ssh/agent: parse constraints when adding keys

Change-Id: I264fc3e3e441d6e5ff7c5aa624eee1018cf9e4de
Reviewed-on: https://go-review.googlesource.com/50811
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/agent/client.go

@@ -57,6 +57,17 @@ type Agent interface {
 	Signers() ([]ssh.Signer, 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
@@ -73,6 +84,9 @@ type AddedKey struct {
 	// 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.
@@ -94,8 +108,9 @@ const (
 	agentAddSmartcardKeyConstrained = 26
 
 	// 3.7 Key constraint identifiers
-	agentConstrainLifetime = 1
-	agentConstrainConfirm  = 2
+	agentConstrainLifetime  = 1
+	agentConstrainConfirm   = 2
+	agentConstrainExtension = 3
 )
 
 // maxAgentResponseBytes is the maximum agent reply size that is accepted. This
@@ -151,6 +166,19 @@ type publicKey struct {
 	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"`
+}
+
 // Key represents a protocol 2 public key as defined in
 // [PROTOCOL.agent], section 2.5.2.
 type Key struct {
@@ -542,11 +570,7 @@ func (c *client) Add(key AddedKey) error {
 	var constraints []byte
 
 	if secs := key.LifetimeSecs; secs != 0 {
-		constraints = append(constraints, agentConstrainLifetime)
-
-		var secsBytes [4]byte
-		binary.BigEndian.PutUint32(secsBytes[:], secs)
-		constraints = append(constraints, secsBytes[:]...)
+		constraints = append(constraints, ssh.Marshal(constrainLifetimeAgentMsg{secs})...)
 	}
 
 	if key.ConfirmBeforeUse {