(subpacket[0])
		subpacket = subpacket[1:]
	case subpacket[0] < 255:
		if len(subpacket) < 2 {
			goto Truncated
		}
		length = uint32(subpacket[0]-192)<<8 + uint32(subpacket[1]) + 192
		subpacket = subpacket[2:]
	default:
		if len(subpacket) < 5 {
			goto Truncated
		}
		length = uint32(subpacket[1])<<24 |
			uint32(subpacket[2])<<16 |
			uint32(subpacket[3])<<8 |
			uint32(subpacket[4])
		subpacket = subpacket[5:]
	}
	if length > uint32(len(subpacket)) {
		goto Truncated
	}
	rest = subpacket[length:]
	subpacket = subpacket[:length]
	if len(subpacket) == 0 {
		err = error.StructuralError("zero length signature subpacket")
		return
	}
	packetType := subpacket[0] & 0x7f
	isCritial := subpacket[0]&0x80 == 0x80
	subpacket = subpacket[1:]
	switch signatureSubpacketType(packetType) {
	case creationTimeSubpacket:
		if !isHashed {
			err = error.StructuralError("signature creation time in non-hashed area")
			return
		}
		if len(subpacket) != 4 {
			err = error.StructuralError("signature creation time not four bytes")
			return
		}
		sig.CreationTime = binary.BigEndian.Uint32(subpacket)
	case signatureExpirationSubpacket:
		// Signature expiration time, section 5.2.3.10
		if !isHashed {
			return
		}
		if len(subpacket) != 4 {
			err = error.StructuralError("expiration subpacket with bad length")
			return
		}
		sig.SigLifetimeSecs = new(uint32)
		*sig.SigLifetimeSecs = binary.BigEndian.Uint32(subpacket)
	case keyExpirySubpacket:
		// Key expiration time, section 5.2.3.6
		if !isHashed {
			return
		}
		if len(subpacket) != 4 {
			err = error.StructuralError("key expiration subpacket with bad length")
			return
		}
		sig.KeyLifetimeSecs = new(uint32)
		*sig.KeyLifetimeSecs = binary.BigEndian.Uint32(subpacket)
	case prefSymmetricAlgosSubpacket:
		// Preferred symmetric algorithms, section 5.2.3.7
		if !isHashed {
			return
		}
		sig.PreferredSymmetric = make([]byte, len(subpacket))
		copy(sig.PreferredSymmetric, subpacket)
	case issuerSubpacket:
		// Issuer, section 5.2.3.5
		if len(subpacket) != 8 {
			err = error.StructuralError("issuer subpacket with bad length")
			return
		}
		sig.IssuerKeyId = new(uint64)
		*sig.IssuerKeyId = binary.BigEndian.Uint64(subpacket)
	case prefHashAlgosSubpacket:
		// Preferred hash algorithms, section 5.2.3.8
		if !isHashed {
			return
		}
		sig.PreferredHash = make([]byte, len(subpacket))
		copy(sig.PreferredHash, subpacket)
	case prefCompressionSubpacket:
		// Preferred compression algorithms, section 5.2.3.9
		if !isHashed {
			return
		}
		sig.PreferredCompression = make([]byte, len(subpacket))
		copy(sig.PreferredCompression, subpacket)
	case primaryUserIdSubpacket:
		// Primary User ID, section 5.2.3.19
		if !isHashed {
			return
		}
		if len(subpacket) != 1 {
			err = error.StructuralError("primary user id subpacket with bad length")
			return
		}
		sig.IsPrimaryId = new(bool)
		if subpacket[0] > 0 {
			*sig.IsPrimaryId = true
		}
	case keyFlagsSubpacket:
		// Key flags, section 5.2.3.21
		if !isHashed {
			return
		}
		if len(subpacket) == 0 {
			err = error.StructuralError("empty key flags subpacket")
			return
		}
		sig.FlagsValid = true
		if subpacket[0]&1 != 0 {
			sig.FlagCertify = true
		}
		if subpacket[0]&2 != 0 {
			sig.FlagSign = true
		}
		if subpacket[0]&4 != 0 {
			sig.FlagEncryptCommunications = true
		}
		if subpacket[0]&8 != 0 {
			sig.FlagEncryptStorage = true
		}

	default:
		if isCritial {
			err = error.UnsupportedError("unknown critical signature subpacket type " + strconv.Itoa(int(packetType)))
			return
		}
	}
	return

