func (pk *PublicKey) parse(r io.Reader) (err os.Error) {
	// RFC 4880, section 5.5.2
	var buf [6]byte
	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}
	if buf[0] != 4 {
		return error.UnsupportedError("public key version")
	}
	pk.CreationTime = uint32(buf[1])<<24 | uint32(buf[2])<<16 | uint32(buf[3])<<8 | uint32(buf[4])
	pk.PubKeyAlgo = PublicKeyAlgorithm(buf[5])
	switch pk.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSAEncryptOnly, PubKeyAlgoRSASignOnly:
		err = pk.parseRSA(r)
	case PubKeyAlgoDSA:
		err = pk.parseDSA(r)
	default:
		err = error.UnsupportedError("public key type")
	}
	if err != nil {
		return
	}

	// RFC 4880, section 12.2
	fingerPrint := sha1.New()
	pk.SerializeSignaturePrefix(fingerPrint)
	pk.Serialize(fingerPrint)
	copy(pk.Fingerprint[:], fingerPrint.Sum())
	pk.KeyId = binary.BigEndian.Uint64(pk.Fingerprint[12:20])

	return
}

func (pk *PublicKey) parse(r io.Reader) (err os.Error) {
	// RFC 4880, section 5.5.2
	var buf [6]byte
	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}
	if buf[0] != 4 {
		return error.UnsupportedError("public key version")
	}
	pk.CreationTime = uint32(buf[1])<<24 | uint32(buf[2])<<16 | uint32(buf[3])<<8 | uint32(buf[4])
	pk.PubKeyAlgo = PublicKeyAlgorithm(buf[5])
	switch pk.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSAEncryptOnly, PubKeyAlgoRSASignOnly:
		err = pk.parseRSA(r)
	case PubKeyAlgoDSA:
		err = pk.parseDSA(r)
	default:
		err = error.UnsupportedError("public key type: " + strconv.Itoa(int(pk.PubKeyAlgo)))
	}
	if err != nil {
		return
	}

	pk.setFingerPrintAndKeyId()
	return
}

func (pk *PrivateKey) parse(r io.Reader) (err os.Error) {
	err = (&pk.PublicKey).parse(r)
	if err != nil {
		return
	}
	var buf [1]byte
	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}

	s2kType := buf[0]

	switch s2kType {
	case 0:
		pk.s2k = nil
		pk.Encrypted = false
	case 254, 255:
		_, err = readFull(r, buf[:])
		if err != nil {
			return
		}
		pk.cipher = CipherFunction(buf[0])
		pk.Encrypted = true
		pk.s2k, err = s2k.Parse(r)
		if err != nil {
			return
		}
		if s2kType == 254 {
			pk.sha1Checksum = true
		}
	default:
		return error.UnsupportedError("deprecated s2k function in private key")
	}

	if pk.Encrypted {
		blockSize := pk.cipher.blockSize()
		if blockSize == 0 {
			return error.UnsupportedError("unsupported cipher in private key: " + strconv.Itoa(int(pk.cipher)))
		}
		pk.iv = make([]byte, blockSize)
		_, err = readFull(r, pk.iv)
		if err != nil {
			return
		}
	}

	pk.encryptedData, err = ioutil.ReadAll(r)
	if err != nil {
		return
	}

	if !pk.Encrypted {
		return pk.parsePrivateKey(pk.encryptedData)
	}

	return
}

// hashForSignature returns a pair of hashes that can be used to verify a
// signature. The signature may specify that the contents of the signed message
// should be preprocessed (i.e. to normalise line endings). Thus this function
// returns two hashes. The second should be used to hash the message itself and
// performs any needed preprocessing.
func hashForSignature(hashId crypto.Hash, sigType packet.SignatureType) (hash.Hash, hash.Hash, os.Error) {
	h := hashId.New()
	if h == nil {
		return nil, nil, error.UnsupportedError("hash not availible: " + strconv.Itoa(int(hashId)))
	}

	switch sigType {
	case packet.SigTypeBinary:
		return h, h, nil
	case packet.SigTypeText:
		return h, NewCanonicalTextHash(h), nil
	}

	return nil, nil, error.UnsupportedError("unsupported signature type: " + strconv.Itoa(int(sigType)))
}

