fix TestValidTerminalMode: missing output from echo SHELL $SHELL

add leading `echo` to have better compatibility

before

```
 go test -run ^TestValidTerminalMode -v
=== RUN   TestValidTerminalMode
    session_test.go:261: echo SHELL $SHELL && stty -a && exit:
        Last login: Thu Jul  6 12:24:38 2023 from 192.168.200.1
SHELL /bin/bashubuntu:~$
        speed 38400 baud; rows 80; columns 40;
        line = 0;
        intr = ^C; quit = ^\; erase = ^?;
        kill = ^U; eof = ^D; eol = <undef>;
        eol2 = <undef>; swtch = <undef>;
        start = ^Q; stop = ^S; susp = ^Z;
        rprnt = ^R; werase = ^W; lnext = ^V;
        discard = ^O; min = 1; time = 0;
        -parenb -parodd -cmspar cs8 -hupcl
        -cstopb cread -clocal -crtscts
        -ignbrk -brkint -ignpar -parmrk -inpck
        -istrip -inlcr -igncr icrnl ixon -ixoff
        -iuclc -ixany -imaxbel -iutf8
        opost -olcuc -ocrnl onlcr -onocr -onlret
        -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
        isig icanon iexten -echo echoe echok
        -echonl -noflsh -xcase -tostop -echoprt
        echoctl echoke -flusho -extproc
        logout
    session_test.go:266: missing output from echo SHELL $SHELL
```

after

```
 go test -run ^TestValidTerminalMode -v
=== RUN   TestValidTerminalMode
    session_test.go:261: echo SHELL $SHELL && stty -a && exit:
        Last login: Thu Jul  6 12:24:38 2023 from 192.168.200.1
        bolian@ubuntu:~$
        SHELL /bin/bash
        speed 38400 baud; rows 80; columns 40;
        line = 0;
        intr = ^C; quit = ^\; erase = ^?;
        kill = ^U; eof = ^D; eol = <undef>;
        eol2 = <undef>; swtch = <undef>;
        start = ^Q; stop = ^S; susp = ^Z;
        rprnt = ^R; werase = ^W; lnext = ^V;
        discard = ^O; min = 1; time = 0;
        -parenb -parodd -cmspar cs8 -hupcl
        -cstopb cread -clocal -crtscts
        -ignbrk -brkint -ignpar -parmrk -inpck
        -istrip -inlcr -igncr icrnl ixon -ixoff
        -iuclc -ixany -imaxbel -iutf8
        opost -olcuc -ocrnl onlcr -onocr -onlret
        -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
        isig icanon iexten -echo echoe echok
        -echonl -noflsh -xcase -tostop -echoprt
        echoctl echoke -flusho -extproc
        logout
--- PASS: TestValidTerminalMode (0.06s)
```

Change-Id: If60c040edb8c78a7d86bf58a6be47636d9e8f173
GitHub-Last-Rev: a2cc1b1af09e47df82fcb8685d829dfed945e8b0
GitHub-Pull-Request: golang/crypto#264
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/508115
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

ssh/client_auth_test.go

@@ -955,3 +955,124 @@ func TestAuthMethodGSSAPIWithMIC(t *testing.T) {
 		}
 	}
 }
+
+func TestCompatibleAlgoAndSignatures(t *testing.T) {
+	type testcase struct {
+		algo       string
+		sigFormat  string
+		compatible bool
+	}
+	testcases := []*testcase{
+		{
+			KeyAlgoRSA,
+			KeyAlgoRSA,
+			true,
+		},
+		{
+			KeyAlgoRSA,
+			KeyAlgoRSASHA256,
+			true,
+		},
+		{
+			KeyAlgoRSA,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			KeyAlgoRSASHA256,
+			KeyAlgoRSA,
+			true,
+		},
+		{
+			KeyAlgoRSASHA512,
+			KeyAlgoRSA,
+			true,
+		},
+		{
+			KeyAlgoRSASHA512,
+			KeyAlgoRSASHA256,
+			true,
+		},
+		{
+			KeyAlgoRSASHA256,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			KeyAlgoRSASHA512,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			CertAlgoRSAv01,
+			KeyAlgoRSA,
+			true,
+		},
+		{
+			CertAlgoRSAv01,
+			KeyAlgoRSASHA256,
+			true,
+		},
+		{
+			CertAlgoRSAv01,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			CertAlgoRSASHA256v01,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			CertAlgoRSASHA512v01,
+			KeyAlgoRSASHA512,
+			true,
+		},
+		{
+			CertAlgoRSASHA512v01,
+			KeyAlgoRSASHA256,
+			true,
+		},
+		{
+			CertAlgoRSASHA256v01,
+			CertAlgoRSAv01,
+			true,
+		},
+		{
+			CertAlgoRSAv01,
+			CertAlgoRSASHA512v01,
+			true,
+		},
+		{
+			KeyAlgoECDSA256,
+			KeyAlgoRSA,
+			false,
+		},
+		{
+			KeyAlgoECDSA256,
+			KeyAlgoECDSA521,
+			false,
+		},
+		{
+			KeyAlgoECDSA256,
+			KeyAlgoECDSA256,
+			true,
+		},
+		{
+			KeyAlgoECDSA256,
+			KeyAlgoED25519,
+			false,
+		},
+		{
+			KeyAlgoED25519,
+			KeyAlgoED25519,
+			true,
+		},
+	}
+
+	for _, c := range testcases {
+		if isAlgoCompatible(c.algo, c.sigFormat) != c.compatible {
+			t.Errorf("algorithm %q, signature format %q, expected compatible to be %t", c.algo, c.sigFormat, c.compatible)
+		}
+	}
+}