Truncated:
	err = error.StructuralError("signature subpacket truncated")
	return
}

// subpacketLengthLength returns the length, in bytes, of an encoded length value.
func subpacketLengthLength(length int) int {
	if length < 192 {
		return 1
	}
	if length < 16320 {
		return 2
	}
	return 5
}

// serializeSubpacketLength marshals the given length into to.
func serializeSubpacketLength(to []byte, length int) int {
	if length < 192 {
		to[0] = byte(length)
		return 1
	}
	if length < 16320 {
		length -= 192
		to[0] = byte(length >> 8)
		to[1] = byte(length)
		return 2
	}
	to[0] = 255
	to[1] = byte(length >> 24)
	to[2] = byte(length >> 16)
	to[3] = byte(length >> 8)
	to[4] = byte(length)
	return 5
}

// subpacketsLength returns the serialized length, in bytes, of the given
// subpackets.
func subpacketsLength(subpackets []outputSubpacket, hashed bool) (length int) {
	for _, subpacket := range subpackets {
		if subpacket.hashed == hashed {
			length += subpacketLengthLength(len(subpacket.contents) + 1)
			length += 1 // type byte
			length += len(subpacket.contents)
		}
	}
	return
}

// serializeSubpackets marshals the given subpackets into to.
func serializeSubpackets(to []byte, subpackets []outputSubpacket, hashed bool) {
	for _, subpacket := range subpackets {
		if subpacket.hashed == hashed {
			n := serializeSubpacketLength(to, len(subpacket.contents)+1)
			to[n] = byte(subpacket.subpacketType)
			to = to[1+n:]
			n = copy(to, subpacket.contents)
			to = to[n:]
		}
	}
	return
}

// buildHashSuffix constructs the HashSuffix member of sig in preparation for signing.
func (sig *Signature) buildHashSuffix() (err os.Error) {
	sig.outSubpackets = sig.buildSubpackets()
	hashedSubpacketsLen := subpacketsLength(sig.outSubpackets, true)

	var ok bool
	l := 6 + hashedSubpacketsLen
	sig.HashSuffix = make([]byte, l+6)
	sig.HashSuffix[0] = 4
	sig.HashSuffix[1] = uint8(sig.SigType)
	sig.HashSuffix[2] = uint8(sig.PubKeyAlgo)
	sig.HashSuffix[3], ok = s2k.HashToHashId(sig.Hash)
	if !ok {
		sig.HashSuffix = nil
		return error.InvalidArgumentError("hash cannot be repesented in OpenPGP: " + strconv.Itoa(int(sig.Hash)))
	}
	sig.HashSuffix[4] = byte(hashedSubpacketsLen >> 8)
	sig.HashSuffix[5] = byte(hashedSubpacketsLen)
	serializeSubpackets(sig.HashSuffix[6:l], sig.outSubpackets, true)
	trailer := sig.HashSuffix[l:]
	trailer[0] = 4
	trailer[1] = 0xff
	trailer[2] = byte(l >> 24)
	trailer[3] = byte(l >> 16)
	trailer[4] = byte(l >> 8)
	trailer[5] = byte(l)
	return
}

func (sig *Signature) signPrepareHash(h hash.Hash) (digest []byte, err os.Error) {
	err = sig.buildHashSuffix()
	if err != nil {
		return
	}

	h.Write(sig.HashSuffix)
	digest = h.Sum()
	copy(sig.HashTag[:], digest)
	return
}

