Proof of concept to add KeyLogWriter to go crypto/tls (NSS key log file output)

go-13057.diff

diff --git a/vendor/crypto/tls/common.go b/vendor/crypto/tls/common.go
index c68ebfe..4ccde9b 100644
--- a/vendor/crypto/tls/common.go
+++ b/vendor/crypto/tls/common.go
@@ -349,6 +349,13 @@ type Config struct {
 	// be used.
 	CurvePreferences []CurveID
 
+	// KeyLogWriter is a debug log stream of all TLS master secrets
+	// as negotiated between client and server.
+	// KeyLogWriter is disabled when nil (default) and must only be
+	// enabled explicitly for debugging as by definition it compromises
+	// the security.
+	KeyLogWriter io.Writer
+
 	serverInitOnce sync.Once // guards calling (*Config).serverInit
 
 	// mutex protects sessionTicketKeys
@@ -562,6 +569,16 @@ func (c *Config) BuildNameToCertificate() {
 	}
 }
 
+// writeKeyLog logs client random and master secret if logging enabled
+// by setting KeyLogWriter.
+func (c *Config) writeKeyLog(clientRandom, masterSecret []byte) error {
+	if c.KeyLogWriter == nil {
+		return nil
+	}
+	_, err := fmt.Fprintf(c.KeyLogWriter, "CLIENT_RANDOM %x %x\n", clientRandom, masterSecret)
+	return err
+}
+
 // A Certificate is a chain of one or more certificates, leaf first.
 type Certificate struct {
 	Certificate [][]byte
diff --git a/vendor/crypto/tls/handshake_client.go b/vendor/crypto/tls/handshake_client.go
index 3c996ac..7c3b328 100644
--- a/vendor/crypto/tls/handshake_client.go
+++ b/vendor/crypto/tls/handshake_client.go
@@ -481,6 +481,10 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 	}
 
 	hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.hello.random, hs.serverHello.random)
+	if err := c.config.writeKeyLog(hs.hello.random, hs.masterSecret); err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
 
 	hs.finishedHash.discardHandshakeBuffer()
 
diff --git a/vendor/crypto/tls/handshake_server.go b/vendor/crypto/tls/handshake_server.go
index e16cddc..691d0b3 100644
--- a/vendor/crypto/tls/handshake_server.go
+++ b/vendor/crypto/tls/handshake_server.go
@@ -459,6 +459,10 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		return err
 	}
 	hs.masterSecret = masterFromPreMasterSecret(c.vers, hs.suite, preMasterSecret, hs.clientHello.random, hs.hello.random)
+	if err := config.writeKeyLog(hs.clientHello.random, hs.masterSecret); err != nil {
+		c.sendAlert(alertInternalError)
+		return err
+	}
 
 	// If we received a client cert in response to our certificate request message,
 	// the client will send us a certificateVerifyMsg immediately after the
diff --git a/vendor/net/http/transport.go b/vendor/net/http/transport.go
index 1e3ea11..41aa30e 100644
--- a/vendor/net/http/transport.go
+++ b/vendor/net/http/transport.go
@@ -1745,6 +1745,7 @@ func cloneTLSConfig(cfg *tls.Config) *tls.Config {
 		MinVersion:               cfg.MinVersion,
 		MaxVersion:               cfg.MaxVersion,
 		CurvePreferences:         cfg.CurvePreferences,
+		KeyLogWriter:             cfg.KeyLogWriter,
 	}
 }
 
@@ -1774,5 +1775,6 @@ func cloneTLSClientConfig(cfg *tls.Config) *tls.Config {
 		MinVersion:               cfg.MinVersion,
 		MaxVersion:               cfg.MaxVersion,
 		CurvePreferences:         cfg.CurvePreferences,
+		KeyLogWriter:             cfg.KeyLogWriter,
 	}
 }