// parseRSA parses RSA public key material from the given Reader. See RFC 4880,
// section 5.5.2.
func (pk *PublicKey) parseRSA(r io.Reader) (err os.Error) {
	pk.n.bytes, pk.n.bitLength, err = readMPI(r)
	if err != nil {
		return
	}
	pk.e.bytes, pk.e.bitLength, err = readMPI(r)
	if err != nil {
		return
	}

	if len(pk.e.bytes) > 3 {
		err = error.UnsupportedError("large public exponent")
		return
	}
	rsa := &rsa.PublicKey{
		N: new(big.Int).SetBytes(pk.n.bytes),
		E: 0,
	}
	for i := 0; i < len(pk.e.bytes); i++ {
		rsa.E <<= 8
		rsa.E |= int(pk.e.bytes[i])
	}
	pk.PublicKey = rsa
	return
}

// Sign signs a message with a 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) Sign(h hash.Hash, priv *PrivateKey) (err os.Error) {
	sig.outSubpackets = sig.buildSubpackets()
	digest, err := sig.signPrepareHash(h)
	if err != nil {
		return
	}

	switch priv.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		sig.RSASignature.bytes, err = rsa.SignPKCS1v15(rand.Reader, priv.PrivateKey.(*rsa.PrivateKey), sig.Hash, digest)
		sig.RSASignature.bitLength = uint16(8 * len(sig.RSASignature.bytes))
	case PubKeyAlgoDSA:
		r, s, err := dsa.Sign(rand.Reader, priv.PrivateKey.(*dsa.PrivateKey), digest)
		if err == nil {
			sig.DSASigR.bytes = r.Bytes()
			sig.DSASigR.bitLength = uint16(8 * len(sig.DSASigR.bytes))
			sig.DSASigS.bytes = s.Bytes()
			sig.DSASigS.bitLength = uint16(8 * len(sig.DSASigS.bytes))
		}
	default:
		err = error.UnsupportedError("public key algorithm: " + strconv.Itoa(int(sig.PubKeyAlgo)))
	}

	return
}

// Decrypt attempts to decrypt an encrypted session key. If it returns nil,
// ske.Key will contain the session key.
func (ske *SymmetricKeyEncrypted) Decrypt(passphrase []byte) os.Error {
	if !ske.Encrypted {
		return nil
	}

	key := make([]byte, ske.CipherFunc.KeySize())
	ske.s2k(key, passphrase)

	if len(ske.encryptedKey) == 0 {
		ske.Key = key
	} else {
		// the IV is all zeros
		iv := make([]byte, ske.CipherFunc.blockSize())
		c := cipher.NewCFBDecrypter(ske.CipherFunc.new(key), iv)
		c.XORKeyStream(ske.encryptedKey, ske.encryptedKey)
		ske.CipherFunc = CipherFunction(ske.encryptedKey[0])
		if ske.CipherFunc.blockSize() == 0 {
			return error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(ske.CipherFunc)))
		}
		ske.CipherFunc = CipherFunction(ske.encryptedKey[0])
		ske.Key = ske.encryptedKey[1:]
		if len(ske.Key)%ske.CipherFunc.blockSize() != 0 {
			ske.Key = nil
			return error.StructuralError("length of decrypted key not a multiple of block size")
		}
	}

	ske.Encrypted = false
	return nil
}

// Parse reads a binary specification for a string-to-key transformation from r
// and returns a function which performs that transform.
func Parse(r io.Reader) (f func(out, in []byte), err os.Error) {
	var buf [9]byte

