// 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 the ssh-agent protocol, and provides both
// a client and a server. The client can talk to a standard ssh-agent
// that uses UNIX sockets, and one could implement an alternative
// ssh-agent process using the sample server.
//
// References:
// [PROTOCOL.agent]: https://tools.ietf.org/html/draft-miller-ssh-agent-00
package agent // import "golang.org/x/crypto/ssh/agent"
import (
"bytes"
"crypto/dsa"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rsa"
"encoding/base64"
"encoding/binary"
"errors"
"fmt"
"io"
"math/big"
"sync"
"crypto"
"golang.org/x/crypto/ed25519"
"golang.org/x/crypto/ssh"
)
// SignatureFlags represent additional flags that can be passed to the signature
// requests an defined in [PROTOCOL.agent] section 4.5.1.
type SignatureFlags uint32
// SignatureFlag values as defined in [PROTOCOL.agent] section 5.3.
const (
SignatureFlagReserved SignatureFlags = 1 << iota
SignatureFlagRsaSha256
SignatureFlagRsaSha512
)
// 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)
// Add adds a private key to the agent.
Add(key AddedKey) 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)
}
type ExtendedAgent interface {
Agent
// SignWithFlags signs like Sign, but allows for additional flags to be sent/received
SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFlags) (*ssh.Signature, error)
// Extension processes a custom extension request. Standard-compliant agents are not
// required to support any extensions, but this method allows agents to implement
// vendor-specific methods or add experimental features. See [PROTOCOL.agent] section 4.7.
// If agent extensions are unsupported entirely this method MUST return an
// ErrExtensionUnsupported error. Similarly, if just the specific extensionType in
// the request is unsupported by the agent then ErrExtensionUnsupported MUST be
// returned.
//
// In the case of success, since [PROTOCOL.agent] section 4.7 specifies that the contents
// of the response are unspecified (including the type of the message), the complete
// response will be returned as a []byte slice, including the "type" byte of the message.
Extension(extensionType string, contents []byte) ([]byte, error)
}
// ConstraintExtension describes an optional constraint defined by users.
type ConstraintExtension struct {
// ExtensionName consist of a UTF-8 string suffixed by the
// implementation domain following the naming scheme defined
// in Section 4.2 of [RFC4251], e.g. "foo@example.com".
ExtensionName string
// ExtensionDetails contains the actual content of the extended
// constraint.
ExtensionDetails []byte
}
// AddedKey describes an SSH key to be added to an Agent.
type AddedKey struct {
// PrivateKey must be a *rsa.PrivateKey, *dsa.PrivateKey or
// *ecdsa.PrivateKey, which will be inserted into the agent.
PrivateKey interface{}
// Certificate, if not nil, is communicated to the agent and will be
// stored with the key.
Certificate *ssh.Certificate
// Comment is an optional, free-form string.
Comment string
// LifetimeSecs, if not zero, is the number of seconds that the
// agent will store the key for.
LifetimeSecs uint32
// ConfirmBeforeUse, if true, requests that the agent confirm with the
// user before each use of this key.
ConfirmBeforeUse bool
// ConstraintExtensions are the experimental or private-use constraints
// defined by users.
ConstraintExtensions []ConstraintExtension
}
// See [PROTOCOL.agent], section 3.
const (
agentRequestV1Identities = 1
agentRemoveAllV1Identities = 9
// 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
agentConstrainExtension = 3
)
// 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"`
}
// 3.7 Key constraint identifiers
type constrainLifetimeAgentMsg struct {
LifetimeSecs uint32 `sshtype:"1"`
}
type constrainExtensionAgentMsg struct {
ExtensionName string `sshtype:"3"`
ExtensionDetails []byte
// Rest is a field used for parsing, not part of message
Rest []byte `ssh:"rest"`
}
// See [PROTOCOL.agent], section 4.7
const agentExtension = 27
const agentExtensionFailure = 28
// ErrExtensionUnsupported indicates that an extension defined in
// [PROTOCOL.agent] section 4.7 is unsupported by the agent. Specifically this
// error indicates that the agent returned a standard SSH_AGENT_FAILURE message
// as the result of a SSH_AGENTC_EXTENSION request. Note that the protocol
// specification (and therefore this error) does not distinguish between a
// specific extension being unsupported and extensions being unsupported entirely.
var ErrExtensionUnsupported = errors.New("agent: extension unsupported")
type extensionAgentMsg struct {
ExtensionType string `sshtype:"27"`
Contents []byte
}
// 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.
func (k *Key) Verify(data []byte, sig *ssh.Signature) error {
pubKey, err := ssh.ParsePublicKey(k.Blob)
if err != nil {
return fmt.Errorf("agent: bad public key: %v", err)
}
return pubKey.Verify(data, sig)
}
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) ExtendedAgent {
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) {
buf, err := c.callRaw(req)
if err != nil {
return nil, err
}
reply, err = unmarshal(buf)
if err != nil {
return nil, clientErr(err)
}
return reply, nil
}
// callRaw sends an RPC to the agent. On success, the raw
// bytes of the response are returned; no unmarshalling is
// performed on the response.
