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@@ -35,6 +35,16 @@ type keyingTransport interface {
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// direction will be effected if a msgNewKeys message is sent
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// or received.
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prepareKeyChange(*algorithms, *kexResult) error
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+
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+ // setStrictMode sets the strict KEX mode, notably triggering
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+ // sequence number resets on sending or receiving msgNewKeys.
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+ // If the sequence number is already > 1 when setStrictMode
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+ // is called, an error is returned.
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+ setStrictMode() error
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+
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+ // setInitialKEXDone indicates to the transport that the initial key exchange
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+ // was completed
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+ setInitialKEXDone()
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}
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// handshakeTransport implements rekeying on top of a keyingTransport
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@@ -100,6 +110,10 @@ type handshakeTransport struct {
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// The session ID or nil if first kex did not complete yet.
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sessionID []byte
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+
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+ // strictMode indicates if the other side of the handshake indicated
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+ // that we should be following the strict KEX protocol restrictions.
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+ strictMode bool
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}
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type pendingKex struct {
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@@ -209,7 +223,10 @@ func (t *handshakeTransport) readLoop() {
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close(t.incoming)
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break
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}
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- if p[0] == msgIgnore || p[0] == msgDebug {
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+ // If this is the first kex, and strict KEX mode is enabled,
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+ // we don't ignore any messages, as they may be used to manipulate
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+ // the packet sequence numbers.
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+ if !(t.sessionID == nil && t.strictMode) && (p[0] == msgIgnore || p[0] == msgDebug) {
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continue
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}
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t.incoming <- p
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@@ -441,6 +458,11 @@ func (t *handshakeTransport) readOnePacket(first bool) ([]byte, error) {
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return successPacket, nil
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}
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+const (
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+ kexStrictClient = "kex-strict-c-v00@openssh.com"
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+ kexStrictServer = "kex-strict-s-v00@openssh.com"
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+)
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+
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// sendKexInit sends a key change message.
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func (t *handshakeTransport) sendKexInit() error {
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t.mu.Lock()
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@@ -454,7 +476,6 @@ func (t *handshakeTransport) sendKexInit() error {
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}
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msg := &kexInitMsg{
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- KexAlgos: t.config.KeyExchanges,
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CiphersClientServer: t.config.Ciphers,
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CiphersServerClient: t.config.Ciphers,
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MACsClientServer: t.config.MACs,
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@@ -464,6 +485,13 @@ func (t *handshakeTransport) sendKexInit() error {
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}
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io.ReadFull(rand.Reader, msg.Cookie[:])
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+ // We mutate the KexAlgos slice, in order to add the kex-strict extension algorithm,
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+ // and possibly to add the ext-info extension algorithm. Since the slice may be the
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+ // user owned KeyExchanges, we create our own slice in order to avoid using user
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+ // owned memory by mistake.
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+ msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+2) // room for kex-strict and ext-info
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+ msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
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+
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isServer := len(t.hostKeys) > 0
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if isServer {
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for _, k := range t.hostKeys {
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@@ -488,17 +516,24 @@ func (t *handshakeTransport) sendKexInit() error {
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msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat)
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}
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}
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+
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+ if t.sessionID == nil {
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+ msg.KexAlgos = append(msg.KexAlgos, kexStrictServer)
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+ }
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} else {
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msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
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// As a client we opt in to receiving SSH_MSG_EXT_INFO so we know what
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// algorithms the server supports for public key authentication. See RFC
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// 8308, Section 2.1.
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+ //
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+ // We also send the strict KEX mode extension algorithm, in order to opt
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+ // into the strict KEX mode.
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if firstKeyExchange := t.sessionID == nil; firstKeyExchange {
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- msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
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- msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
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msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
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+ msg.KexAlgos = append(msg.KexAlgos, kexStrictClient)
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}
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+
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}
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packet := Marshal(msg)
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@@ -604,6 +639,13 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
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return err
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}
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+ if t.sessionID == nil && ((isClient && contains(serverInit.KexAlgos, kexStrictServer)) || (!isClient && contains(clientInit.KexAlgos, kexStrictClient))) {
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+ t.strictMode = true
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+ if err := t.conn.setStrictMode(); err != nil {
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+ return err
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+ }
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+ }
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+
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// We don't send FirstKexFollows, but we handle receiving it.
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//
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// RFC 4253 section 7 defines the kex and the agreement method for
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@@ -679,6 +721,12 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
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return unexpectedMessageError(msgNewKeys, packet[0])
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}
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+ if firstKeyExchange {
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+ // Indicates to the transport that the first key exchange is completed
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+ // after receiving SSH_MSG_NEWKEYS.
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+ t.conn.setInitialKEXDone()
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+ }
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+
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return nil
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}
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