ssh: invert algorithm choices on the server

At the protocol level, SSH lets client and server specify different
algorithms for the read and write half of the connection. This has
never worked correctly, as Client-to-Server was always interpreted as
the "write" side, even if we were the server.

This has never been a problem because, apparently, there are no
clients that insist on different algorithm choices running against Go
SSH servers.

Since the SSH package does not expose a mechanism to specify
algorithms for read/write separately, there is end-to-end for this
change, so add a unittest instead.

Change-Id: Ie3aa781630a3bb7a3b0e3754cb67b3ce12581544
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/172538
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/common_test.go

@@ -0,0 +1,176 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestFindAgreedAlgorithms(t *testing.T) {
+	initKex := func(k *kexInitMsg) {
+		if k.KexAlgos == nil {
+			k.KexAlgos = []string{"kex1"}
+		}
+		if k.ServerHostKeyAlgos == nil {
+			k.ServerHostKeyAlgos = []string{"hostkey1"}
+		}
+		if k.CiphersClientServer == nil {
+			k.CiphersClientServer = []string{"cipher1"}
+
+		}
+		if k.CiphersServerClient == nil {
+			k.CiphersServerClient = []string{"cipher1"}
+
+		}
+		if k.MACsClientServer == nil {
+			k.MACsClientServer = []string{"mac1"}
+
+		}
+		if k.MACsServerClient == nil {
+			k.MACsServerClient = []string{"mac1"}
+
+		}
+		if k.CompressionClientServer == nil {
+			k.CompressionClientServer = []string{"compression1"}
+
+		}
+		if k.CompressionServerClient == nil {
+			k.CompressionServerClient = []string{"compression1"}
+
+		}
+		if k.LanguagesClientServer == nil {
+			k.LanguagesClientServer = []string{"language1"}
+
+		}
+		if k.LanguagesServerClient == nil {
+			k.LanguagesServerClient = []string{"language1"}
+
+		}
+	}
+
+	initDirAlgs := func(a *directionAlgorithms) {
+		if a.Cipher == "" {
+			a.Cipher = "cipher1"
+		}
+		if a.MAC == "" {
+			a.MAC = "mac1"
+		}
+		if a.Compression == "" {
+			a.Compression = "compression1"
+		}
+	}
+
+	initAlgs := func(a *algorithms) {
+		if a.kex == "" {
+			a.kex = "kex1"
+		}
+		if a.hostKey == "" {
+			a.hostKey = "hostkey1"
+		}
+		initDirAlgs(&a.r)
+		initDirAlgs(&a.w)
+	}
+
+	type testcase struct {
+		name                   string
+		clientIn, serverIn     kexInitMsg
+		wantClient, wantServer algorithms
+		wantErr                bool
+	}
+
+	cases := []testcase{
+		testcase{
+			name: "standard",
+		},
+
+		testcase{
+			name: "no common hostkey",
+			serverIn: kexInitMsg{
+				ServerHostKeyAlgos: []string{"hostkey2"},
+			},
+			wantErr: true,
+		},
+
+		testcase{
+			name: "no common kex",
+			serverIn: kexInitMsg{
+				KexAlgos: []string{"kex2"},
+			},
+			wantErr: true,
+		},
+
+		testcase{
+			name: "no common cipher",
+			serverIn: kexInitMsg{
+				CiphersClientServer: []string{"cipher2"},
+			},
+			wantErr: true,
+		},
+
+		testcase{
+			name: "client decides cipher",
+			serverIn: kexInitMsg{
+				CiphersClientServer: []string{"cipher1", "cipher2"},
+				CiphersServerClient: []string{"cipher2", "cipher3"},
+			},
+			clientIn: kexInitMsg{
+				CiphersClientServer: []string{"cipher2", "cipher1"},
+				CiphersServerClient: []string{"cipher3", "cipher2"},
+			},
+			wantClient: algorithms{
+				r: directionAlgorithms{
+					Cipher: "cipher3",
+				},
+				w: directionAlgorithms{
+					Cipher: "cipher2",
+				},
+			},
+			wantServer: algorithms{
+				w: directionAlgorithms{
+					Cipher: "cipher3",
+				},
+				r: directionAlgorithms{
+					Cipher: "cipher2",
+				},
+			},
+		},
+
+		// TODO(hanwen): fix and add tests for AEAD ignoring
+		// the MACs field
+	}
+
+	for i := range cases {
+		initKex(&cases[i].clientIn)
+		initKex(&cases[i].serverIn)
+		initAlgs(&cases[i].wantClient)
+		initAlgs(&cases[i].wantServer)
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			serverAlgs, serverErr := findAgreedAlgorithms(false, &c.clientIn, &c.serverIn)
+			clientAlgs, clientErr := findAgreedAlgorithms(true, &c.clientIn, &c.serverIn)
+
+			serverHasErr := serverErr != nil
+			clientHasErr := clientErr != nil
+			if c.wantErr != serverHasErr || c.wantErr != clientHasErr {
+				t.Fatalf("got client/server error (%v, %v), want hasError %v",
+					clientErr, serverErr, c.wantErr)
+
+			}
+			if c.wantErr {
+				return
+			}
+
+			if !reflect.DeepEqual(serverAlgs, &c.wantServer) {
+				t.Errorf("server: got algs %#v, want %#v", serverAlgs, &c.wantServer)
+			}
+			if !reflect.DeepEqual(clientAlgs, &c.wantClient) {
+				t.Errorf("server: got algs %#v, want %#v", clientAlgs, &c.wantClient)
+			}
+		})
+	}
+}