ssh: use the configured rand in kex init

Change-Id: I4f89c395886b9dd07b584d1fcf1a0f2df215b91b
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/644435
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: drakkan

ssh/handshake.go

@@ -5,7 +5,6 @@
 package ssh
 
 import (
-	"crypto/rand"
 	"errors"
 	"fmt"
 	"io"
@@ -501,7 +500,7 @@ func (t *handshakeTransport) sendKexInit() error {
 		CompressionClientServer: supportedCompressions,
 		CompressionServerClient: supportedCompressions,
 	}
-	io.ReadFull(rand.Reader, msg.Cookie[:])
+	io.ReadFull(t.config.Rand, msg.Cookie[:])
 
 	// We mutate the KexAlgos slice, in order to add the kex-strict extension algorithm,
 	// and possibly to add the ext-info extension algorithm. Since the slice may be the