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// 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://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.agent
*/
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)
}
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