func (c *client) callRaw(req []byte) (reply []byte, 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(errors.New("response too large"))
}
buf := make([]byte, respSize)
if _, err = io.ReadFull(c.conn, buf); err != nil {
return nil, clientErr(err)
}
return buf, nil
}
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) {
return c.SignWithFlags(key, data, 0)
}
func (c *client) SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFlags) (*ssh.Signature, error) {
req := ssh.Marshal(signRequestAgentMsg{
KeyBlob: key.Marshal(),
Data: data,
Flags: uint32(flags),
})
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)
case agentV1IdentitiesAnswer:
msg = new(agentV1IdentityMsg)
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|25"`
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
Constraints []byte `ssh:"rest"`
}
type dsaKeyMsg struct {
Type string `sshtype:"17|25"`
P *big.Int
Q *big.Int
G *big.Int
Y *big.Int
X *big.Int
Comments string
Constraints []byte `ssh:"rest"`
}
type ecdsaKeyMsg struct {
Type string `sshtype:"17|25"`
Curve string
KeyBytes []byte
D *big.Int
Comments string
Constraints []byte `ssh:"rest"`
}
type ed25519KeyMsg struct {
Type string `sshtype:"17|25"`
Pub []byte
Priv []byte
Comments string
Constraints []byte `ssh:"rest"`
}
// Insert adds a private key to the agent.
func (c *client) insertKey(s interface{}, comment string, constraints []byte) 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,
Constraints: constraints,
})
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,
Constraints: constraints,
})
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,
Constraints: constraints,
})
case *ed25519.PrivateKey:
req = ssh.Marshal(ed25519KeyMsg{
Type: ssh.KeyAlgoED25519,
Pub: []byte(*k)[32:],
Priv: []byte(*k),
Comments: comment,
Constraints: constraints,
})
default:
return fmt.Errorf("agent: unsupported key type %T", s)
}
// if constraints are present then the message type needs to be changed.
if len(constraints) != 0 {
req[0] = agentAddIDConstrained
}
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|25"`
CertBytes []byte
D *big.Int
Iqmp *big.Int // IQMP = Inverse Q Mod P
P *big.Int
Q *big.Int
Comments string
Constraints []byte `ssh:"rest"`
}
type dsaCertMsg struct {
Type string `sshtype:"17|25"`
CertBytes []byte
X *big.Int
Comments string
Constraints []byte `ssh:"rest"`
}
type ecdsaCertMsg struct {
Type string `sshtype:"17|25"`
CertBytes []byte
D *big.Int
Comments string
Constraints []byte `ssh:"rest"`
}
type ed25519CertMsg struct {
Type string `sshtype:"17|25"`
CertBytes []byte
Pub []byte
Priv []byte
Comments string
Constraints []byte `ssh:"rest"`
}
// Add adds a private key to the agent. If a certificate is given,
// that certificate is added instead as public key.
func (c *client) Add(key AddedKey) error {
var constraints []byte
if secs := key.LifetimeSecs; secs != 0 {
constraints = append(constraints, ssh.Marshal(constrainLifetimeAgentMsg{secs})...)
}
if key.ConfirmBeforeUse {
constraints = append(constraints, agentConstrainConfirm)
}
cert := key.Certificate
if cert == nil {
return c.insertKey(key.PrivateKey, key.Comment, constraints)
}
return c.insertCert(key.PrivateKey, cert, key.Comment, constraints)
}
func (c *client) insertCert(s interface{}, cert *ssh.Certificate, comment string, constraints []byte) 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,
Constraints: constraints,
})
case *dsa.PrivateKey:
req = ssh.Marshal(dsaCertMsg{
Type: cert.Type(),
CertBytes: cert.Marshal(),
X: k.X,
Comments: comment,
Constraints: constraints,
})
case *ecdsa.PrivateKey:
req = ssh.Marshal(ecdsaCertMsg{
Type: cert.Type(),
CertBytes: cert.Marshal(),
D: k.D,
Comments: comment,
Constraints: constraints,
})
case *ed25519.PrivateKey:
req = ssh.Marshal(ed25519CertMsg{
Type: cert.Type(),
CertBytes: cert.Marshal(),
Pub: []byte(*k)[32:],
Priv: []byte(*k),
Comments: comment,
Constraints: constraints,
})
default:
return fmt.Errorf("agent: unsupported key type %T", s)
}
// if constraints are present then the message type needs to be changed.
if len(constraints) != 0 {
req[0] = agentAddIDConstrained
}
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)
}
func (s *agentKeyringSigner) SignWithOpts(rand io.Reader, data []byte, opts crypto.SignerOpts) (*ssh.Signature, error) {
var flags SignatureFlags
if opts != nil {
switch opts.HashFunc() {
case crypto.SHA256:
flags = SignatureFlagRsaSha256
case crypto.SHA512:
flags = SignatureFlagRsaSha512
}
}
return s.agent.SignWithFlags(s.pub, data, flags)
}
// Calls an extension method. It is up to the agent implementation as to whether or not
// any particular extension is supported and may always return an error. Because the
// type of the response is up to the implementation, this returns the bytes of the
// response and does not attempt any type of unmarshalling.
func (c *client) Extension(extensionType string, contents []byte) ([]byte, error) {
req := ssh.Marshal(extensionAgentMsg{
ExtensionType: extensionType,
Contents: contents,
})
buf, err := c.callRaw(req)
if err != nil {
return nil, err
}
if len(buf) == 0 {
return nil, errors.New("agent: failure; empty response")
}
// [PROTOCOL.agent] section 4.7 indicates that an SSH_AGENT_FAILURE message
// represents an agent that does not support the extension
if buf[0] == agentFailure {
return nil, ErrExtensionUnsupported
}
if buf[0] == agentExtensionFailure {
return nil, errors.New("agent: generic extension failure")
}
return buf, nil
}