	_, err = io.ReadFull(r, buf[:2])
	if err != nil {
		return
	}

	hash, ok := HashIdToHash(buf[1])
	if !ok {
		return nil, error.UnsupportedError("hash for S2K function: " + strconv.Itoa(int(buf[1])))
	}
	h := hash.New()
	if h == nil {
		return nil, error.UnsupportedError("hash not available: " + strconv.Itoa(int(hash)))
	}

	switch buf[0] {
	case 1:
		f := func(out, in []byte) {
			Simple(out, h, in)
		}
		return f, nil
	case 2:
		_, err := io.ReadFull(r, buf[:8])
		if err != nil {
			return
		}
		f := func(out, in []byte) {
			Salted(out, h, in, buf[:8])
		}
		return f, nil
	case 3:
		_, err := io.ReadFull(r, buf[:9])
		if err != nil {
			return
		}
		count := (16 + int(buf[8]&15)) << (uint32(buf[8]>>4) + 6)
		f := func(out, in []byte) {
			Iterated(out, h, in, buf[:8], count)
		}
		return f, nil
	}

	return nil, error.UnsupportedError("S2K function")
}

// readSignedMessage reads a possibly signed message if mdin is non-zero then
// that structure is updated and returned. Otherwise a fresh MessageDetails is
// used.
func readSignedMessage(packets *packet.Reader, mdin *MessageDetails, keyring KeyRing) (md *MessageDetails, err os.Error) {
	if mdin == nil {
		mdin = new(MessageDetails)
	}
	md = mdin

	var p packet.Packet
	var h hash.Hash
	var wrappedHash hash.Hash
FindLiteralData:
	for {
		p, err = packets.Next()
		if err != nil {
			return nil, err
		}
		switch p := p.(type) {
		case *packet.Compressed:
			packets.Push(p.Body)
		case *packet.OnePassSignature:
			if !p.IsLast {
				return nil, error.UnsupportedError("nested signatures")
			}

			h, wrappedHash, err = hashForSignature(p.Hash, p.SigType)
			if err != nil {
				md = nil
				return
			}

			md.IsSigned = true
			md.SignedByKeyId = p.KeyId
			keys := keyring.KeysById(p.KeyId)
			for i, key := range keys {
				if key.SelfSignature.FlagsValid && !key.SelfSignature.FlagSign {
					continue
				}
				md.SignedBy = &keys[i]
				break
			}
		case *packet.LiteralData:
			md.LiteralData = p
			break FindLiteralData
		}
	}

	if md.SignedBy != nil {
		md.UnverifiedBody = &signatureCheckReader{packets, h, wrappedHash, md}
	} else if md.decrypted != nil {
		md.UnverifiedBody = checkReader{md}
	} else {
		md.UnverifiedBody = md.LiteralData.Body
	}

	return md, nil
}

// Parse reads a binary specification for a string-to-key transformation from r
// and returns a function which performs that transform.
func Parse(r io.Reader) (f func(out, in []byte), err os.Error) {
	var buf [9]byte

	_, err = io.ReadFull(r, buf[:2])
	if err != nil {
		return
	}

	h := hashFuncFromType(buf[1])
	if h == nil {
		return nil, error.UnsupportedError("hash for S2K function")
	}

	switch buf[0] {
	case 1:
		f := func(out, in []byte) {
			Simple(out, h, in)
		}
		return f, nil
	case 2:
		_, err := io.ReadFull(r, buf[:8])
		if err != nil {
			return
		}
		f := func(out, in []byte) {
			Salted(out, h, in, buf[:8])
		}
		return f, nil
	case 3:
		_, err := io.ReadFull(r, buf[:9])
		if err != nil {
			return
		}
		count := (16 + int(buf[8]&15)) << (uint32(buf[8]>>4) + 6)
		f := func(out, in []byte) {
			Iterated(out, h, in, buf[:8], count)
		}
		return f, nil
	}

	return nil, error.UnsupportedError("S2K function")
}

func (ske *SymmetricKeyEncrypted) parse(r io.Reader) (err os.Error) {
	// RFC 4880, section 5.3.
	var buf [2]byte
	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}
	if buf[0] != symmetricKeyEncryptedVersion {
		return error.UnsupportedError("SymmetricKeyEncrypted version")
	}
	ske.CipherFunc = CipherFunction(buf[1])

	if ske.CipherFunc.KeySize() == 0 {
		return error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(buf[1])))
	}

