client.go

// Copyright 2012 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 agent implements a client to an ssh-agent daemon.

References:
  [PROTOCOL.agent]:    http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.agent?rev=HEAD
*/
package agent

import (
	"bytes"
	"crypto/dsa"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rsa"
	"encoding/base64"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"math/big"
	"sync"

	"golang.org/x/crypto/ssh"
)

// Agent represents the capabilities of an ssh-agent.
type Agent interface {
	// List returns the identities known to the agent.
	List() ([]*Key, error)

	// Sign has the agent sign the data using a protocol 2 key as defined
	// in [PROTOCOL.agent] section 2.6.2.
	Sign(key ssh.PublicKey, data []byte) (*ssh.Signature, error)

	// Insert adds a private key to the agent. If a certificate
	// is given, that certificate is added as public key.
	Add(s interface{}, cert *ssh.Certificate, comment string) error

	// Remove removes all identities with the given public key.
	Remove(key ssh.PublicKey) error

	// RemoveAll removes all identities.
	RemoveAll() error

	// Lock locks the agent. Sign and Remove will fail, and List will empty an empty list.
	Lock(passphrase []byte) error

	// Unlock undoes the effect of Lock
	Unlock(passphrase []byte) error

	// Signers returns signers for all the known keys.
	Signers() ([]ssh.Signer, error)
}

// See [PROTOCOL.agent], section 3.
const (
	agentRequestV1Identities = 1

	// 3.2 Requests from client to agent for protocol 2 key operations
	agentAddIdentity         = 17
	agentRemoveIdentity      = 18
	agentRemoveAllIdentities = 19
	agentAddIdConstrained    = 25

	// 3.3 Key-type independent requests from client to agent
	agentAddSmartcardKey            = 20
	agentRemoveSmartcardKey         = 21
	agentLock                       = 22
	agentUnlock                     = 23
	agentAddSmartcardKeyConstrained = 26

	// 3.7 Key constraint identifiers
	agentConstrainLifetime = 1
	agentConstrainConfirm  = 2
)

// maxAgentResponseBytes is the maximum agent reply size that is accepted. This
// is a sanity check, not a limit in the spec.
const maxAgentResponseBytes = 16 << 20

// Agent messages:
// These structures mirror the wire format of the corresponding ssh agent
// messages found in [PROTOCOL.agent].

// 3.4 Generic replies from agent to client
const agentFailure = 5

type failureAgentMsg struct{}

const agentSuccess = 6

type successAgentMsg struct{}

// See [PROTOCOL.agent], section 2.5.2.
const agentRequestIdentities = 11

type requestIdentitiesAgentMsg struct{}

// See [PROTOCOL.agent], section 2.5.2.
const agentIdentitiesAnswer = 12

type identitiesAnswerAgentMsg struct {
	NumKeys uint32 `sshtype:"12"`
	Keys    []byte `ssh:"rest"`
}

// See [PROTOCOL.agent], section 2.6.2.
const agentSignRequest = 13

type signRequestAgentMsg struct {
	KeyBlob []byte `sshtype:"13"`
	Data    []byte
	Flags   uint32
}

// See [PROTOCOL.agent], section 2.6.2.

// 3.6 Replies from agent to client for protocol 2 key operations
const agentSignResponse = 14

type signResponseAgentMsg struct {
	SigBlob []byte `sshtype:"14"`
}

type publicKey struct {
	Format string
	Rest   []byte `ssh:"rest"`
}

// Key represents a protocol 2 public key as defined in
// [PROTOCOL.agent], section 2.5.2.
type Key struct {
	Format  string
	Blob    []byte
	Comment string
}

func clientErr(err error) error {
	return fmt.Errorf("agent: client error: %v", err)
}

