ssh: allow to configure public key auth algorithms on the server side

Fixes golang/go#61244

Change-Id: I29b43e379cf0cdb07b0d6935666491b997157e73
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/510775
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Commit-Queue: Nicola Murino <nicola.murino@gmail.com>
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/server_test.go

@@ -0,0 +1,85 @@
+// Copyright 2023 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 (
+	"testing"
+)
+
+func TestClientAuthRestrictedPublicKeyAlgos(t *testing.T) {
+	for _, tt := range []struct {
+		name      string
+		key       Signer
+		wantError bool
+	}{
+		{"rsa", testSigners["rsa"], false},
+		{"dsa", testSigners["dsa"], true},
+		{"ed25519", testSigners["ed25519"], true},
+	} {
+		c1, c2, err := netPipe()
+		if err != nil {
+			t.Fatalf("netPipe: %v", err)
+		}
+		defer c1.Close()
+		defer c2.Close()
+		serverConf := &ServerConfig{
+			PublicKeyAuthAlgorithms: []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512},
+			PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+				return nil, nil
+			},
+		}
+		serverConf.AddHostKey(testSigners["ecdsap256"])
+
+		done := make(chan struct{})
+		go func() {
+			defer close(done)
+			NewServerConn(c1, serverConf)
+		}()
+
+		clientConf := ClientConfig{
+			User: "user",
+			Auth: []AuthMethod{
+				PublicKeys(tt.key),
+			},
+			HostKeyCallback: InsecureIgnoreHostKey(),
+		}
+
+		_, _, _, err = NewClientConn(c2, "", &clientConf)
+		if err != nil {
+			if !tt.wantError {
+				t.Errorf("%s: got unexpected error %q", tt.name, err.Error())
+			}
+		} else if tt.wantError {
+			t.Errorf("%s: succeeded, but want error", tt.name)
+		}
+		<-done
+	}
+}
+
+func TestNewServerConnValidationErrors(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := &ServerConfig{
+		PublicKeyAuthAlgorithms: []string{CertAlgoRSAv01},
+	}
+	_, _, _, err = NewServerConn(c1, serverConf)
+	if err == nil {
+		t.Fatal("NewServerConn with invalid public key auth algorithms succeeded")
+	}
+	serverConf = &ServerConfig{
+		Config: Config{
+			KeyExchanges: []string{kexAlgoDHGEXSHA256},
+		},
+	}
+	_, _, _, err = NewServerConn(c1, serverConf)
+	if err == nil {
+		t.Fatal("NewServerConn with unsupported key exchange succeeded")
+	}
+}