	ske.s2k, err = s2k.Parse(r)
	if err != nil {
		return
	}

	encryptedKey := make([]byte, maxSessionKeySizeInBytes)
	// The session key may follow. We just have to try and read to find
	// out. If it exists then we limit it to maxSessionKeySizeInBytes.
	n, err := readFull(r, encryptedKey)
	if err != nil && err != io.ErrUnexpectedEOF {
		return
	}
	err = nil
	if n != 0 {
		if n == maxSessionKeySizeInBytes {
			return error.UnsupportedError("oversized encrypted session key")
		}
		ske.encryptedKey = encryptedKey[:n]
	}

	ske.Encrypted = true

	return
}

// SerializeSymmetricKeyEncrypted serializes a symmetric key packet to w. The
// packet contains a random session key, encrypted by a key derived from the
// given passphrase. The session key is returned and must be passed to
// SerializeSymmetricallyEncrypted.
func SerializeSymmetricKeyEncrypted(w io.Writer, rand io.Reader, passphrase []byte, cipherFunc CipherFunction) (key []byte, err os.Error) {
	keySize := cipherFunc.KeySize()
	if keySize == 0 {
		return nil, error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(cipherFunc)))
	}

	s2kBuf := new(bytes.Buffer)
	keyEncryptingKey := make([]byte, keySize)
	// s2k.Serialize salts and stretches the passphrase, and writes the
	// resulting key to keyEncryptingKey and the s2k descriptor to s2kBuf.
	err = s2k.Serialize(s2kBuf, keyEncryptingKey, rand, passphrase)
	if err != nil {
		return
	}
	s2kBytes := s2kBuf.Bytes()

	packetLength := 2 /* header */ + len(s2kBytes) + 1 /* cipher type */ + keySize
	err = serializeHeader(w, packetTypeSymmetricKeyEncrypted, packetLength)
	if err != nil {
		return
	}

	var buf [2]byte
	buf[0] = symmetricKeyEncryptedVersion
	buf[1] = byte(cipherFunc)
	_, err = w.Write(buf[:])
	if err != nil {
		return
	}
	_, err = w.Write(s2kBytes)
	if err != nil {
		return
	}

	sessionKey := make([]byte, keySize)
	_, err = io.ReadFull(rand, sessionKey)
	if err != nil {
		return
	}
	iv := make([]byte, cipherFunc.blockSize())
	c := cipher.NewCFBEncrypter(cipherFunc.new(keyEncryptingKey), iv)
	encryptedCipherAndKey := make([]byte, keySize+1)
	c.XORKeyStream(encryptedCipherAndKey, buf[1:])
	c.XORKeyStream(encryptedCipherAndKey[1:], sessionKey)
	_, err = w.Write(encryptedCipherAndKey)
	if err != nil {
		return
	}

	key = sessionKey
	return
}

// keySignatureHash returns a Hash of the message that needs to be signed for
// pk to assert a subkey relationship to signed.
func keySignatureHash(pk, signed *PublicKey, sig *Signature) (h hash.Hash, err os.Error) {
	h = sig.Hash.New()
	if h == nil {
		return nil, error.UnsupportedError("hash function")
	}

	// RFC 4880, section 5.2.4
	pk.SerializeSignaturePrefix(h)
	pk.serializeWithoutHeaders(h)
	signed.SerializeSignaturePrefix(h)
	signed.serializeWithoutHeaders(h)
	return
}

// VerifyKeySignature returns nil iff sig is a valid signature, make by this
// public key, of the public key in signed.
func (pk *PublicKey) VerifyKeySignature(signed *PublicKey, sig *Signature) (err os.Error) {
	h := sig.Hash.New()
	if h == nil {
		return error.UnsupportedError("hash function")
	}

	// RFC 4880, section 5.2.4
	pk.SerializeSignaturePrefix(h)
	pk.Serialize(h)
	signed.SerializeSignaturePrefix(h)
	signed.Serialize(h)

	return pk.VerifySignature(h, sig)
}

func (ops *OnePassSignature) parse(r io.Reader) (err error) {
	var buf [13]byte

	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}
	if buf[0] != onePassSignatureVersion {
		err = error_.UnsupportedError("one-pass-signature packet version " + strconv.Itoa(int(buf[0])))
	}

