ssh: permit empty but non-nil HostKeyAlgorithms, KeyExchanges, Ciphers, MACs

The documentation of ClientConfig.HostKeyAlgorithms says:

  If empty, a reasonable default is used.

However, if HostKeyAlgorithms is empty but non-nil, the reasonable
default won't be applied, and the SSH handshake will fail. Fix this by
changing the nil check to a length check.

While here, do the same for KeyExchanges, Ciphers, MACs in Config.

Change-Id: I1b3d2d2159f3c7d57a9a690ad04f1f6818b39a5c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/800080
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>

ssh/common.go

diff --git a/ssh/common.go b/ssh/common.go
index 4024712..aed0fd9 100644
--- a/ssh/common.go
+++ b/ssh/common.go
@@ -544,7 +544,7 @@
 	if c.Rand == nil {
 		c.Rand = rand.Reader
 	}
-	if c.Ciphers == nil {
+	if len(c.Ciphers) == 0 {
 		c.Ciphers = defaultCiphers
 	}
 	var ciphers []string
@@ -556,7 +556,7 @@
 	}
 	c.Ciphers = ciphers
 
-	if c.KeyExchanges == nil {
+	if len(c.KeyExchanges) == 0 {
 		c.KeyExchanges = defaultKexAlgos
 	}
 	var kexs []string
@@ -571,7 +571,7 @@
 	}
 	c.KeyExchanges = kexs
 
-	if c.MACs == nil {
+	if len(c.MACs) == 0 {
 		c.MACs = defaultMACs
 	}
 	var macs []string

ssh/handshake.go

diff --git a/ssh/handshake.go b/ssh/handshake.go
index 4be3cbb..711a7f7 100644
--- a/ssh/handshake.go
+++ b/ssh/handshake.go
@@ -162,7 +162,7 @@
 	t.remoteAddr = addr
 	t.hostKeyCallback = config.HostKeyCallback
 	t.bannerCallback = config.BannerCallback
-	if config.HostKeyAlgorithms != nil {
+	if len(config.HostKeyAlgorithms) > 0 {
 		t.hostKeyAlgorithms = config.HostKeyAlgorithms
 	} else {
 		t.hostKeyAlgorithms = defaultHostKeyAlgos