// String returns the storage form of an agent key with the format, base64
// encoded serialized key, and the comment if it is not empty.
func (k *Key) String() string {
	s := string(k.Format) + " " + base64.StdEncoding.EncodeToString(k.Blob)

	if k.Comment != "" {
		s += " " + k.Comment
	}

	return s
}

// Type returns the public key type.
func (k *Key) Type() string {
	return k.Format
}

// Marshal returns key blob to satisfy the ssh.PublicKey interface.
func (k *Key) Marshal() []byte {
	return k.Blob
}

// Verify satisfies the ssh.PublicKey interface, but is not
// implemented for agent keys.
func (k *Key) Verify(data []byte, sig *ssh.Signature) error {
	return errors.New("agent: agent key does not know how to verify")
}

type wireKey struct {
	Format string
	Rest   []byte `ssh:"rest"`
}

func parseKey(in []byte) (out *Key, rest []byte, err error) {
	var record struct {
		Blob    []byte
		Comment string
		Rest    []byte `ssh:"rest"`
	}

	if err := ssh.Unmarshal(in, &record); err != nil {
		return nil, nil, err
	}

	var wk wireKey
	if err := ssh.Unmarshal(record.Blob, &wk); err != nil {
		return nil, nil, err
	}

	return &Key{
		Format:  wk.Format,
		Blob:    record.Blob,
		Comment: record.Comment,
	}, record.Rest, nil
}

// client is a client for an ssh-agent process.
type client struct {
	// conn is typically a *net.UnixConn
	conn io.ReadWriter
	// mu is used to prevent concurrent access to the agent
	mu sync.Mutex
}

// NewClient returns an Agent that talks to an ssh-agent process over
// the given connection.
func NewClient(rw io.ReadWriter) Agent {
	return &client{conn: rw}
}

// call sends an RPC to the agent. On success, the reply is
// unmarshaled into reply and replyType is set to the first byte of
// the reply, which contains the type of the message.
func (c *client) call(req []byte) (reply interface{}, err error) {
	c.mu.Lock()
	defer c.mu.Unlock()

	msg := make([]byte, 4+len(req))
	binary.BigEndian.PutUint32(msg, uint32(len(req)))
	copy(msg[4:], req)
	if _, err = c.conn.Write(msg); err != nil {
		return nil, clientErr(err)
	}

	var respSizeBuf [4]byte
	if _, err = io.ReadFull(c.conn, respSizeBuf[:]); err != nil {
		return nil, clientErr(err)
	}
	respSize := binary.BigEndian.Uint32(respSizeBuf[:])
	if respSize > maxAgentResponseBytes {
		return nil, clientErr(err)
	}

	buf := make([]byte, respSize)
	if _, err = io.ReadFull(c.conn, buf); err != nil {
		return nil, clientErr(err)
	}
	reply, err = unmarshal(buf)
	if err != nil {
		return nil, clientErr(err)
	}
	return reply, err
}

func (c *client) simpleCall(req []byte) error {
	resp, err := c.call(req)
	if err != nil {
		return err
	}
	if _, ok := resp.(*successAgentMsg); ok {
		return nil
	}
	return errors.New("agent: failure")
}

func (c *client) RemoveAll() error {
	return c.simpleCall([]byte{agentRemoveAllIdentities})
}

func (c *client) Remove(key ssh.PublicKey) error {
	req := ssh.Marshal(&agentRemoveIdentityMsg{
		KeyBlob: key.Marshal(),
	})
	return c.simpleCall(req)
}

func (c *client) Lock(passphrase []byte) error {
	req := ssh.Marshal(&agentLockMsg{
		Passphrase: passphrase,
	})
	return c.simpleCall(req)
}

func (c *client) Unlock(passphrase []byte) error {
	req := ssh.Marshal(&agentUnlockMsg{
		Passphrase: passphrase,
	})
	return c.simpleCall(req)
}

