func addSubkey(e *Entity, packets *packet.Reader, pub *packet.PublicKey, priv *packet.PrivateKey) os.Error {
	var subKey Subkey
	subKey.PublicKey = pub
	subKey.PrivateKey = priv
	p, err := packets.Next()
	if err == os.EOF {
		return io.ErrUnexpectedEOF
	}
	if err != nil {
		return error.StructuralError("subkey signature invalid: " + err.String())
	}
	var ok bool
	subKey.Sig, ok = p.(*packet.Signature)
	if !ok {
		return error.StructuralError("subkey packet not followed by signature")
	}
	if subKey.Sig.SigType != packet.SigTypeSubkeyBinding {
		return error.StructuralError("subkey signature with wrong type")
	}
	err = e.PrimaryKey.VerifyKeySignature(subKey.PublicKey, subKey.Sig)
	if err != nil {
		return error.StructuralError("subkey signature invalid: " + err.String())
	}
	e.Subkeys = append(e.Subkeys, subKey)
	return nil
}

// 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
}

// readToNextPublicKey reads packets until the start of the entity and leaves
// the first packet of the new entity in the Reader.
func readToNextPublicKey(packets *packet.Reader) (err os.Error) {
	var p packet.Packet
	for {
		p, err = packets.Next()
		if err == os.EOF {
			return
		} else if err != nil {
			if _, ok := err.(error.UnsupportedError); ok {
				err = nil
				continue
			}
			return
		}

		if pk, ok := p.(*packet.PublicKey); ok && !pk.IsSubkey {
			packets.Unread(p)
			return
		}
	}

	panic("unreachable")
}

// readEntity reads an entity (public key, identities, subkeys etc) from the
// given Reader.
func readEntity(packets *packet.Reader) (*Entity, os.Error) {
	e := new(Entity)
	e.Identities = make(map[string]*Identity)

	p, err := packets.Next()
	if err != nil {
		return nil, err
	}

	var ok bool
	if e.PrimaryKey, ok = p.(*packet.PublicKey); !ok {
		if e.PrivateKey, ok = p.(*packet.PrivateKey); !ok {
			packets.Unread(p)
			return nil, error.StructuralError("first packet was not a public/private key")
		} else {
			e.PrimaryKey = &e.PrivateKey.PublicKey
		}
	}

	var current *Identity
EachPacket:
	for {
		p, err := packets.Next()
		if err == os.EOF {
			break
		} else if err != nil {
			return nil, err
		}

		switch pkt := p.(type) {
		case *packet.UserId:
			current = new(Identity)
			current.Name = pkt.Id
			current.UserId = pkt
			e.Identities[pkt.Id] = current

			for {
				p, err = packets.Next()
				if err == os.EOF {
					return nil, io.ErrUnexpectedEOF
				} else if err != nil {
					return nil, err
				}

				sig, ok := p.(*packet.Signature)
				if !ok {
					return nil, error.StructuralError("user ID packet not followed by self-signature")
				}

				if sig.SigType == packet.SigTypePositiveCert && sig.IssuerKeyId != nil && *sig.IssuerKeyId == e.PrimaryKey.KeyId {
					if err = e.PrimaryKey.VerifyUserIdSignature(pkt.Id, sig); err != nil {
						return nil, error.StructuralError("user ID self-signature invalid: " + err.String())
					}
					current.SelfSignature = sig
					break
				}
				current.Signatures = append(current.Signatures, sig)
			}
		case *packet.Signature:
			if current == nil {
				return nil, error.StructuralError("signature packet found before user id packet")
			}
			current.Signatures = append(current.Signatures, pkt)
		case *packet.PrivateKey:
			if pkt.IsSubkey == false {
				packets.Unread(p)
				break EachPacket
			}
			err = addSubkey(e, packets, &pkt.PublicKey, pkt)
			if err != nil {
				return nil, err
			}
		case *packet.PublicKey:
			if pkt.IsSubkey == false {
				packets.Unread(p)
				break EachPacket
			}
			err = addSubkey(e, packets, pkt, nil)
			if err != nil {
				return nil, err
			}
		default:
			// we ignore unknown packets
		}
	}

	if len(e.Identities) == 0 {
		return nil, error.StructuralError("entity without any identities")
	}

	return e, nil
}