ssh/common.go

@@ -85,7 +85,7 @@ var supportedHostKeyAlgos = []string{
 // This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
 // because they have reached the end of their useful life.
 var supportedMACs = []string{
-	"hmac-sha2-512-etm@openssh.com", "hmac-sha2-256-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
+	"hmac-sha2-256-etm@openssh.com", "hmac-sha2-512-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
 }
 
 var supportedCompressions = []string{compressionNone}
@@ -119,6 +119,13 @@ func algorithmsForKeyFormat(keyFormat string) []string {
 	}
 }
 
+// isRSA returns whether algo is a supported RSA algorithm, including certificate
+// algorithms.
+func isRSA(algo string) bool {
+	algos := algorithmsForKeyFormat(KeyAlgoRSA)
+	return contains(algos, underlyingAlgo(algo))
+}
+
 // supportedPubKeyAuthAlgos specifies the supported client public key
 // authentication algorithms. Note that this doesn't include certificate types
 // since those use the underlying algorithm. This list is sent to the client if

ssh/server.go

@@ -370,6 +370,25 @@ func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *c
 	return authErr, perms, nil
 }
 
+// isAlgoCompatible checks if the signature format is compatible with the
+// selected algorithm taking into account edge cases that occur with old
+// clients.
+func isAlgoCompatible(algo, sigFormat string) bool {
+	// Compatibility for old clients.
+	//
+	// For certificate authentication with OpenSSH 7.2-7.7 signature format can
+	// be rsa-sha2-256 or rsa-sha2-512 for the algorithm
+	// ssh-rsa-cert-v01@openssh.com.
+	//
+	// With gpg-agent < 2.2.6 the algorithm can be rsa-sha2-256 or rsa-sha2-512
+	// for signature format ssh-rsa.
+	if isRSA(algo) && isRSA(sigFormat) {
+		return true
+	}
+	// Standard case: the underlying algorithm must match the signature format.
+	return underlyingAlgo(algo) == sigFormat
+}
+
 // ServerAuthError represents server authentication errors and is
 // sometimes returned by NewServerConn. It appends any authentication
 // errors that may occur, and is returned if all of the authentication
@@ -567,7 +586,7 @@ userAuthLoop:
 					authErr = fmt.Errorf("ssh: algorithm %q not accepted", sig.Format)
 					break
 				}
-				if underlyingAlgo(algo) != sig.Format {
+				if !isAlgoCompatible(algo, sig.Format) {
 					authErr = fmt.Errorf("ssh: signature %q not compatible with selected algorithm %q", sig.Format, algo)
 					break
 				}

ssh/test/session_test.go

@@ -248,7 +248,7 @@ func TestValidTerminalMode(t *testing.T) {
 		t.Fatalf("session failed: %s", err)
 	}
 
-	if _, err := io.WriteString(stdin, "echo SHELL $SHELL && stty -a && exit\n"); err != nil {
+	if _, err := io.WriteString(stdin, "echo && echo SHELL $SHELL && stty -a && exit\n"); err != nil {
 		t.Fatal(err)
 	}
 
@@ -258,7 +258,7 @@ func TestValidTerminalMode(t *testing.T) {
 	}
 
 	if testing.Verbose() {
-		t.Logf("echo SHELL $SHELL && stty -a && exit:\n%s", buf)
+		t.Logf("echo && echo SHELL $SHELL && stty -a && exit:\n%s", buf)
 	}
 
 	shellLine := regexp.MustCompile("(?m)^SHELL (.*)$").FindStringSubmatch(buf.String())