// SignRSA signs a message with an RSA private key. The hash, h, must contain
// the hash of the message to be signed and will be mutated by this function.
// On success, the signature is stored in sig. Call Serialize to write it out.
func (sig *Signature) SignRSA(h hash.Hash, priv *rsa.PrivateKey) (err os.Error) {
	digest, err := sig.signPrepareHash(h)
	if err != nil {
		return
	}
	sig.RSASignature, err = rsa.SignPKCS1v15(rand.Reader, priv, sig.Hash, digest)
	return
}

// SignDSA signs a message with a DSA private key. The hash, h, must contain
// the hash of the message to be signed and will be mutated by this function.
// On success, the signature is stored in sig. Call Serialize to write it out.
func (sig *Signature) SignDSA(h hash.Hash, priv *dsa.PrivateKey) (err os.Error) {
	digest, err := sig.signPrepareHash(h)
	if err != nil {
		return
	}
	sig.DSASigR, sig.DSASigS, err = dsa.Sign(rand.Reader, priv, digest)
	return
}

// Serialize marshals sig to w. SignRSA or SignDSA must have been called first.
func (sig *Signature) Serialize(w io.Writer) (err os.Error) {
	if sig.RSASignature == nil && sig.DSASigR == nil {
		return error.InvalidArgumentError("Signature: need to call SignRSA or SignDSA before Serialize")
	}

	sigLength := 0
	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		sigLength = len(sig.RSASignature)
	case PubKeyAlgoDSA:
		sigLength = 2 /* MPI length */
		sigLength += (sig.DSASigR.BitLen() + 7) / 8
		sigLength += 2 /* MPI length */
		sigLength += (sig.DSASigS.BitLen() + 7) / 8
	default:
		panic("impossible")
	}

	unhashedSubpacketsLen := subpacketsLength(sig.outSubpackets, false)
	length := len(sig.HashSuffix) - 6 /* trailer not included */ +
		2 /* length of unhashed subpackets */ + unhashedSubpacketsLen +
		2 /* hash tag */ + 2 /* length of signature MPI */ + sigLength
	err = serializeHeader(w, packetTypeSignature, length)
	if err != nil {
		return
	}

	_, err = w.Write(sig.HashSuffix[:len(sig.HashSuffix)-6])
	if err != nil {
		return
	}

	unhashedSubpackets := make([]byte, 2+unhashedSubpacketsLen)
	unhashedSubpackets[0] = byte(unhashedSubpacketsLen >> 8)
	unhashedSubpackets[1] = byte(unhashedSubpacketsLen)
	serializeSubpackets(unhashedSubpackets[2:], sig.outSubpackets, false)

	_, err = w.Write(unhashedSubpackets)
	if err != nil {
		return
	}
	_, err = w.Write(sig.HashTag[:])
	if err != nil {
		return
	}

	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		err = writeMPI(w, 8*uint16(len(sig.RSASignature)), sig.RSASignature)
	case PubKeyAlgoDSA:
		err = writeBig(w, sig.DSASigR)
		if err == nil {
			err = writeBig(w, sig.DSASigS)
		}
	default:
		panic("impossible")
	}
	return
}

// outputSubpacket represents a subpacket to be marshaled.
type outputSubpacket struct {
	hashed        bool // true if this subpacket is in the hashed area.
	subpacketType signatureSubpacketType
	contents      []byte
}

func (sig *Signature) buildSubpackets() (subpackets []outputSubpacket) {
	creationTime := make([]byte, 4)
	creationTime[0] = byte(sig.CreationTime >> 24)
	creationTime[1] = byte(sig.CreationTime >> 16)
	creationTime[2] = byte(sig.CreationTime >> 8)
	creationTime[3] = byte(sig.CreationTime)
	subpackets = append(subpackets, outputSubpacket{true, creationTimeSubpacket, creationTime})

	if sig.IssuerKeyId != nil {
		keyId := make([]byte, 8)
		binary.BigEndian.PutUint64(keyId, *sig.IssuerKeyId)
		subpackets = append(subpackets, outputSubpacket{true, issuerSubpacket, keyId})
	}

	return
}