fix: detect RSA public keys against modern OpenSSH (issue #48)

auth/halfauth_research.go

@@ -29,3 +29,8 @@ func (s *HalfSigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
 	copy(s.SignData, data)
 	return nil, ErrHalfAuth
 }
+
+// SignWithAlgorithm makes HalfSigner an ssh.AlgorithmSigner so RSA keys are probed with rsa-sha2-* instead of SHA-1 ssh-rsa.
+func (s *HalfSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*ssh.Signature, error) {
+	return s.Sign(rand, data)
+}

auth/results.go

@@ -70,22 +70,31 @@ func (r *AuthResult) SupportsHostKey(t string) bool {
 }
 
 func (r *AuthResult) SupportsPubKeyType(t string) bool {
-	// Example: ssh-ed25519,sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-256,rsa-sha2-512,ssh-rsa,ssh-dss
 	okTypes, ok := r.Extensions["server-sig-algs"]
-	if !ok {
-		// Assume all types are supported unless the server
-		// has told us otherwise via the extension.
+	if !ok || strings.TrimSpace(okTypes) == "" {
 		return true
 	}
+	// ssh-rsa keys are usable when the server advertises any RSA signature algo;
+	// modern OpenSSH (>= 8.8) only offers rsa-sha2-256/rsa-sha2-512, not ssh-rsa.
+	wanted := sigAlgosForKeyType(t)
 	for kt := range strings.SplitSeq(okTypes, ",") {
 		kt = strings.TrimSpace(kt)
-		if strings.EqualFold(kt, t) {
-			return true
+		for _, w := range wanted {
+			if strings.EqualFold(kt, w) {
+				return true
+			}
 		}
 	}
 	return false
 }
 
+func sigAlgosForKeyType(t string) []string {
+	if t == "ssh-rsa" {
+		return []string{"ssh-rsa", "rsa-sha2-256", "rsa-sha2-512"}
+	}
+	return []string{t}
+}
+
 func (r *AuthResult) AddVuln(v VulnResult) {
 	r.Vulns = append(r.Vulns, v)
 }

auth/results_test.go

@@ -0,0 +1,27 @@
+package auth
+
+import "testing"
+
+func TestSupportsPubKeyType(t *testing.T) {
+	const modern = "ssh-ed25519,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256"
+	const legacy = "ssh-ed25519,rsa-sha2-256,rsa-sha2-512,ssh-rsa"
+
+	cases := []struct {
+		sigAlgs string
+		keyType string
+		want    bool
+	}{
+		{modern, "ssh-rsa", true},
+		{modern, "ssh-ed25519", true},
+		{legacy, "ssh-rsa", true},
+		{"ssh-ed25519", "ssh-rsa", false},
+		{modern, "ecdsa-sha2-nistp521", false},
+	}
+
+	for _, tc := range cases {
+		r := &AuthResult{Extensions: map[string]string{"server-sig-algs": tc.sigAlgs}}
+		if got := r.SupportsPubKeyType(tc.keyType); got != tc.want {
+			t.Errorf("SupportsPubKeyType(%q) with %q = %v, want %v", tc.keyType, tc.sigAlgs, got, tc.want)
+		}
+	}
+}