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

	ops.SigType = SignatureType(buf[1])
	ops.PubKeyAlgo = PublicKeyAlgorithm(buf[3])
	ops.KeyId = binary.BigEndian.Uint64(buf[4:12])
	ops.IsLast = buf[12] != 0
	return
}

func (se *SymmetricallyEncrypted) parse(r io.Reader) os.Error {
	if se.MDC {
		// See RFC 4880, section 5.13.
		var buf [1]byte
		_, err := readFull(r, buf[:])
		if err != nil {
			return err
		}
		if buf[0] != symmetricallyEncryptedVersion {
			return error.UnsupportedError("unknown SymmetricallyEncrypted version")
		}
	}
	se.contents = r
	return nil
}

// Sign signs a message with a 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) Sign(h hash.Hash, priv *PrivateKey) (err os.Error) {
	digest, err := sig.signPrepareHash(h)
	if err != nil {
		return
	}

	switch priv.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		sig.RSASignature, err = rsa.SignPKCS1v15(rand.Reader, priv.PrivateKey.(*rsa.PrivateKey), sig.Hash, digest)
	case PubKeyAlgoDSA:
		sig.DSASigR, sig.DSASigS, err = dsa.Sign(rand.Reader, priv.PrivateKey.(*dsa.PrivateKey), digest)
	default:
		err = error.UnsupportedError("public key algorithm: " + strconv.Itoa(int(sig.PubKeyAlgo)))
	}

	return
}

func (e *EncryptedKey) parse(r io.Reader) (err os.Error) {
	var buf [10]byte
	_, err = readFull(r, buf[:])
	if err != nil {
		return
	}
	if buf[0] != 3 {
		return error.UnsupportedError("unknown EncryptedKey version " + strconv.Itoa(int(buf[0])))
	}
	e.KeyId = binary.BigEndian.Uint64(buf[1:9])
	e.Algo = PublicKeyAlgorithm(buf[9])
	if e.Algo == PubKeyAlgoRSA || e.Algo == PubKeyAlgoRSAEncryptOnly {
		e.Encrypted, _, err = readMPI(r)
	}
	_, err = consumeAll(r)
	return
}

func (c *Compressed) parse(r io.Reader) error {
	var buf [1]byte
	_, err := readFull(r, buf[:])
	if err != nil {
		return err
	}

	switch buf[0] {
	case 1:
		c.Body = flate.NewReader(r)
	case 2:
		c.Body, err = zlib.NewReader(r)
	default:
		err = error_.UnsupportedError("unknown compression algorithm: " + strconv.Itoa(int(buf[0])))
	}

	return err
}

// Decrypt returns a ReadCloser, from which the decrypted contents of the
// packet can be read. An incorrect key can, with high probability, be detected
// immediately and this will result in a KeyIncorrect error being returned.
func (se *SymmetricallyEncrypted) Decrypt(c CipherFunction, key []byte) (io.ReadCloser, os.Error) {
	keySize := c.KeySize()
	if keySize == 0 {
		return nil, error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(c)))
	}
	if len(key) != keySize {
		return nil, error.InvalidArgumentError("SymmetricallyEncrypted: incorrect key length")
	}

	if se.prefix == nil {
		se.prefix = make([]byte, c.blockSize()+2)
		_, err := readFull(se.contents, se.prefix)
		if err != nil {
			return nil, err
		}
	} else if len(se.prefix) != c.blockSize()+2 {
		return nil, error.InvalidArgumentError("can't try ciphers with different block lengths")
	}

	ocfbResync := cipher.OCFBResync
	if se.MDC {
		// MDC packets use a different form of OCFB mode.
		ocfbResync = cipher.OCFBNoResync
	}

	s := cipher.NewOCFBDecrypter(c.new(key), se.prefix, ocfbResync)
	if s == nil {
		return nil, error.KeyIncorrectError
	}

	plaintext := cipher.StreamReader{S: s, R: se.contents}

	if se.MDC {
		// MDC packets have an embedded hash that we need to check.
		h := sha1.New()
		h.Write(se.prefix)
		return &seMDCReader{in: plaintext, h: h}, nil
	}

	// Otherwise, we just need to wrap plaintext so that it's a valid ReadCloser.
	return seReader{plaintext}, nil
}