feat: authentication support fallback (#431)

src/main/java/app/termora/CombinedKeyIdentityProvider.java

@@ -0,0 +1,69 @@
+package app.termora;
+
+import org.apache.sshd.common.keyprovider.KeyIdentityProvider;
+import org.apache.sshd.common.session.SessionContext;
+
+import java.io.IOException;
+import java.security.GeneralSecurityException;
+import java.security.KeyPair;
+import java.util.*;
+
+public class CombinedKeyIdentityProvider implements KeyIdentityProvider {
+
+    private final List<KeyIdentityProvider> providers = new ArrayList<>();
+
+    @Override
+    public Iterable<KeyPair> loadKeys(SessionContext context) {
+        return () -> new Iterator<>() {
+
+            private final Iterator<KeyIdentityProvider> factories = providers
+                    .iterator();
+            private Iterator<KeyPair> current;
+
+            private Boolean hasElement;
+
+            @Override
+            public boolean hasNext() {
+                if (hasElement != null) {
+                    return hasElement;
+                }
+                while (current == null || !current.hasNext()) {
+                    if (factories.hasNext()) {
+                        try {
+                            current = factories.next().loadKeys(context)
+                                    .iterator();
+                        } catch (IOException | GeneralSecurityException e) {
+                            throw new RuntimeException(e);
+                        }
+                    } else {
+                        current = null;
+                        hasElement = Boolean.FALSE;
+                        return false;
+                    }
+                }
+                hasElement = Boolean.TRUE;
+                return true;
+            }
+
+            @Override
+            public KeyPair next() {
+                if ((hasElement == null && !hasNext()) || !hasElement) {
+                    throw new NoSuchElementException();
+                }
+                hasElement = null;
+                KeyPair result;
+                try {
+                    result = current.next();
+                } catch (NoSuchElementException e) {
+                    result = null;
+                }
+                return result;
+            }
+
+        };
+    }
+
+    public void addKeyKeyIdentityProvider(KeyIdentityProvider provider) {
+        providers.add(Objects.requireNonNull(provider));
+    }
+}