// List returns the identities known to the agent.
func (c *client) List() ([]*Key, error) {
	// see [PROTOCOL.agent] section 2.5.2.
	req := []byte{agentRequestIdentities}

	msg, err := c.call(req)
	if err != nil {
		return nil, err
	}

	switch msg := msg.(type) {
	case *identitiesAnswerAgentMsg:
		if msg.NumKeys > maxAgentResponseBytes/8 {
			return nil, errors.New("agent: too many keys in agent reply")
		}
		keys := make([]*Key, msg.NumKeys)
		data := msg.Keys
		for i := uint32(0); i < msg.NumKeys; i++ {
			var key *Key
			var err error
			if key, data, err = parseKey(data); err != nil {
				return nil, err
			}
			keys[i] = key
		}
		return keys, nil
	case *failureAgentMsg:
		return nil, errors.New("agent: failed to list keys")
	}
	panic("unreachable")
}

// Sign has the agent sign the data using a protocol 2 key as defined
// in [PROTOCOL.agent] section 2.6.2.
func (c *client) Sign(key ssh.PublicKey, data []byte) (*ssh.Signature, error) {
	req := ssh.Marshal(signRequestAgentMsg{
		KeyBlob: key.Marshal(),
		Data:    data,
	})

	msg, err := c.call(req)
	if err != nil {
		return nil, err
	}

	switch msg := msg.(type) {
	case *signResponseAgentMsg:
		var sig ssh.Signature
		if err := ssh.Unmarshal(msg.SigBlob, &sig); err != nil {
			return nil, err
		}

		return &sig, nil
	case *failureAgentMsg:
		return nil, errors.New("agent: failed to sign challenge")
	}
	panic("unreachable")
}

// unmarshal parses an agent message in packet, returning the parsed
// form and the message type of packet.
func unmarshal(packet []byte) (interface{}, error) {
	if len(packet) < 1 {
		return nil, errors.New("agent: empty packet")
	}
	var msg interface{}
	switch packet[0] {
	case agentFailure:
		return new(failureAgentMsg), nil
	case agentSuccess:
		return new(successAgentMsg), nil
	case agentIdentitiesAnswer:
		msg = new(identitiesAnswerAgentMsg)
	case agentSignResponse:
		msg = new(signResponseAgentMsg)
	default:
		return nil, fmt.Errorf("agent: unknown type tag %d", packet[0])
	}
	if err := ssh.Unmarshal(packet, msg); err != nil {
		return nil, err
	}
	return msg, nil
}

type rsaKeyMsg struct {
	Type     string `sshtype:"17"`
	N        *big.Int
	E        *big.Int
	D        *big.Int
	Iqmp     *big.Int // IQMP = Inverse Q Mod P
	P        *big.Int
	Q        *big.Int
	Comments string
}

type dsaKeyMsg struct {
	Type     string `sshtype:"17"`
	P        *big.Int
	Q        *big.Int
	G        *big.Int
	Y        *big.Int
	X        *big.Int
	Comments string
}

type ecdsaKeyMsg struct {
	Type     string `sshtype:"17"`
	Curve    string
	KeyBytes []byte
	D        *big.Int
	Comments string
}

// Insert adds a private key to the agent.
func (c *client) insertKey(s interface{}, comment string) error {
	var req []byte
	switch k := s.(type) {
	case *rsa.PrivateKey:
		if len(k.Primes) != 2 {
			return fmt.Errorf("agent: unsupported RSA key with %d primes", len(k.Primes))
		}
		k.Precompute()
		req = ssh.Marshal(rsaKeyMsg{
			Type:     ssh.KeyAlgoRSA,
			N:        k.N,
			E:        big.NewInt(int64(k.E)),
			D:        k.D,
			Iqmp:     k.Precomputed.Qinv,
			P:        k.Primes[0],
			Q:        k.Primes[1],
			Comments: comment,
		})
	case *dsa.PrivateKey:
		req = ssh.Marshal(dsaKeyMsg{
			Type:     ssh.KeyAlgoDSA,
			P:        k.P,
			Q:        k.Q,
			G:        k.G,
			Y:        k.Y,
			X:        k.X,
			Comments: comment,
		})
	case *ecdsa.PrivateKey:
		nistID := fmt.Sprintf("nistp%d", k.Params().BitSize)
		req = ssh.Marshal(ecdsaKeyMsg{
			Type:     "ecdsa-sha2-" + nistID,
			Curve:    nistID,
			KeyBytes: elliptic.Marshal(k.Curve, k.X, k.Y),
			D:        k.D,
			Comments: comment,
		})
	default:
		return fmt.Errorf("agent: unsupported key type %T", s)
	}
	resp, err := c.call(req)
	if err != nil {
		return err
	}
	if _, ok := resp.(*successAgentMsg); ok {
		return nil
	}
	return errors.New("agent: failure")
}

type rsaCertMsg struct {
	Type      string `sshtype:"17"`
	CertBytes []byte
	D         *big.Int
	Iqmp      *big.Int // IQMP = Inverse Q Mod P
	P         *big.Int
	Q         *big.Int
	Comments  string
}

type dsaCertMsg struct {
	Type      string `sshtype:"17"`
	CertBytes []byte
	X         *big.Int
	Comments  string
}

type ecdsaCertMsg struct {
	Type      string `sshtype:"17"`
	CertBytes []byte
	D         *big.Int
	Comments  string
}

// Insert adds a private key to the agent. If a certificate is given,
// that certificate is added instead as public key.
func (c *client) Add(s interface{}, cert *ssh.Certificate, comment string) error {
	if cert == nil {
		return c.insertKey(s, comment)
	} else {
		return c.insertCert(s, cert, comment)
	}
}

func (c *client) insertCert(s interface{}, cert *ssh.Certificate, comment string) error {
	var req []byte
	switch k := s.(type) {
	case *rsa.PrivateKey:
		if len(k.Primes) != 2 {
			return fmt.Errorf("agent: unsupported RSA key with %d primes", len(k.Primes))
		}
		k.Precompute()
		req = ssh.Marshal(rsaCertMsg{
			Type:      cert.Type(),
			CertBytes: cert.Marshal(),
			D:         k.D,
			Iqmp:      k.Precomputed.Qinv,
			P:         k.Primes[0],
			Q:         k.Primes[1],
			Comments:  comment,
		})
	case *dsa.PrivateKey:
		req = ssh.Marshal(dsaCertMsg{
			Type:      cert.Type(),
			CertBytes: cert.Marshal(),
			X:         k.X,
			Comments:  comment,
		})
	case *ecdsa.PrivateKey:
		req = ssh.Marshal(ecdsaCertMsg{
			Type:      cert.Type(),
			CertBytes: cert.Marshal(),
			D:         k.D,
			Comments:  comment,
		})
	default:
		return fmt.Errorf("agent: unsupported key type %T", s)
	}

	signer, err := ssh.NewSignerFromKey(s)
	if err != nil {
		return err
	}
	if bytes.Compare(cert.Key.Marshal(), signer.PublicKey().Marshal()) != 0 {
		return errors.New("agent: signer and cert have different public key")
	}

	resp, err := c.call(req)
	if err != nil {
		return err
	}
	if _, ok := resp.(*successAgentMsg); ok {
		return nil
	}
	return errors.New("agent: failure")
}

// Signers provides a callback for client authentication.
func (c *client) Signers() ([]ssh.Signer, error) {
	keys, err := c.List()
	if err != nil {
		return nil, err
	}

	var result []ssh.Signer
	for _, k := range keys {
		result = append(result, &agentKeyringSigner{c, k})
	}
	return result, nil
}

type agentKeyringSigner struct {
	agent *client
	pub   ssh.PublicKey
}

func (s *agentKeyringSigner) PublicKey() ssh.PublicKey {
	return s.pub
}

func (s *agentKeyringSigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
	// The agent has its own entropy source, so the rand argument is ignored.
	return s.agent.Sign(s.pub, data)
}